feat: live operating-point fill + -6 dB refs on the gain curve

Add a per-channel input_level meter (post pre-gain, peak-with-decay) and use it
to shade the gain curve: a translucent fill under the curve from the floor up to
the current input, its right edge riding the curve (width = input, height =
output), plus a dot at the operating point. Add -6 dBFS reference lines on both
axes with a tick label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -34,12 +34,12 @@ Built with **Rust** + **NIH-plug** (VST3 + CLAP output) + **egui** for the UI.
 ```
 Input
   └─ Crossover filterbank (Linkwitz-Riley LR4 @ each crossover freq)
-       ├─ Band 1 (low)   → look-ahead delay → compressor VCA → gain stage ─┐ (bypassable)
-       ├─ Band 2 (mid)   → look-ahead delay → compressor VCA → gain stage ─┤ (bypassable)
-       └─ Band 3 (high)  → look-ahead delay → compressor VCA → gain stage ─┤ (bypassable)
+       ├─ Band 1 (low)   → pre-gain → look-ahead delay → compressor VCA → makeup ─┐ (dry/wet mix)
+       ├─ Band 2 (mid)   → pre-gain → look-ahead delay → compressor VCA → makeup ─┤ (dry/wet mix)
+       └─ Band 3 (high)  → pre-gain → look-ahead delay → compressor VCA → makeup ─┤ (dry/wet mix)
                                                                                   │
-       Sum of bands ◄──────────────────────────────────────────────------┘
-         └─ 'All' channel → look-ahead delay → compressor VCA → gain stage
+       Sum of bands ◄─────────────────────────────────────────────────────------┘
+         └─ 'All' channel → pre-gain → look-ahead delay → compressor VCA → makeup
               └─ output brickwall limiter (true-peak, 4x oversampled) → output
 ```
  
@@ -62,20 +62,21 @@ a first-class mode, not an afterthought.
 - Bands sum phase-coherently to flat **magnitude** (the sum is an all-pass; lower bands get an all-pass at each later crossover to match phase — not a bit-exact time-domain null)
 - Crossover frequencies are user-adjustable parameters
 ### Per-Band Compressor
+- **Pre-gain (drive)**: scales the band *before* the detector, so it pushes harder into compression and feeds the sum/limiter hotter — a mild "compressed semi-distortion" without a dedicated saturator. Applied in the wiring (the compressor itself is untouched). Pairs with makeup for full input/output gain-staging
 - Level detection: switchable peak / RMS (RMS window currently hardcoded small; can be exposed later)
 - Gain computer: threshold, ratio, soft knee
 - Attack / release envelopes (logarithmic ballistics)
-- Makeup gain per band
+- Makeup gain per band (−24…+24 dB — attenuates as well as boosts)
 - Look-ahead: circular delay buffer on the audio path; detector reads ahead
 ### 'All' Aggregate Channel
 - Structurally **identical to a per-band compressor** — reuse the same comp/lim code/params, just fed the summed signal instead of a filtered band
 - Runs after the three bands are summed, before the output brickwall limiter
-- Bands are individually bypassable; with all three bypassed the (phase-coherent) crossover sum equals the dry input, so the 'All' channel alone acts as a full-band comp/lim
+- Bands have a per-channel dry/wet **mix** (parallel compression); at 0% (or all three dry) the (phase-coherent) crossover sum equals the dry input, so the 'All' channel alone acts as a full-band comp/lim
 - Has its own look-ahead; the plugin reports a single **constant** total latency (the fixed band + 'All' look-ahead), set once — see Latency below
 ### Output Limiter
 - Brickwall, ceiling = 0 dBFS or user-defined (`output_ceiling`). Look-ahead + sliding-max peak detection + a ceiling clamp guarantee the output never exceeds the ceiling
 - Short attack (≤ 0.1 ms), auto-release (release time user-set)
-- **Sample-peak today**; 4x-oversampled true-peak (inter-sample) detection is the remaining Stage-4 work
+- **True-peak**: 4× polyphase oversampling estimates the inter-sample peak (detection only — the upsampled signal is discarded); the limiter targets a 0.3 dB margin under the ceiling to cover the 4× residual
 ### Latency
 - Reported via `context.set_latency_samples()` in `initialize()` — **never** from `process()`; renegotiating latency mid-stream crashes some hosts (FL included)
 - Reported latency is a **constant** (the max look-ahead); the look-ahead control only moves the detector tap within that fixed delay
@@ -85,21 +86,29 @@ a first-class mode, not an afterthought.
 ## Parameters
  
 ### Global
-- `input_gain` — pre-gain before filterbank (dB)
 - `output_ceiling` — brickwall ceiling (dBFS, default 0.0)
 - `limiter_release_ms` — output limiter release time
 - `look_ahead_ms` — look-ahead time (0–5 ms). Reported latency is **constant** (the max look-ahead); the knob only moves the detector tap within that fixed delay, so it is safe to adjust during playback (changing reported latency mid-stream crashes some hosts, FL included)
 - `crossover_low_hz` — low/mid crossover frequency
 - `crossover_high_hz` — mid/high crossover frequency
+
+> **Crossover automation caveat:** the lo ≤ hi limit is enforced in the **editor only** (the two
+> are independent params). Host automation writes them directly, so it can drive lo past hi and
+> momentarily invert the mid band. The DSP clamps to a monotonic split so it won't break audio,
+> but FL's automation can misbehave once inverted. Not fixed by design — just don't automate the
+> two across each other.
 ### Per-Channel Compressor (× 4: low, mid, high, **all** — one `#[nested]` params struct reused)
+- `pre_gain_db` — drive into the compressor (−24…+36 dB, smoothed)
 - `detection` — peak / RMS level detection
+- `low_slope` — low-level shaper slope at the silence floor (1 = unity, >1 fans up/boost, <1 fans down/cut). **Serial**: reshapes the level *before* the threshold, so a boost can lift quiet material up into compression
+- `low_curve` — bends the low shaper toward a bounded saturation (0% = straight line) so the serial composition doesn't run away
 - `threshold_db`
 - `ratio` — 1.0 (off) to ∞ (limiting)
+- `knee_db` — soft knee width
 - `attack_ms`
 - `release_ms`
-- `knee_db` — soft knee width
-- `makeup_gain_db`
-- `bypass` — per-channel bypass (bypassing low+mid+high = simple full-band comp via the 'all' channel)
+- `makeup_db` — makeup gain (−24…+24 dB)
+- `mix` — per-channel dry/wet mix (parallel compression); 0% = dry (a clean bypass), 100% = fully processed. Bands at 0% → simple full-band comp via the 'all' channel
 
 The 'all' channel uses the same struct so its UI and DSP are identical to a band; it just sits after the band sum.
 ---
@@ -110,24 +119,37 @@ Target layout (✅ = exists today; the rest is planned):
 
 ```
 src/
-  lib.rs            # ✅ Plugin trait + Params + egui editor (all inline for now)
-  params.rs         #    (planned) split Params out of lib.rs
+  lib.rs            # ✅ Plugin trait + DSP wiring + process()
+  params.rs         # ✅ Params structs, defaults, build_settings()
+  editor.rs         # ✅ egui editor: meter panel + rolling plot (drawn via Painter) + slider columns
+  meters.rs         # ✅ lock-free Meters (atomics): decayed bar values + raw plot feed
   dsp/
     mod.rs          # ✅ module declarations
     compressor.rs   # ✅ full-band comp: peak/RMS detector, gain computer, ballistics, look-ahead delay
     crossover.rs    # ✅ LR4 3-band filterbank with all-pass phase compensation
     biquad.rs       # ✅ generic biquad (Transposed Direct Form II)
-    limiter.rs      # ✅ look-ahead brickwall limiter (sample-peak; true-peak pending)
-    delay.rs        #    (planned) look-ahead delay (currently inside compressor.rs / limiter.rs)
-    oversampler.rs  #    (planned) 4x oversampler for true-peak detection
-  editor/
-    mod.rs          #    (planned) egui editor split out of lib.rs
-    widgets/
-      gain_curve.rs #    (planned) custom egui Widget: gain curve display
-      band_meter.rs #    (planned) per-band gain reduction meter
-      level_meter.rs#    (planned) input/output level meter
+    limiter.rs      # ✅ look-ahead brickwall limiter (true-peak via oversampler)
+    oversampler.rs  # ✅ 4x polyphase oversampler for true-peak detection (detection-only)
 ```
 
+The editor lives in an `editor/` module — one file per visualiser widget (each owns its GUI
+state), with `mod.rs` as the aggregator/layout. Drawn directly with egui's `Painter`.
+
+```
+src/
+  editor/
+    mod.rs            # aggregator: create(), EditorState, layout, placeholder slider columns
+    meter.rs          # |L | GR | R| level + gain-reduction bars + per-channel ceiling lamp
+    plot.rs           # rolling in/out/GR scope (200 Hz ring feed) + ceiling-hit markers
+    crossover.rs      # log-freq strip with draggable crossover handles + number boxes
+    gain_curve.rs     # static gain-curve display (out vs in) for the selected channel
+```
+
+Remaining UI work: replace the placeholder per-channel slider columns in `mod.rs` with the real
+layout.
+
+Deferred until the redesign — no need to split prematurely while the layout is still a placeholder.
+ 
 ---
  
 ## Build Steps
@@ -176,11 +198,13 @@ is essential — without it FL silently skips a plugin it has seen before.)
  
 Work through these stages in order — each stage produces a loadable, audible plugin.
 
-**Status (2026-06-19):** Stages 1–3 plus the **base-rate brickwall limiter** (Stage 4a) are done:
-3-band LR4 crossover → per-band compressors (peak/RMS) → 'All' channel → look-ahead brickwall
-limiter, with a basic 4-column UI. **Next: Stage 4b — 4× oversampling for true-peak (inter-sample)
-limiting** (deferred as the CPU-heavy part). DSP is in `src/dsp/` (`biquad.rs`, `crossover.rs`,
-`compressor.rs`, `limiter.rs`); params and the egui editor are still inline in `src/lib.rs`.
+**Status (2026-06-25):** Stages 1–4 done — the full signal chain works: 3-band LR4 crossover →
+per-band pre-gain + compressors (peak/RMS) → per-channel dry/wet mix → 'All' channel → **true-peak
+brickwall limiter** (4× oversampled detection). `lib.rs` is split into `params.rs`, `meters.rs`, and
+an `editor/` widget module. Stage 6 visualisers are essentially complete: per-channel **|L | GR | R|
+meters** + **per-channel ceiling lamps**, a **rolling in/out/GR plot** (200 Hz ring feed, flow-speed,
+ceiling-hit markers), **draggable crossover handles**, and a **static gain-curve display**. **Next:
+replace the placeholder slider columns with the real UI layout.**
  
 ### Stage 1 — Skeleton plugin ✅
 - [x] NIH-plug "passthrough" compiling and loading in DAW
@@ -200,23 +224,25 @@ limiting** (deferred as the CPU-heavy part). DSP is in `src/dsp/` (`biquad.rs`,
 - [x] Apply per-band compressor to each band
 - [x] Sum bands back together
 - [x] Run the summed signal through the 'All' channel compressor before output
-### Stage 4 — Output brickwall limiter + oversampler  *(4a done; 4b = oversampling)*
+### Stage 4 — Output brickwall limiter + oversampler ✅
 - [x] Look-ahead delay (circular buffer) — inside `compressor.rs` and `limiter.rs`, no separate `delay.rs`
 - [x] Wire look-ahead: detector reads N samples ahead of the VCA
-- [x] Report latency — `context.set_latency_samples()` once; now the constant three-stage total (bands + 'All' + limiter)
-- [x] Brickwall output limiter (`limiter.rs`): look-ahead + sliding-max peak detect + ceiling clamp guarantee; limits **sample** peaks
-- [ ] `oversampler.rs` (4x, polyphase FIR / windowed sinc)  ⬅ next
-- [ ] True-peak (inter-sample) limiting on top of the brickwall, via the oversampler
+- [x] Report latency — `context.set_latency_samples()` once; constant three-stage total (bands + 'All' + limiter)
+- [x] Brickwall output limiter (`limiter.rs`): look-ahead + sliding-max + ceiling clamp guarantee
+- [x] `oversampler.rs` — 4× polyphase windowed-sinc, detection-only (returns the inter-sample max)
+- [x] True-peak limiting: limiter peak = max(sample, inter-sample); targets a 0.3 dB margin under the ceiling for the 4× residual
 ### Stage 5 — Basic egui UI  *(basic version done early)*
 - [x] Add `nih_plug_egui` editor
 - [x] Sliders for all current parameters (`ParamSlider` grid)
-- [ ] Per-band bypass toggles *(partial — single-band bypass present; per-band arrives with Stage 3)*
+- [x] Per-channel dry/wet mix (parallel compression; replaced the bypass toggle)
 - [x] Confirm UI controls update DSP in real time
 ### Stage 6 — Custom visualisations
-- [ ] `level_meter.rs` — input/output RMS + peak meters
-- [ ] `band_meter.rs` — per-band gain reduction meters (vertical bars)
-- [ ] `gain_curve.rs` — static gain curve display per band (threshold/ratio/knee)
-- [ ] Draggable crossover handles on a frequency display
+- [x] Per-channel level meters (output level, `|L | GR | R|` cluster)
+- [x] Per-channel gain-reduction meters (vertical bars) + latching ceiling lamp
+- [x] Rolling in/out/gain-reduction plot (per-channel tabs, flow-speed selector)
+- [x] Static gain-curve display (out vs in; includes pre-gain + makeup) for the selected channel
+- [x] Draggable crossover handles on a log-frequency display (with number boxes)
+- [ ] Replace the placeholder slider columns with the real UI
 ---
  
 ## Key Implementation Notes
@@ -239,8 +265,16 @@ far below audibility. We therefore do **not** set the register ourselves or flus
 NIH-plug provides `Smoother` — use it for all gain/threshold params to avoid zipper noise.
  
 ### Thread safety
-Params are atomics. The editor and audio thread communicate only through params and
-`Arc<Mutex<...>>` meter data. Never pass DSP state to the UI directly.
+Params are atomics. The editor and audio thread communicate only through params and a shared
+`Arc<Meters>` (`meters.rs`) — never a mutex on the audio path. Two lock-free feeds, both gated on
+the editor being open:
+- **Bar meters** — decayed atomic scalars, one store per block; the editor reads them each frame.
+- **Scrolling plot** — a single-producer/single-consumer `ScopeRing` of buckets clocked at
+  ~200 Hz, so the plot's horizontal resolution is decoupled from the ~60 fps repaint. The editor
+  drains all new buckets each frame. The scope is **transport-gated** (advances only while playing)
+  so it freezes rather than scrolling silence when the host is stopped/paused.
+
+Never pass DSP state to the UI directly.
  
 ### VST3 licensing
 You must accept Steinberg's VST3 SDK licence before distributing VST3 binaries.

deploy.ps1

@@ -74,11 +74,22 @@ if (Test-Path -LiteralPath '$clapSrc') { Copy-Item -LiteralPath '$clapSrc' -Dest
     Start-Process powershell -Verb RunAs -Wait -ArgumentList '-NoProfile','-EncodedCommand',$enc
 }
 
-$check = Join-Path $vst3Dst "$vst3Name\Contents\x86_64-win\$vst3Name"
-if (Test-Path $check) {
-    Write-Host "VST3 installed OK -> $check" -ForegroundColor Green
-    Write-Host "In FL Studio: Manage plugins -> 'Rescan previously failed plugins' -> Find installed plugins." -ForegroundColor Yellow
-}
-else {
-    throw "Install verification failed: $check not found"
+# Verify each bundle was actually refreshed. A plugin currently loaded in a DAW keeps its
+# binary locked, so the copy fails silently and leaves a STALE install — re-scanning then runs
+# old code. Compare install vs build timestamps to catch exactly that.
+function Test-Installed($label, $src, $dst) {
+    if (-not (Test-Path $src)) { return }  # nothing was built for this format
+    if (-not (Test-Path $dst)) { throw "$label install verification failed: $dst not found" }
+    $srcT = (Get-Item $src).LastWriteTime
+    $dstT = (Get-Item $dst).LastWriteTime
+    if ($dstT -lt $srcT) {
+        Write-Warning "$label is STALE (installed $dstT < built $srcT). It's almost certainly loaded in your DAW (file locked). Close the plugin/DAW and re-run deploy."
+    }
+    else {
+        Write-Host "$label installed OK -> $dst" -ForegroundColor Green
+    }
 }
+
+Test-Installed "VST3" (Join-Path $vst3Src "Contents\x86_64-win\$vst3Name") (Join-Path $vst3Dst "$vst3Name\Contents\x86_64-win\$vst3Name")
+Test-Installed "CLAP" $clapSrc (Join-Path $clapDst $clapName)
+Write-Host "In FL Studio: Manage plugins -> 'Rescan previously failed plugins' -> Find installed plugins." -ForegroundColor Yellow

src/dsp/compressor.rs

@@ -26,6 +26,16 @@ const MAX_CHANNELS: usize = 2;
 /// ~ -240 dBFS; keeps `log10` away from zero without affecting audible levels.
 const LEVEL_EPS: f32 = 1e-12;
 
+/// Silence-floor anchor for the below-threshold shaping: at/below this level the gain change is 0
+/// (silence stays silence), and the low region fans up/down from here toward the threshold. Matches
+/// the editor gain-curve's display floor.
+const LOW_ANCHOR_DB: f32 = -60.0;
+
+/// Max bulge (dB) the low-shaper curvature adds at the MIDDLE of the low region, at `|low_curve|`=1.
+/// Bipolar: positive bulges up (boost the quiet middle), negative bulges down (suppress). Zero at
+/// both ends (silence floor and the knee), so it never moves those anchors.
+const LOW_BULGE_MAX_DB: f32 = 12.0;
+
 /// Hardcoded RMS averaging window (one-pole time constant). Deliberately small; can be
 /// promoted to a parameter later.
 const RMS_WINDOW_MS: f32 = 5.0;
@@ -40,6 +50,11 @@ pub struct CompressorSettings {
     pub threshold_db: f32,
     pub ratio: f32,
     pub knee_db: f32,
+    /// Low shaper slope at the silence floor (1 = unity; >1 fans up/boost, <1 fans down/cut).
+    /// Reshapes the level the compressor sees (serial), anchored at the floor.
+    pub low_slope: f32,
+    /// Low shaper curvature, 0..1 (0 = straight line, 1 = max bend toward bounded saturation).
+    pub low_curve: f32,
     /// One-pole coefficient for the attack ramp (see [`Compressor::time_to_coef`]).
     pub attack_coef: f32,
     /// One-pole coefficient for the release ramp.
@@ -50,7 +65,9 @@ pub struct CompressorSettings {
     pub lookahead_samples: usize,
     /// `true` = RMS detection (running power average), `false` = naive sample peak.
     pub use_rms: bool,
-    pub bypass: bool,
+    /// Dry/wet blend, 0..=1. 1 = fully compressed (incl. makeup), 0 = dry passthrough (bypass).
+    /// Parallel: dry and wet share the same delayed input, so the mix is phase-aligned.
+    pub mix: f32,
 }
 
 pub struct Compressor {
@@ -67,7 +84,8 @@ pub struct Compressor {
     mean_sq: f32,
     rms_coef: f32,
 
-    /// Smooth decoupled peak-detector state, expressed as dB of **attenuation** (>= 0).
+    /// Smooth decoupled peak-detector state, in dB of attenuation (signed: usually >= 0, but can go
+    /// negative = boost when `low_slope < 1`). The `max()` recurrence makes cut fast / boost slow.
     y1: f32, // release branch (peak-with-decay)
     yl: f32, // attack-smoothed output
 }
@@ -129,29 +147,68 @@ impl Compressor {
         }
     }
 
-    /// Static compressor curve. Returns gain reduction in dB (<= 0) for an input `level_db`.
-    /// Quadratic soft knee of width `knee_db`, centred on `threshold_db`.
-    fn gain_computer(level_db: f32, threshold_db: f32, ratio: f32, knee_db: f32) -> f32 {
+    /// Pure compressor transfer (threshold / ratio / quadratic soft knee). Returns gain reduction
+    /// in dB (<= 0) for an input `level_db`.
+    fn comp_gain_db(level_db: f32, threshold_db: f32, ratio: f32, knee_db: f32) -> f32 {
         let slope = 1.0 / ratio - 1.0; // <= 0 for ratio >= 1
         let over = level_db - threshold_db;
-
         if knee_db > 0.0 && 2.0 * over.abs() <= knee_db {
-            // Inside the knee: a parabola joining the two regions with a continuous slope.
             let x = over + knee_db * 0.5; // 0..knee
             slope * x * x / (2.0 * knee_db)
         } else if over > 0.0 {
-            // Above the knee (also covers the hard-knee case): linear region.
             slope * over
         } else {
             0.0
         }
     }
 
+    /// Low-level shaper gain in dB. Anchored at BOTH the silence floor ([`LOW_ANCHOR_DB`]) and the
+    /// knee (threshold). `low_slope` tilts the straight line between those anchors (1 = unity);
+    /// `low_curve` (-1..1) bulges that line in the middle without moving either endpoint — positive
+    /// bulges up (boost the quiet middle), negative down (suppress). Reshapes the level the
+    /// compressor then sees.
+    fn low_gain_db(level_db: f32, threshold_db: f32, low_slope: f32, low_curve: f32) -> f32 {
+        let d = level_db - LOW_ANCHOR_DB;
+        if d <= 0.0 {
+            return 0.0;
+        }
+        let span = (threshold_db - LOW_ANCHOR_DB).max(1.0); // floor -> threshold width
+        let t = (d / span).min(1.0); // normalized position, clamped at the knee
+        // Straight line anchored at the floor (t=0 -> 0) and the knee (t=1 -> (slope-1)*span).
+        let slope_line = (low_slope - 1.0) * span * t;
+        // Bipolar bulge: 0 at both ends, peaks (4·t·(1-t) = 1) at the middle.
+        let bulge = low_curve * LOW_BULGE_MAX_DB * 4.0 * t * (1.0 - t);
+        slope_line + bulge
+    }
+
+    /// Full static curve, **serial**: the low shaper reshapes the level, then the compressor's
+    /// threshold sees the shaped level. Returns total gain in dB (signed: negative = cut, positive
+    /// = boost). `gain = low + comp(level + low)`. Shared with the editor's gain-curve display —
+    /// single source of truth.
+    pub fn gain_computer(
+        level_db: f32,
+        threshold_db: f32,
+        ratio: f32,
+        knee_db: f32,
+        low_slope: f32,
+        low_curve: f32,
+    ) -> f32 {
+        let low = Self::low_gain_db(level_db, threshold_db, low_slope, low_curve);
+        low + Self::comp_gain_db(level_db + low, threshold_db, ratio, knee_db)
+    }
+
     /// The plugin's fixed reported latency in samples (the constant audio delay).
     pub fn latency(&self) -> u32 {
         self.fixed_delay as u32
     }
 
+    /// Current gain reduction being applied, in dB (>= 0), excluding makeup. For metering.
+    /// This is the smoothed detector output `yl`, so it tracks the visible needle, not the
+    /// instantaneous static curve.
+    pub fn gain_reduction_db(&self) -> f32 {
+        self.yl
+    }
+
     /// Process one sample frame in place: `input[ch]` -> `output[ch]`.
     ///
     /// `input` and `output` are short stack slices (one value per channel), so this
@@ -181,18 +238,22 @@ impl Compressor {
             peak = peak.max(self.delay[ch][det_pos].abs());
         }
 
-        // 4) Gain computer + ballistics. On bypass we keep the delay aligned (so toggling
-        //    bypass doesn't shift timing) but apply unity gain and no makeup.
-        let gain_lin = if set.bypass {
-            1.0
-        } else {
-            // RMS = running mean of the linked squared level over a fixed window. Updated
-            // whenever active (regardless of mode) so switching peak<->RMS is seamless.
+        // 4) Gain computer + ballistics. The detector ALWAYS runs (even at mix 0) so metering
+        //    reflects the wet gain reduction regardless of the dry/wet blend.
+        //    RMS = running mean of the linked squared level over a fixed window. Updated whenever
+        //    active (regardless of mode) so switching peak<->RMS is seamless.
         self.mean_sq = self.rms_coef * self.mean_sq + (1.0 - self.rms_coef) * peak * peak;
         let detector = if set.use_rms { self.mean_sq.sqrt() } else { peak };
         let level_db = 20.0 * (detector + LEVEL_EPS).log10();
         // Desired attenuation in dB, as a positive quantity.
-            let target = -Self::gain_computer(level_db, set.threshold_db, set.ratio, set.knee_db);
+        let target = -Self::gain_computer(
+            level_db,
+            set.threshold_db,
+            set.ratio,
+            set.knee_db,
+            set.low_slope,
+            set.low_curve,
+        );
 
         // Smooth, decoupled peak detector (Giannoulis eq. 17–18) on the attenuation:
         //   y1 = max(target, release-smoothed y1)   (fast up / slow down "peak hold")
@@ -200,13 +261,14 @@ impl Compressor {
         self.y1 = target.max(set.release_coef * self.y1 + (1.0 - set.release_coef) * target);
         self.yl = set.attack_coef * self.yl + (1.0 - set.attack_coef) * self.y1;
 
-            let total_db = set.makeup_db - self.yl;
-            10.0f32.powf(total_db / 20.0)
-        };
+        let wet_gain = 10.0f32.powf((set.makeup_db - self.yl) / 20.0);
 
-        // 5) Output = delayed input (always `fixed_delay` old) * gain.
+        // 5) Dry/wet mix (parallel compression). Both paths use the same delayed input, so the
+        //    blend is phase-aligned. mix = 0 -> dry passthrough (clean bypass), mix = 1 -> wet.
+        let mix = set.mix.clamp(0.0, 1.0);
+        let blend = (1.0 - mix) + mix * wet_gain;
         for ch in 0..n {
-            output[ch] = self.delay[ch][out_pos] * gain_lin;
+            output[ch] = self.delay[ch][out_pos] * blend;
         }
 
         // 6) Advance the write head.
@@ -234,20 +296,41 @@ mod tests {
             makeup_db: 0.0,
             lookahead_samples: 0,
             use_rms: false,
-            bypass: false,
+            mix: 1.0,
+            low_slope: 1.0,
+            low_curve: 0.0,
         }
     }
 
     #[test]
     fn below_threshold_is_untouched() {
         // -30 dB input, -20 dB threshold -> no reduction.
-        assert_eq!(Compressor::gain_computer(-30.0, -20.0, 4.0, 6.0), 0.0);
+        assert_eq!(Compressor::gain_computer(-30.0, -20.0, 4.0, 6.0, 1.0, 0.0), 0.0);
+    }
+
+    #[test]
+    fn low_shaper_serial_slope_and_bipolar_bulge() {
+        let thr = -18.0;
+        // Serial slope-only (no curve): boost lifts -30 by 30 dB to 0 dB -> 18 dB over threshold,
+        // comp pulls back (1/4 - 1)*18 = -13.5 -> net 16.5.
+        assert_close(Compressor::gain_computer(-30.0, thr, 4.0, 0.0, 2.0, 0.0), 16.5, 1e-3);
+        // Cut (slope 0.5) -> -15 dB; shaped to -45, still below threshold -> net -15.
+        assert_close(Compressor::gain_computer(-30.0, thr, 4.0, 0.0, 0.5, 0.0), -15.0, 1e-3);
+
+        // Curvature is a BIPOLAR bulge at the middle of the low region (unity slope here).
+        let mid = -39.0; // middle of [-60, -18]
+        let flat = Compressor::gain_computer(mid, thr, 4.0, 0.0, 1.0, 0.0);
+        let up = Compressor::gain_computer(mid, thr, 4.0, 0.0, 1.0, 1.0);
+        let down = Compressor::gain_computer(mid, thr, 4.0, 0.0, 1.0, -1.0);
+        assert!(up > flat && flat > down, "bipolar bulge expected: {down} < {flat} < {up}");
+        // Endpoints are unaffected by curvature (silence anchored).
+        assert_close(Compressor::gain_computer(-60.0, thr, 4.0, 0.0, 1.0, 1.0), 0.0, 1e-6);
     }
 
     #[test]
     fn above_knee_follows_ratio() {
         // 10 dB over threshold at 4:1 -> output only 2.5 dB over -> 7.5 dB reduction.
-        let r = Compressor::gain_computer(-10.0, -20.0, 4.0, 0.0);
+        let r = Compressor::gain_computer(-10.0, -20.0, 4.0, 0.0, 1.0, 0.0);
         assert_close(r, -7.5, 1e-4);
     }
 
@@ -256,11 +339,11 @@ mod tests {
         // At the upper knee edge the soft-knee and linear formulas must agree.
         let (t, ratio, knee) = (0.0, 4.0, 6.0);
         let edge = t + knee / 2.0;
-        let knee_val = Compressor::gain_computer(edge, t, ratio, knee);
+        let knee_val = Compressor::gain_computer(edge, t, ratio, knee, 1.0, 0.0);
         let linear_val = (1.0 / ratio - 1.0) * (edge - t);
         assert_close(knee_val, linear_val, 1e-4);
         // At the lower edge there is still no reduction.
-        assert_close(Compressor::gain_computer(t - knee / 2.0, t, ratio, knee), 0.0, 1e-6);
+        assert_close(Compressor::gain_computer(t - knee / 2.0, t, ratio, knee, 1.0, 0.0), 0.0, 1e-6);
     }
 
     #[test]
@@ -329,7 +412,7 @@ mod tests {
         for &l in &[0usize, d / 2, d] {
             comp.reset();
             let mut set = settings(0.0, 1.0, 0.0);
-            set.bypass = true; // unity gain -> isolate the delay behaviour
+            set.mix = 0.0; // dry passthrough -> isolate the delay behaviour
             set.lookahead_samples = l;
 
             let mut out = [0.0f32];

src/dsp/limiter.rs

@@ -8,11 +8,16 @@
 //! Detection is stereo-linked (one gain for all channels). This stage limits **sample** peaks at
 //! the base rate; true-peak (inter-sample) limiting via oversampling is a later addition.
 
+use super::oversampler::Oversampler;
+
 const MAX_CHANNELS: usize = 2;
 /// Fixed look-ahead — also this stage's constant latency contribution.
 const LOOKAHEAD_MS: f32 = 1.5;
 /// Near-instant attack; the look-ahead gives it time to act before the peak arrives.
 const ATTACK_MS: f32 = 0.05;
+/// The 4× true-peak detector can still under-read by a few tenths of a dB near Nyquist, so we
+/// target a hair below the ceiling to keep the actual inter-sample peak under it.
+const TRUE_PEAK_MARGIN_DB: f32 = 0.3;
 
 fn time_to_coef(time_ms: f32, sample_rate: f32) -> f32 {
     if time_ms <= 0.0 {
@@ -33,6 +38,8 @@ pub struct Limiter {
     /// Current smoothed gain (<= 1).
     gain: f32,
     attack_coef: f32,
+    /// 4× interpolator for true-peak (inter-sample) detection.
+    oversampler: Oversampler,
 }
 
 impl Default for Limiter {
@@ -45,6 +52,7 @@ impl Default for Limiter {
             fixed_delay: 0,
             gain: 1.0,
             attack_coef: 0.0,
+            oversampler: Oversampler::new(),
         }
     }
 }
@@ -62,6 +70,7 @@ impl Limiter {
         let channels = num_channels.clamp(1, MAX_CHANNELS);
         self.delay = vec![vec![0.0; self.capacity]; channels];
         self.peaks = vec![0.0; self.capacity];
+        self.oversampler.prepare(channels);
         self.reset();
     }
 
@@ -70,6 +79,7 @@ impl Limiter {
             ch.iter_mut().for_each(|s| *s = 0.0);
         }
         self.peaks.iter_mut().for_each(|p| *p = 0.0);
+        self.oversampler.reset();
         self.write_pos = 0;
         self.gain = 1.0;
     }
@@ -79,6 +89,12 @@ impl Limiter {
         self.fixed_delay as u32
     }
 
+    /// Current limiter gain reduction in dB (>= 0). `gain` is linear (<= 1); expressed here as a
+    /// positive dB amount for the ceiling lamp / metering.
+    pub fn gain_reduction_db(&self) -> f32 {
+        -20.0 * self.gain.max(1e-9).log10()
+    }
+
     /// Limit one frame in place: `input[ch]` -> `output[ch]`.
     ///
     /// `ceiling` is linear gain (e.g. `util::db_to_gain(ceiling_db)`); `release_coef` comes from a
@@ -86,11 +102,15 @@ impl Limiter {
     pub fn process(&mut self, input: &[f32], output: &mut [f32], ceiling: f32, release_coef: f32) {
         let n = input.len().min(self.delay.len());
 
-        // Linked peak of the current input.
-        let mut peak = 0.0f32;
+        // Detector = max of the sample peak and the 4× true-peak (inter-sample) estimate.
+        let mut sample_peak = 0.0f32;
         for &x in &input[..n] {
-            peak = peak.max(x.abs());
+            sample_peak = sample_peak.max(x.abs());
         }
+        let peak = sample_peak.max(self.oversampler.max_true_peak(&input[..n]));
+
+        // Target a hair below the ceiling so the (slightly under-read) true peak stays under it.
+        let target_ceiling = ceiling * 10.0f32.powf(-TRUE_PEAK_MARGIN_DB / 20.0);
 
         // Write into the ring.
         for ch in 0..n {
@@ -105,7 +125,11 @@ impl Limiter {
         for &p in &self.peaks {
             window_max = window_max.max(p);
         }
-        let target = if window_max > ceiling { ceiling / window_max } else { 1.0 };
+        let target = if window_max > target_ceiling {
+            target_ceiling / window_max
+        } else {
+            1.0
+        };
 
         // Decoupled smoothing: fast attack down, slow release up.
         self.gain = if target < self.gain {

src/dsp/mod.rs

@@ -1,10 +1,11 @@
 //! DSP building blocks for Codename 206.
 //!
-//! Stage 2 introduces the full-band compressor (also the engine that will be reused
-//! per band and for the 'All' aggregate channel — see README.md). Later stages add the
-//! crossover filterbank, output limiter, and oversampler alongside it.
+//! The signal chain: a `crossover` filterbank splits into bands, each band (plus the summed
+//! 'All' channel) runs a `compressor`, and a `limiter` (with a true-peak `oversampler` detector)
+//! is the final stage. `biquad` is the shared filter primitive the crossover is built from.
 
 pub mod biquad;
 pub mod compressor;
 pub mod crossover;
 pub mod limiter;
+pub mod oversampler;

src/dsp/oversampler.rs

@@ -0,0 +1,149 @@
+//! 4× polyphase interpolation for **true-peak (inter-sample) detection only**.
+//!
+//! A band-limited signal can overshoot its sample values between samples, so the digital sample
+//! peak under-reads the real (post-DAC) peak. We reconstruct the 4× grid with a polyphase
+//! windowed-sinc interpolator and report only the maximum magnitude found — the interpolated
+//! samples themselves are discarded. The audio path is untouched; this just feeds a better peak
+//! estimate into the limiter.
+//!
+//! Speed: the prototype `PHASES * TAPS_PER_PHASE`-tap low-pass is split into `PHASES` sub-filters
+//! of `TAPS_PER_PHASE` taps, each run at the base rate (no zero-stuffed multiplies). Cost is
+//! `PHASES * TAPS_PER_PHASE` MACs per input sample per channel — about one base-rate FIR.
+//!
+//! The interpolation point sits at the centre of the tap window, so the estimate lags the input by
+//! ~`TAPS_PER_PHASE/2` samples. That is far smaller than the limiter's look-ahead, which absorbs it
+//! — so this adds no reported latency.
+
+const PHASES: usize = 4;
+const TAPS_PER_PHASE: usize = 12;
+const MAX_CHANNELS: usize = 2;
+
+/// Build the normalised polyphase coefficients: a windowed-sinc prototype split into `PHASES`
+/// sub-filters, each normalised to unity DC gain so reconstruction preserves amplitude.
+fn build_coefficients() -> [[f32; TAPS_PER_PHASE]; PHASES] {
+    use std::f32::consts::PI;
+    let n = PHASES * TAPS_PER_PHASE;
+    let center = (n as f32 - 1.0) / 2.0;
+
+    let mut proto = [0.0f32; PHASES * TAPS_PER_PHASE];
+    for (m, p) in proto.iter_mut().enumerate() {
+        // Sinc low-pass at the base-rate Nyquist (cutoff = 1/PHASES of the oversampled rate).
+        let x = (m as f32 - center) / PHASES as f32;
+        let sinc = if x.abs() < 1e-7 { 1.0 } else { (PI * x).sin() / (PI * x) };
+        // Blackman window.
+        let t = m as f32 / (n as f32 - 1.0);
+        let window = 0.42 - 0.5 * (2.0 * PI * t).cos() + 0.08 * (4.0 * PI * t).cos();
+        *p = sinc * window;
+    }
+
+    let mut coeffs = [[0.0f32; TAPS_PER_PHASE]; PHASES];
+    for (phase, row) in coeffs.iter_mut().enumerate() {
+        let mut sum = 0.0;
+        for (k, c) in row.iter_mut().enumerate() {
+            *c = proto[k * PHASES + phase];
+            sum += *c;
+        }
+        if sum.abs() > 1e-12 {
+            for c in row.iter_mut() {
+                *c /= sum; // unity DC per phase -> amplitude-preserving
+            }
+        }
+    }
+    coeffs
+}
+
+pub struct Oversampler {
+    coeffs: [[f32; TAPS_PER_PHASE]; PHASES],
+    /// Per-channel circular history of the last `TAPS_PER_PHASE` input samples.
+    history: Vec<[f32; TAPS_PER_PHASE]>,
+    pos: usize,
+}
+
+impl Default for Oversampler {
+    fn default() -> Self {
+        Self {
+            coeffs: build_coefficients(),
+            history: Vec::new(),
+            pos: 0,
+        }
+    }
+}
+
+impl Oversampler {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn prepare(&mut self, num_channels: usize) {
+        let channels = num_channels.clamp(1, MAX_CHANNELS);
+        self.history = vec![[0.0; TAPS_PER_PHASE]; channels];
+        self.reset();
+    }
+
+    pub fn reset(&mut self) {
+        for ch in &mut self.history {
+            *ch = [0.0; TAPS_PER_PHASE];
+        }
+        self.pos = 0;
+    }
+
+    /// Feed one input frame; return the maximum inter-sample magnitude across all channels and the
+    /// 4× grid (the reconstructed samples are not kept).
+    pub fn max_true_peak(&mut self, input: &[f32]) -> f32 {
+        let n = input.len().min(self.history.len());
+        let slot = self.pos % TAPS_PER_PHASE;
+        for ch in 0..n {
+            self.history[ch][slot] = input[ch];
+        }
+
+        let mut peak = 0.0f32;
+        for ch in 0..n {
+            let hist = &self.history[ch];
+            for phase in &self.coeffs {
+                let mut acc = 0.0f32;
+                for (k, &c) in phase.iter().enumerate() {
+                    // k = 0 is the newest sample, increasing k goes back in time.
+                    let idx = (self.pos + TAPS_PER_PHASE - k) % TAPS_PER_PHASE;
+                    acc += c * hist[idx];
+                }
+                peak = peak.max(acc.abs());
+            }
+        }
+
+        self.pos += 1;
+        peak
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::f32::consts::PI;
+
+    #[test]
+    fn detects_inter_sample_overshoot() {
+        // A unit sine at fs/4 phased so every sample lands at ±0.707 while the true peak is 1.0
+        // (a classic ~3 dB inter-sample overshoot). The detector must see well above 0.707.
+        let mut os = Oversampler::new();
+        os.prepare(1);
+        let mut detected = 0.0f32;
+        for n in 0..2_000 {
+            let x = (PI * n as f32 / 2.0 + PI / 4.0).sin(); // sin(πn/2 + π/4)
+            detected = detected.max(os.max_true_peak(&[x]));
+        }
+        assert!(detected > 0.9, "missed inter-sample peak: {detected}");
+        assert!(detected < 1.1, "implausible overshoot: {detected}");
+    }
+
+    #[test]
+    fn preserves_amplitude_of_constant() {
+        // Unity-DC normalisation: a constant signal reconstructs at its own level.
+        let mut os = Oversampler::new();
+        os.prepare(1);
+        let mut last = 0.0f32;
+        for _ in 0..200 {
+            last = os.max_true_peak(&[0.5]);
+        }
+        assert!((last - 0.5).abs() < 0.02, "amplitude not preserved: {last}");
+    }
+}

src/editor/crossover.rs

@@ -0,0 +1,141 @@
+//! Draggable crossover handles — replaces the two plain crossover sliders.
+//!
+//! A horizontal log-frequency strip split into LOW / MID / HIGH by two draggable handles, plus a
+//! number box per crossover (double-click to type an exact value). The handles enforce a dynamic
+//! limit: lo/mid can never exceed mid/hi (and vice-versa), on top of each param's own range.
+
+use nih_plug::prelude::*;
+use nih_plug_egui::egui::{
+    self, pos2, vec2, Align2, Color32, CornerRadius, DragValue, FontId, Painter, Rect, Sense, Stroke,
+};
+
+use crate::params::Codename206Params;
+
+/// Height of the handle strip.
+const GRAPH_H: f32 = 54.0;
+/// Displayed frequency axis (log), independent of the params' own ranges.
+const DISP_MIN_HZ: f32 = 20.0;
+const DISP_MAX_HZ: f32 = 20_000.0;
+
+fn log_span() -> f32 {
+    DISP_MAX_HZ.ln() - DISP_MIN_HZ.ln()
+}
+
+fn fmt_hz(hz: f32) -> String {
+    if hz >= 1000.0 {
+        format!("{:.2} kHz", hz / 1000.0)
+    } else {
+        format!("{:.0} Hz", hz)
+    }
+}
+
+pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, setter: &ParamSetter) {
+    let lo = params.crossover_low_hz.value();
+    let hi = params.crossover_high_hz.value();
+    // Each param's own min/max (normalized 0/1 map to the range ends).
+    let lo_min = params.crossover_low_hz.preview_plain(0.0);
+    let lo_max = params.crossover_low_hz.preview_plain(1.0);
+    let hi_min = params.crossover_high_hz.preview_plain(0.0);
+    let hi_max = params.crossover_high_hz.preview_plain(1.0);
+    // Dynamic limits so the two never cross: lo <= hi.
+    let lo_upper = lo_max.min(hi);
+    let hi_lower = hi_min.max(lo);
+
+    ui.label("Crossover");
+
+    let (rect, _) =
+        ui.allocate_exact_size(vec2(ui.available_width(), GRAPH_H), Sense::hover());
+    let p = ui.painter_at(rect);
+    let (left, right, top, bottom) =
+        (rect.left() + 2.0, rect.right() - 2.0, rect.top() + 2.0, rect.bottom() - 2.0);
+    let width = right - left;
+    let x_for = |hz: f32| left + (hz.max(1.0).ln() - DISP_MIN_HZ.ln()) / log_span() * width;
+    let xlo = x_for(lo);
+    let xhi = x_for(hi);
+
+    // Three band regions.
+    p.rect_filled(Rect::from_min_max(pos2(left, top), pos2(xlo, bottom)), CornerRadius::ZERO, Color32::from_rgb(28, 38, 52));
+    p.rect_filled(Rect::from_min_max(pos2(xlo, top), pos2(xhi, bottom)), CornerRadius::ZERO, Color32::from_rgb(30, 48, 36));
+    p.rect_filled(Rect::from_min_max(pos2(xhi, top), pos2(right, bottom)), CornerRadius::ZERO, Color32::from_rgb(52, 38, 30));
+
+    let band_label = |cx: f32, t: &str| {
+        p.text(pos2(cx, top + 2.0), Align2::CENTER_TOP, t, FontId::proportional(11.0), Color32::from_gray(160));
+    };
+    band_label((left + xlo) * 0.5, "LOW");
+    band_label((xlo + xhi) * 0.5, "MID");
+    band_label((xhi + right) * 0.5, "HIGH");
+
+    handle(ui, &p, setter, "xover_lo", xlo, lo, &params.crossover_low_hz, lo_min, lo_upper, left, width, top, bottom, Color32::from_rgb(120, 170, 230));
+    handle(ui, &p, setter, "xover_hi", xhi, hi, &params.crossover_high_hz, hi_lower, hi_max, left, width, top, bottom, Color32::from_rgb(230, 150, 120));
+
+    // Number boxes (double-click to type). Clamped to the same dynamic limits.
+    ui.horizontal(|ui| {
+        ui.label("Lo/Mid");
+        let mut v = lo as f64;
+        if ui
+            .add(DragValue::new(&mut v).range(lo_min as f64..=lo_upper as f64).speed(0.5).suffix(" Hz"))
+            .changed()
+        {
+            set_clamped(setter, &params.crossover_low_hz, v as f32, lo_min, lo_upper);
+        }
+        ui.add_space(16.0);
+        ui.label("Mid/Hi");
+        let mut v = hi as f64;
+        if ui
+            .add(DragValue::new(&mut v).range(hi_lower as f64..=hi_max as f64).speed(2.0).suffix(" Hz"))
+            .changed()
+        {
+            set_clamped(setter, &params.crossover_high_hz, v as f32, hi_lower, hi_max);
+        }
+    });
+}
+
+/// One draggable vertical handle. Drives `param` from the pointer's x (log-frequency), clamped to
+/// `[min, max]` (which already encodes the dynamic lo<=hi limit), with proper begin/end gestures.
+#[allow(clippy::too_many_arguments)]
+fn handle(
+    ui: &egui::Ui,
+    p: &Painter,
+    setter: &ParamSetter,
+    id_salt: &str,
+    x: f32,
+    hz: f32,
+    param: &FloatParam,
+    min: f32,
+    max: f32,
+    left: f32,
+    width: f32,
+    top: f32,
+    bottom: f32,
+    color: Color32,
+) {
+    let hit = Rect::from_min_max(pos2(x - 5.0, top), pos2(x + 5.0, bottom));
+    let resp = ui
+        .interact(hit, ui.id().with(id_salt), Sense::drag())
+        .on_hover_cursor(egui::CursorIcon::ResizeHorizontal);
+
+    if resp.drag_started() {
+        setter.begin_set_parameter(param);
+    }
+    if resp.dragged() {
+        if let Some(pos) = resp.interact_pointer_pos() {
+            let frac = ((pos.x - left) / width).clamp(0.0, 1.0);
+            let new = (DISP_MIN_HZ.ln() + frac * log_span()).exp().clamp(min, max);
+            setter.set_parameter(param, new);
+        }
+    }
+    if resp.drag_stopped() {
+        setter.end_set_parameter(param);
+    }
+
+    let col = if resp.dragged() || resp.hovered() { Color32::WHITE } else { color };
+    p.line_segment([pos2(x, top), pos2(x, bottom)], Stroke::new(2.0, col));
+    p.text(pos2(x, bottom - 1.0), Align2::CENTER_BOTTOM, fmt_hz(hz), FontId::proportional(10.0), Color32::from_gray(220));
+}
+
+/// Set a param to `value` clamped to `[min, max]`, wrapped in its own gesture (for the number box).
+fn set_clamped(setter: &ParamSetter, param: &FloatParam, value: f32, min: f32, max: f32) {
+    setter.begin_set_parameter(param);
+    setter.set_parameter(param, value.clamp(min, max));
+    setter.end_set_parameter(param);
+}

src/editor/gain_curve.rs

@@ -0,0 +1,106 @@
+//! Static gain-curve display: output level vs input level for the channel currently selected in
+//! the plot. Plots the wet transfer `out = (in + pre_gain) + gain_reduction(...) + makeup` (mix not
+//! folded in; output clamped at 0 dBFS), plus a live **operating-point fill** under the curve up to
+//! the channel's current input level — its right edge rides the curve (width = input, height = out).
+
+use nih_plug::prelude::util;
+use nih_plug_egui::egui::{self, pos2, vec2, Align2, Color32, CornerRadius, FontId, Sense, Stroke};
+use std::sync::atomic::Ordering;
+
+use crate::dsp::compressor::Compressor;
+use crate::meters::Meters;
+use crate::params::Codename206Params;
+
+/// Side length of the square plot.
+const CURVE_SIZE: f32 = 150.0;
+/// dB extent of both axes (bottom/left = FLOOR_DB, top/right = 0 dBFS).
+const FLOOR_DB: f32 = -60.0;
+
+pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, selected: usize, meters: &Meters) {
+    let labels = ["LOW", "MID", "HIGH", "ALL"];
+    let ch = selected.min(3);
+    let cp = match ch {
+        0 => &params.low,
+        1 => &params.mid,
+        2 => &params.high,
+        _ => &params.all,
+    };
+    let pre = cp.pre_gain_db.value();
+    let threshold = cp.threshold_db.value();
+    let ratio = cp.ratio.value();
+    let knee = cp.knee_db.value();
+    let low_slope = cp.low_slope.value();
+    let low_curve = cp.low_curve.value();
+    let makeup = cp.makeup_db.value();
+
+    ui.label(format!("Curve: {}", labels[ch]));
+    let (rect, _) = ui.allocate_exact_size(vec2(CURVE_SIZE, CURVE_SIZE), Sense::hover());
+    let p = ui.painter_at(rect);
+    p.rect_filled(rect, CornerRadius::ZERO, Color32::from_rgb(16, 16, 20));
+
+    let inset = 2.0;
+    let (left, right, top, bottom) =
+        (rect.left() + inset, rect.right() - inset, rect.top() + inset, rect.bottom() - inset);
+    let w = right - left;
+    let h = bottom - top;
+    let x_for = |db: f32| left + (db - FLOOR_DB) / -FLOOR_DB * w;
+    let y_for = |db: f32| bottom - (db - FLOOR_DB) / -FLOOR_DB * h;
+
+    // -6 dBFS reference lines on both axes.
+    let g6 = Color32::from_gray(38);
+    let x6 = x_for(-6.0);
+    let y6 = y_for(-6.0);
+    p.line_segment([pos2(x6, top), pos2(x6, bottom)], Stroke::new(1.0, g6));
+    p.line_segment([pos2(left, y6), pos2(right, y6)], Stroke::new(1.0, g6));
+
+    // Unity reference (out = in), bottom-left to top-right.
+    p.line_segment([pos2(left, bottom), pos2(right, top)], Stroke::new(1.0, Color32::from_gray(45)));
+    // Threshold marker on the input axis — shifted left by pre-gain (the comp sees in + pre).
+    let tx = x_for((threshold - pre).clamp(FLOOR_DB, 0.0));
+    p.line_segment([pos2(tx, top), pos2(tx, bottom)], Stroke::new(1.0, Color32::from_rgb(80, 60, 45)));
+
+    // Full wet transfer: drive into the comp, then makeup. (Mix not folded in.)
+    let n = 96;
+    let mut pts = Vec::with_capacity(n + 1);
+    for i in 0..=n {
+        let in_db = FLOOR_DB + (i as f32 / n as f32) * -FLOOR_DB; // external input, -60..0
+        let driven = in_db + pre;
+        let gr = Compressor::gain_computer(driven, threshold, ratio, knee, low_slope, low_curve); // signed dB
+        let out_db = (driven + gr + makeup).clamp(FLOOR_DB, 0.0);
+        pts.push(pos2(x_for(in_db), y_for(out_db)));
+    }
+    // Operating-point fill: shade under the curve from the floor up to the current input level.
+    let driven_now = util::gain_to_db(meters.input_level[ch].load(Ordering::Relaxed));
+    let ext_in = (driven_now - pre).clamp(FLOOR_DB, 0.0); // external input -> curve x
+    let x_now = x_for(ext_in);
+    let fill_col = Color32::from_rgba_unmultiplied(120, 200, 160, 45);
+    for seg in pts.windows(2) {
+        let a = seg[0];
+        let mut b = seg[1];
+        if a.x >= x_now {
+            break;
+        }
+        if b.x > x_now {
+            let f = ((x_now - a.x) / (b.x - a.x)).clamp(0.0, 1.0); // clip the last quad at x_now
+            b = pos2(x_now, a.y + (b.y - a.y) * f);
+        }
+        p.add(egui::Shape::convex_polygon(
+            vec![pos2(a.x, bottom), a, b, pos2(b.x, bottom)],
+            fill_col,
+            Stroke::NONE,
+        ));
+    }
+
+    p.add(egui::Shape::line(pts, Stroke::new(1.6, Color32::from_rgb(120, 200, 160))));
+
+    // Operating-point dot, on the curve at the current input.
+    let driven = ext_in + pre;
+    let gr = Compressor::gain_computer(driven, threshold, ratio, knee, low_slope, low_curve);
+    let out_op = (driven + gr + makeup).clamp(FLOOR_DB, 0.0);
+    p.circle_filled(pos2(x_now, y_for(out_op)), 3.0, Color32::from_rgb(235, 240, 235));
+
+    // Corner dB ticks + the -6 dB reference.
+    p.text(pos2(left + 1.0, top + 1.0), Align2::LEFT_TOP, "0", FontId::proportional(9.0), Color32::from_gray(90));
+    p.text(pos2(left + 1.0, bottom - 1.0), Align2::LEFT_BOTTOM, "-60", FontId::proportional(9.0), Color32::from_gray(90));
+    p.text(pos2(x6 + 2.0, bottom - 1.0), Align2::LEFT_BOTTOM, "-6", FontId::proportional(9.0), Color32::from_gray(80));
+}

src/editor/meter.rs

@@ -0,0 +1,143 @@
+//! Per-channel level + gain-reduction meters, each with its own latching ceiling/over lamp.
+//!
+//! Each channel is a `|L | GR | R|` cluster (output level left/right, mono gain reduction in the
+//! middle) topped by a lamp. The lamp latches when the channel's output reaches 0 dBFS (a hot /
+//! "over" warning — useful when pre-gain drives a band hard); for the ALL channel it also lights
+//! when the output limiter is actually catching peaks. It holds, then clears after `LAMP_HOLD_S`
+//! or on a click. Fed by the lock-free [`Meters`] state the audio thread publishes each block.
+
+use nih_plug::prelude::*;
+use nih_plug_egui::egui::{
+    self, pos2, vec2, Align2, Color32, CornerRadius, CursorIcon, FontId, Painter, Rect, Sense,
+};
+use std::sync::atomic::Ordering;
+
+use super::METER_FLOOR_DB;
+use crate::meters::{Meters, NUM_CHANNELS};
+
+/// Full-scale of the gain-reduction bar (fills downward from the top).
+const GR_FULL_DB: f32 = 24.0;
+/// Output level (dBFS) at/above which a channel's lamp latches on.
+const OVER_DB: f32 = 0.0;
+/// Limiter gain reduction (dB) above which the ALL channel's lamp also latches on.
+const LAMP_TRIGGER_DB: f32 = 0.1;
+/// How long a lamp stays lit after the most recent trigger (seconds).
+const LAMP_HOLD_S: f64 = 3.0;
+/// Height of the meter panel.
+const METER_PANEL_H: f32 = 140.0;
+
+/// GUI-side state for the meter panel: one lamp latch per channel.
+pub(super) struct MeterState {
+    /// egui time (seconds) of each channel's most recent lamp trigger, while latched on.
+    /// `None` = lamp off (never triggered, expired, or dismissed by a click).
+    ceiling_trigger: [Option<f64>; NUM_CHANNELS],
+}
+
+impl Default for MeterState {
+    fn default() -> Self {
+        Self { ceiling_trigger: [None; NUM_CHANNELS] }
+    }
+}
+
+/// Draw the meter panel: a `|L | GR | R|` cluster + a latching over/ceiling lamp per channel.
+pub(super) fn draw(ui: &mut egui::Ui, meters: &Meters, state: &mut MeterState) {
+    let labels = ["LOW", "MID", "HIGH", "ALL"];
+    let now = ui.ctx().input(|i| i.time);
+    let (rect, _) =
+        ui.allocate_exact_size(vec2(ui.available_width(), METER_PANEL_H), Sense::hover());
+    let p = ui.painter_at(rect);
+    p.rect_filled(rect, CornerRadius::ZERO, Color32::from_rgb(20, 20, 24));
+
+    let top = rect.top() + 22.0; // leave a row at the top for the lamps
+    let bottom = rect.bottom() - 18.0; // and a row at the bottom for the labels
+    let cell_w = rect.width() / NUM_CHANNELS as f32;
+    // Three bars per cluster, so they're narrower than a two-bar layout.
+    let bar_w = (cell_w * 0.17).min(14.0);
+    let gap = (cell_w * 0.05).min(5.0);
+
+    for i in 0..NUM_CHANNELS {
+        let cell_left = rect.left() + i as f32 * cell_w;
+        let group_w = bar_w * 3.0 + gap * 2.0;
+        let bx = cell_left + (cell_w - group_w) * 0.5;
+
+        // L / R output level (upward); colour warns as it nears 0 dBFS.
+        let l_db = util::gain_to_db(meters.level_l[i].load(Ordering::Relaxed));
+        let r_db = util::gain_to_db(meters.level_r[i].load(Ordering::Relaxed));
+        let l_frac = ((l_db - METER_FLOOR_DB) / -METER_FLOOR_DB).clamp(0.0, 1.0);
+        let r_frac = ((r_db - METER_FLOOR_DB) / -METER_FLOOR_DB).clamp(0.0, 1.0);
+
+        // Mono gain reduction (downward from the top).
+        let gr_db = meters.gain_reduction_db[i].load(Ordering::Relaxed);
+        let gr_frac = (gr_db / GR_FULL_DB).clamp(0.0, 1.0);
+
+        v_bar(&p, bx, bar_w, top, bottom, l_frac, level_color(l_db), false);
+        v_bar(&p, bx + bar_w + gap, bar_w, top, bottom, gr_frac, Color32::from_rgb(240, 150, 60), true);
+        v_bar(&p, bx + 2.0 * (bar_w + gap), bar_w, top, bottom, r_frac, level_color(r_db), false);
+
+        p.text(
+            pos2(cell_left + cell_w * 0.5, rect.bottom() - 2.0),
+            Align2::CENTER_BOTTOM,
+            labels[i],
+            FontId::proportional(12.0),
+            Color32::from_gray(200),
+        );
+
+        // Per-channel lamp: latch on output reaching 0 dBFS; the ALL channel also latches when the
+        // output limiter is catching peaks (the true master-ceiling event).
+        let over_db = l_db.max(r_db);
+        let mut triggered = over_db >= OVER_DB;
+        if i == NUM_CHANNELS - 1 {
+            triggered |= meters.limiter_gr_db.load(Ordering::Relaxed) > LAMP_TRIGGER_DB;
+        }
+        if triggered {
+            state.ceiling_trigger[i] = Some(now);
+        }
+
+        let lamp_center = pos2(cell_left + cell_w * 0.5, rect.top() + 11.0);
+        let lamp_rect = Rect::from_center_size(lamp_center, vec2(18.0, 18.0));
+        let resp = ui
+            .interact(lamp_rect, ui.id().with(("ceiling_lamp", i)), Sense::click())
+            .on_hover_cursor(CursorIcon::PointingHand)
+            .on_hover_text("Output reached 0 dBFS — click to clear");
+        if resp.clicked() {
+            state.ceiling_trigger[i] = None;
+        }
+        if let Some(t) = state.ceiling_trigger[i] {
+            if now - t >= LAMP_HOLD_S {
+                state.ceiling_trigger[i] = None;
+            }
+        }
+        let lamp = if state.ceiling_trigger[i].is_some() {
+            Color32::from_rgb(255, 40, 40)
+        } else {
+            Color32::from_rgb(40, 12, 12)
+        };
+        p.circle_filled(lamp_center, 5.0, lamp);
+    }
+}
+
+/// Draw a vertical bar within `[top, bottom]`. `frac` is 0..1; `from_top` fills downward from the
+/// top (gain reduction) instead of upward from the bottom (level).
+fn v_bar(p: &Painter, x: f32, w: f32, top: f32, bottom: f32, frac: f32, fill: Color32, from_top: bool) {
+    let track = Color32::from_rgb(34, 34, 40);
+    p.rect_filled(Rect::from_min_max(pos2(x, top), pos2(x + w, bottom)), CornerRadius::ZERO, track);
+
+    let h = (bottom - top) * frac.clamp(0.0, 1.0);
+    let filled = if from_top {
+        Rect::from_min_max(pos2(x, top), pos2(x + w, top + h))
+    } else {
+        Rect::from_min_max(pos2(x, bottom - h), pos2(x + w, bottom))
+    };
+    p.rect_filled(filled, CornerRadius::ZERO, fill);
+}
+
+/// Level-bar colour: green below -6 dB, yellow approaching, red near 0 dBFS.
+fn level_color(db: f32) -> Color32 {
+    if db >= -1.0 {
+        Color32::from_rgb(235, 70, 60)
+    } else if db >= -6.0 {
+        Color32::from_rgb(230, 200, 70)
+    } else {
+        Color32::from_rgb(90, 200, 110)
+    }
+}

src/editor/mod.rs

@@ -0,0 +1,116 @@
+//! egui editor: assembly + control layout.
+//!
+//! The aggregator. Builds the editor window and lays out the heading, the meter panel
+//! ([`meter`]), the rolling plot ([`plot`]), and the (placeholder) per-channel slider columns.
+//! Each visualiser owns its GUI state and drawing in its submodule; this module wires them
+//! together and holds the shared [`EditorState`]. When the UI is redesigned the slider columns
+//! get replaced and the visualisers stay as self-contained widgets.
+
+use nih_plug::prelude::*;
+use nih_plug_egui::{
+    create_egui_editor,
+    egui::{self, Vec2},
+    resizable_window::ResizableWindow,
+    widgets,
+};
+use std::sync::Arc;
+
+use crate::meters::Meters;
+use crate::params::{Codename206Params, CompressorParams};
+use crate::Codename206;
+
+mod crossover;
+mod gain_curve;
+mod meter;
+mod plot;
+
+/// Bottom of the dB scale shared by the meters and the plot (top is 0 dBFS).
+const METER_FLOOR_DB: f32 = -60.0;
+
+/// GUI-side editor state (not persisted): the per-widget state for the meter panel and the plot.
+#[derive(Default)]
+struct EditorState {
+    meter: meter::MeterState,
+    plot: plot::PlotState,
+}
+
+/// Build the plugin editor over shared handles to the params and meter state.
+pub(crate) fn create(params: Arc<Codename206Params>, meters: Arc<Meters>) -> Option<Box<dyn Editor>> {
+    let egui_state = params.editor_state.clone();
+    create_egui_editor(
+        params.editor_state.clone(),
+        EditorState::default(),
+        |_, _| {},
+        move |egui_ctx, setter, state| {
+            // Keep frames coming so the meters animate and the lamp can time out while open.
+            egui_ctx.request_repaint();
+
+            // One column of controls for a single compressor channel (placeholder layout).
+            let band_col = |ui: &mut egui::Ui, title: &str, p: &CompressorParams| {
+                // Roughly in signal order: input drive -> low shaper -> compressor -> output.
+                ui.strong(title);
+                ui.label("Pre-gain");
+                ui.add(widgets::ParamSlider::for_param(&p.pre_gain_db, setter));
+                ui.add(widgets::ParamSlider::for_param(&p.detection, setter));
+                ui.label("Low Slope");
+                ui.add(widgets::ParamSlider::for_param(&p.low_slope, setter));
+                ui.label("Low Curve");
+                ui.add(widgets::ParamSlider::for_param(&p.low_curve, setter));
+                ui.label("Threshold");
+                ui.add(widgets::ParamSlider::for_param(&p.threshold_db, setter));
+                ui.label("Ratio");
+                ui.add(widgets::ParamSlider::for_param(&p.ratio, setter));
+                ui.label("Knee");
+                ui.add(widgets::ParamSlider::for_param(&p.knee_db, setter));
+                ui.label("Attack");
+                ui.add(widgets::ParamSlider::for_param(&p.attack_ms, setter));
+                ui.label("Release");
+                ui.add(widgets::ParamSlider::for_param(&p.release_ms, setter));
+                ui.label("Makeup");
+                ui.add(widgets::ParamSlider::for_param(&p.makeup_db, setter));
+                ui.label("Mix");
+                ui.add(widgets::ParamSlider::for_param(&p.mix, setter));
+            };
+
+            // Resizable window; vertical scroll so every control stays reachable even when the
+            // window is small. (Placeholder layout — the redesign will replace the slider columns.)
+            ResizableWindow::new("editor")
+                .min_size(Vec2::new(480.0, 320.0))
+                .show(egui_ctx, egui_state.as_ref(), |ui| {
+                    egui::ScrollArea::vertical().show(ui, |ui| {
+                        ui.heading(Codename206::NAME);
+                        meter::draw(ui, &meters, &mut state.meter);
+                        ui.separator();
+                        // Gain curve (left, square) beside the scrolling plot (right, fills the rest).
+                        let selected = state.plot.selected;
+                        ui.horizontal_top(|ui| {
+                            ui.vertical(|ui| gain_curve::draw(ui, &params, selected, &meters));
+                            ui.vertical(|ui| plot::draw(ui, &meters, &mut state.plot));
+                        });
+                        ui.separator();
+                        crossover::draw(ui, &params, setter);
+                        ui.separator();
+                        // Global controls stacked vertically so they never overflow sideways.
+                        egui::Grid::new("globals").num_columns(2).show(ui, |ui| {
+                            ui.label("Look-ahead");
+                            ui.add(widgets::ParamSlider::for_param(&params.look_ahead_ms, setter));
+                            ui.end_row();
+                            ui.label("Ceiling");
+                            ui.add(widgets::ParamSlider::for_param(&params.output_ceiling_db, setter));
+                            ui.end_row();
+                            ui.label("Lim Release");
+                            ui.add(widgets::ParamSlider::for_param(&params.limiter_release_ms, setter));
+                            ui.end_row();
+                        });
+                        ui.separator();
+                        ui.columns(4, |cols| {
+                            band_col(&mut cols[0], "LOW", &params.low);
+                            band_col(&mut cols[1], "MID", &params.mid);
+                            band_col(&mut cols[2], "HIGH", &params.high);
+                            band_col(&mut cols[3], "ALL", &params.all);
+                        });
+                    });
+                });
+        },
+    )
+}

src/editor/plot.rs

@@ -0,0 +1,226 @@
+//! Rolling input/output/gain-reduction plot with per-channel tabs and a flow-speed selector.
+//!
+//! Histories for all four channels run continuously (cheap), so switching tabs shows that
+//! channel's existing history. It's fed by draining the audio thread's scope ring
+//! ([`Meters::scope`], clocked at `BUCKET_HZ`), so the horizontal resolution is set by the bucket
+//! rate rather than the editor frame rate. Buckets are folded into `PLOT_N` columns
+//! (peak-preserving); `window_s` (the flow speed) sets how many buckets span each column.
+
+use nih_plug::prelude::*;
+use nih_plug_egui::egui::{self, pos2, vec2, Align2, Color32, CornerRadius, FontId, Rect, Sense, Stroke};
+
+use super::METER_FLOOR_DB;
+use crate::meters::{Meters, BUCKET_HZ, NUM_CHANNELS};
+
+/// Height of the plot panel.
+const PLOT_PANEL_H: f32 = 150.0;
+/// Number of columns held in the history ring.
+const PLOT_N: usize = 256;
+
+const COLOR_IN: Color32 = Color32::from_rgb(90, 170, 235);
+const COLOR_OUT: Color32 = Color32::from_rgb(90, 200, 110);
+const COLOR_GR: Color32 = Color32::from_rgb(240, 150, 60);
+
+/// Rolling history for all channels: a per-channel ring of (in_db, out_db, gr_db) columns.
+struct PlotHistory {
+    in_db: [[f32; PLOT_N]; NUM_CHANNELS],
+    out_db: [[f32; PLOT_N]; NUM_CHANNELS],
+    gr_db: [[f32; PLOT_N]; NUM_CHANNELS],
+    /// Per-column flag: the output limiter hit the ceiling somewhere in this column.
+    hit: [bool; PLOT_N],
+    write: usize,
+    len: usize,
+}
+
+impl Default for PlotHistory {
+    fn default() -> Self {
+        Self {
+            in_db: [[METER_FLOOR_DB; PLOT_N]; NUM_CHANNELS],
+            out_db: [[METER_FLOOR_DB; PLOT_N]; NUM_CHANNELS],
+            gr_db: [[0.0; PLOT_N]; NUM_CHANNELS],
+            hit: [false; PLOT_N],
+            write: 0,
+            len: 0,
+        }
+    }
+}
+
+impl PlotHistory {
+    /// Append one column of (in_db, out_db, gr_db) per channel, plus the ceiling-hit flag.
+    fn push(&mut self, samples: &[(f32, f32, f32); NUM_CHANNELS], hit: bool) {
+        for i in 0..NUM_CHANNELS {
+            self.in_db[i][self.write] = samples[i].0;
+            self.out_db[i][self.write] = samples[i].1;
+            self.gr_db[i][self.write] = samples[i].2;
+        }
+        self.hit[self.write] = hit;
+        self.write = (self.write + 1) % PLOT_N;
+        self.len = (self.len + 1).min(PLOT_N);
+    }
+}
+
+/// GUI-side state for the plot: selected channel, history ring, ring-drain cursor, and the
+/// column being assembled from drained buckets.
+pub(super) struct PlotState {
+    /// Channel shown in the plot (0..NUM_CHANNELS: low/mid/high/all). Also drives the gain curve.
+    pub(super) selected: usize,
+    history: PlotHistory,
+    /// Seconds of history shown across the full plot width — the flow speed (smaller = faster).
+    window_s: f64,
+    /// Read position into the scope ring; `None` until the first frame (then starts at "now").
+    cursor: Option<u64>,
+    /// Per-channel max accumulator (in_db, out_db, gr_db) for the column currently being built.
+    col_acc: [(f32, f32, f32); NUM_CHANNELS],
+    /// Ceiling-hit flag accumulated for the column currently being built.
+    col_hit: bool,
+    /// Buckets folded into the current column so far (fractional — a column may span <1 bucket).
+    col_fill: f64,
+}
+
+impl Default for PlotState {
+    fn default() -> Self {
+        Self {
+            selected: 0,
+            history: PlotHistory::default(),
+            window_s: 5.0,
+            cursor: None,
+            col_acc: [(METER_FLOOR_DB, METER_FLOOR_DB, 0.0); NUM_CHANNELS],
+            col_hit: false,
+            col_fill: 0.0,
+        }
+    }
+}
+
+/// Draw the scrolling in/out/gain-reduction plot for the selected channel, plus the channel tabs
+/// and flow-speed selector. History for all channels advances every frame regardless of the tab.
+pub(super) fn draw(ui: &mut egui::Ui, meters: &Meters, state: &mut PlotState) {
+    // Buckets that make up one column at the current flow speed (may be fractional).
+    let buckets_per_col = (state.window_s * BUCKET_HZ as f64 / PLOT_N as f64).max(1e-6);
+
+    // Drain every bucket produced since the last frame (audio-clocked), folding them into columns.
+    // A fresh cursor starts at "now" so we don't replay stale buckets.
+    {
+        let w0 = meters.scope.write_index();
+        let cursor = state.cursor.get_or_insert(w0);
+        let history = &mut state.history;
+        let col_acc = &mut state.col_acc;
+        let col_hit = &mut state.col_hit;
+        let col_fill = &mut state.col_fill;
+        meters.scope.drain(cursor, |in_lin, out_lin, gr_db, hit| {
+            for ch in 0..NUM_CHANNELS {
+                col_acc[ch].0 = col_acc[ch].0.max(util::gain_to_db(in_lin[ch]));
+                col_acc[ch].1 = col_acc[ch].1.max(util::gain_to_db(out_lin[ch]));
+                col_acc[ch].2 = col_acc[ch].2.max(gr_db[ch]);
+            }
+            *col_hit |= hit > 0.5;
+            *col_fill += 1.0;
+            while *col_fill >= buckets_per_col {
+                history.push(col_acc, *col_hit);
+                *col_acc = [(METER_FLOOR_DB, METER_FLOOR_DB, 0.0); NUM_CHANNELS];
+                *col_hit = false;
+                *col_fill -= buckets_per_col;
+            }
+        });
+    }
+
+    // Channel tabs + flow-speed selector + legend.
+    let labels = ["LOW", "MID", "HIGH", "ALL"];
+    let speeds = [2.0f64, 5.0, 15.0, 45.0];
+    ui.horizontal(|ui| {
+        ui.label("Plot:");
+        for (i, l) in labels.iter().enumerate() {
+            ui.selectable_value(&mut state.selected, i, *l);
+        }
+        ui.separator();
+        ui.label("Speed:");
+        let mut speed_changed = false;
+        for &w in &speeds {
+            if ui.selectable_value(&mut state.window_s, w, format!("{w:.0}s")).changed() {
+                speed_changed = true;
+            }
+        }
+        if speed_changed {
+            // Cadence changed: start the history fresh so the time axis is consistent.
+            state.history = PlotHistory::default();
+            state.col_acc = [(METER_FLOOR_DB, METER_FLOOR_DB, 0.0); NUM_CHANNELS];
+            state.col_hit = false;
+            state.col_fill = 0.0;
+        }
+        ui.separator();
+        ui.colored_label(COLOR_IN, "in");
+        ui.colored_label(COLOR_OUT, "out");
+        ui.colored_label(COLOR_GR, "GR");
+    });
+
+    let (rect, _) =
+        ui.allocate_exact_size(vec2(ui.available_width(), PLOT_PANEL_H), Sense::hover());
+    let p = ui.painter_at(rect);
+    p.rect_filled(rect, CornerRadius::ZERO, Color32::from_rgb(16, 16, 20));
+
+    let (top, bottom, left, right) =
+        (rect.top() + 4.0, rect.bottom() - 4.0, rect.left() + 4.0, rect.right() - 4.0);
+    let width = right - left;
+    let y_for_db = |db: f32| -> f32 {
+        let frac = ((db - METER_FLOOR_DB) / -METER_FLOOR_DB).clamp(0.0, 1.0);
+        bottom - frac * (bottom - top)
+    };
+
+    // Gridlines (dB).
+    for &g in &[0.0f32, -12.0, -24.0, -48.0] {
+        let y = y_for_db(g);
+        p.line_segment([pos2(left, y), pos2(right, y)], Stroke::new(1.0, Color32::from_gray(40)));
+        p.text(
+            pos2(left + 2.0, y),
+            Align2::LEFT_BOTTOM,
+            format!("{g:.0}"),
+            FontId::proportional(9.0),
+            Color32::from_gray(90),
+        );
+    }
+
+    let c = state.selected.min(NUM_CHANNELS - 1);
+    let (write, len) = (state.history.write, state.history.len);
+    if len >= 2 {
+        let draw_series = |series: &[f32; PLOT_N], to_db: &dyn Fn(f32) -> f32, color: Color32, fill: bool| {
+            let mut pts = Vec::with_capacity(len);
+            for k in 0..len {
+                let idx = (write + PLOT_N - len + k) % PLOT_N;
+                let pos = (PLOT_N - len + k) as f32 / (PLOT_N - 1) as f32; // newest hugs the right
+                pts.push(pos2(left + pos * width, y_for_db(to_db(series[idx]))));
+            }
+            if fill {
+                // Fill as a strip of per-segment convex quads down to the baseline. A single
+                // concave polygon mis-tessellates in egui (it fans from one corner, leaving stray
+                // triangles), so build convex pieces — one box per time unit — instead.
+                let fill_col = Color32::from_rgba_unmultiplied(color.r(), color.g(), color.b(), 40);
+                for seg in pts.windows(2) {
+                    let (a, b) = (seg[0], seg[1]);
+                    p.add(egui::Shape::convex_polygon(
+                        vec![pos2(a.x, bottom), pos2(a.x, a.y), pos2(b.x, b.y), pos2(b.x, bottom)],
+                        fill_col,
+                        Stroke::NONE,
+                    ));
+                }
+            }
+            p.add(egui::Shape::line(pts, Stroke::new(1.5, color)));
+        };
+        draw_series(&state.history.in_db[c], &|db| db, COLOR_IN, true);
+        draw_series(&state.history.out_db[c], &|db| db, COLOR_OUT, true);
+        // GR hangs from the 0 dB line: a reduction of X dB is drawn at the -X gridline.
+        draw_series(&state.history.gr_db[c], &|gr| -gr, COLOR_GR, false);
+
+        // Ceiling-hit markers: a short red tick at the TOP for any column where the output limiter
+        // hit the ceiling (global — shown on every channel's view). One column wide, so runs of
+        // hits merge into a continuous segment and a lone hit is just a dot. Nothing otherwise.
+        let dx = width / (PLOT_N - 1) as f32;
+        let marker = Color32::from_rgb(235, 45, 45);
+        for k in 0..len {
+            let idx = (write + PLOT_N - len + k) % PLOT_N;
+            if state.history.hit[idx] {
+                let pos = (PLOT_N - len + k) as f32 / (PLOT_N - 1) as f32;
+                let x = left + pos * width;
+                p.rect_filled(Rect::from_min_max(pos2(x, top), pos2(x + dx, top + 3.0)), CornerRadius::ZERO, marker);
+            }
+        }
+    }
+}

src/lib.rs

@@ -1,11 +1,20 @@
 use nih_plug::prelude::*;
-use nih_plug_egui::{create_egui_editor, egui, widgets, EguiState};
 use std::sync::Arc;
 
 mod dsp;
-use dsp::compressor::{Compressor, CompressorSettings, MAX_LOOKAHEAD_MS};
+mod editor;
+mod meters;
+mod params;
+
+use dsp::compressor::{Compressor, MAX_LOOKAHEAD_MS};
 use dsp::crossover::Crossover;
 use dsp::limiter::Limiter;
+use meters::Meters;
+use params::{build_settings, Codename206Params};
+
+/// Peak-meter fall: after this long of silence the bars decay by 12 dB. (Matches nih-plug's
+/// gain-gui example feel.)
+const METER_DECAY_MS: f64 = 150.0;
 
 /// Band indices into the compressor array: low, mid, high, then the 'All' aggregate channel.
 const LOW: usize = 0;
@@ -13,17 +22,6 @@ const MID: usize = 1;
 const HIGH: usize = 2;
 const ALL: usize = 3;
 
-/// Level-detection mode for a compressor's detector.
-#[derive(Enum, PartialEq, Clone, Copy)]
-enum DetectionMode {
-    #[id = "peak"]
-    #[name = "Peak"]
-    Peak,
-    #[id = "rms"]
-    #[name = "RMS"]
-    Rms,
-}
-
 /// Codename 206 — Stage 3: 3-band crossover + per-band compressors summed into an 'All' channel.
 ///
 /// Signal: input → LR4 crossover → {low, mid, high} each through their own compressor → sum →
@@ -37,59 +35,26 @@ struct Codename206 {
     comps: [Compressor; 4],
     /// Output brickwall limiter (final stage).
     limiter: Limiter,
+    /// Lock-free meter state shared with the editor.
+    meters: Arc<Meters>,
+    /// Per-sample decay factor for the meter peak-hold (computed from the sample rate; raised to
+    /// the block length when applied once per block in `process`).
+    meter_decay_weight: f32,
+
+    /// Per-channel max accumulators for the plot bucket currently being built (in/out linear, GR
+    /// dB). Persist across blocks since a bucket spans many samples.
+    scope_in: [f32; 4],
+    scope_out: [f32; 4],
+    scope_gr: [f32; 4],
+    /// Max output-limiter gain reduction seen in the current bucket (for the ceiling-hit marker).
+    scope_hit: f32,
+    /// Samples accumulated into the current bucket, and the bucket length (= sample_rate / BUCKET_HZ).
+    scope_samples: usize,
+    scope_bucket_len: usize,
 }
 
-#[derive(Params)]
-struct Codename206Params {
-    #[persist = "editor-state"]
-    editor_state: Arc<EguiState>,
-
-    /// Low/Mid crossover frequency.
-    #[id = "xover_lo"]
-    pub crossover_low_hz: FloatParam,
-    /// Mid/High crossover frequency.
-    #[id = "xover_hi"]
-    pub crossover_high_hz: FloatParam,
-    /// Global look-ahead time (constant reported latency — safe to adjust during playback).
-    #[id = "lookahead"]
-    pub look_ahead_ms: FloatParam,
-
-    /// Output brickwall ceiling (the limiter never lets output exceed this).
-    #[id = "ceiling"]
-    pub output_ceiling_db: FloatParam,
-    /// Output limiter release time.
-    #[id = "lim_rel"]
-    pub limiter_release_ms: FloatParam,
-
-    #[nested(id_prefix = "low", group = "Low")]
-    pub low: CompressorParams,
-    #[nested(id_prefix = "mid", group = "Mid")]
-    pub mid: CompressorParams,
-    #[nested(id_prefix = "high", group = "High")]
-    pub high: CompressorParams,
-    #[nested(id_prefix = "all", group = "All")]
-    pub all: CompressorParams,
-}
-
-#[derive(Params)]
-struct CompressorParams {
-    #[id = "detect"]
-    pub detection: EnumParam<DetectionMode>,
-    #[id = "thresh"]
-    pub threshold_db: FloatParam,
-    #[id = "ratio"]
-    pub ratio: FloatParam,
-    #[id = "knee"]
-    pub knee_db: FloatParam,
-    #[id = "attack"]
-    pub attack_ms: FloatParam,
-    #[id = "release"]
-    pub release_ms: FloatParam,
-    #[id = "makeup"]
-    pub makeup_db: FloatParam,
-    #[id = "bypass"]
-    pub bypass: BoolParam,
-}
+/// Limiter gain reduction (dB) above which a plot bucket is flagged as hitting the ceiling.
+const CEILING_HIT_GR_DB: f32 = 0.1;
 
 impl Default for Codename206 {
     fn default() -> Self {
@@ -99,131 +64,18 @@ impl Default for Codename206 {
             crossover: Crossover::new(),
             comps: [Compressor::new(), Compressor::new(), Compressor::new(), Compressor::new()],
             limiter: Limiter::new(),
+            meters: Arc::new(Meters::default()),
+            meter_decay_weight: 1.0,
+            scope_in: [0.0; 4],
+            scope_out: [0.0; 4],
+            scope_gr: [0.0; 4],
+            scope_hit: 0.0,
+            scope_samples: 0,
+            scope_bucket_len: 1,
         }
     }
 }
 
-impl Default for Codename206Params {
-    fn default() -> Self {
-        Self {
-            editor_state: EguiState::from_size(760, 520),
-
-            crossover_low_hz: FloatParam::new(
-                "Crossover Lo/Mid",
-                200.0,
-                FloatRange::Skewed { min: 30.0, max: 1_000.0, factor: FloatRange::skew_factor(-1.0) },
-            )
-            .with_value_to_string(formatters::v2s_f32_hz_then_khz(0))
-            .with_string_to_value(formatters::s2v_f32_hz_then_khz()),
-
-            crossover_high_hz: FloatParam::new(
-                "Crossover Mid/Hi",
-                2_500.0,
-                FloatRange::Skewed { min: 500.0, max: 18_000.0, factor: FloatRange::skew_factor(-1.0) },
-            )
-            .with_value_to_string(formatters::v2s_f32_hz_then_khz(0))
-            .with_string_to_value(formatters::s2v_f32_hz_then_khz()),
-
-            look_ahead_ms: FloatParam::new(
-                "Look-ahead",
-                2.0,
-                FloatRange::Linear { min: 0.0, max: MAX_LOOKAHEAD_MS },
-            )
-            .with_unit(" ms")
-            .with_value_to_string(formatters::v2s_f32_rounded(2)),
-
-            output_ceiling_db: FloatParam::new(
-                "Ceiling",
-                0.0,
-                FloatRange::Linear { min: -24.0, max: 0.0 },
-            )
-            .with_unit(" dB")
-            .with_value_to_string(formatters::v2s_f32_rounded(1)),
-
-            limiter_release_ms: FloatParam::new(
-                "Limiter Release",
-                100.0,
-                FloatRange::Skewed { min: 1.0, max: 1_000.0, factor: FloatRange::skew_factor(-2.0) },
-            )
-            .with_unit(" ms")
-            .with_value_to_string(formatters::v2s_f32_rounded(1)),
-
-            low: CompressorParams::default(),
-            mid: CompressorParams::default(),
-            high: CompressorParams::default(),
-            all: CompressorParams::default(),
-        }
-    }
-}
-
-impl Default for CompressorParams {
-    fn default() -> Self {
-        Self {
-            detection: EnumParam::new("Detection", DetectionMode::Peak),
-
-            threshold_db: FloatParam::new(
-                "Threshold",
-                -18.0,
-                FloatRange::Linear { min: -60.0, max: 0.0 },
-            )
-            .with_unit(" dB")
-            .with_value_to_string(formatters::v2s_f32_rounded(1)),
-
-            ratio: FloatParam::new(
-                "Ratio",
-                2.0,
-                FloatRange::Skewed { min: 1.0, max: 20.0, factor: FloatRange::skew_factor(-1.0) },
-            )
-            .with_value_to_string(Arc::new(|v| format!("{v:.2} : 1")))
-            .with_string_to_value(Arc::new(|s| {
-                s.split(':').next().and_then(|x| x.trim().parse::<f32>().ok())
-            })),
-
-            knee_db: FloatParam::new("Knee", 6.0, FloatRange::Linear { min: 0.0, max: 24.0 })
-                .with_unit(" dB")
-                .with_value_to_string(formatters::v2s_f32_rounded(1)),
-
-            attack_ms: FloatParam::new(
-                "Attack",
-                10.0,
-                FloatRange::Skewed { min: 0.0, max: 100.0, factor: FloatRange::skew_factor(-2.0) },
-            )
-            .with_unit(" ms")
-            .with_value_to_string(formatters::v2s_f32_rounded(2)),
-
-            release_ms: FloatParam::new(
-                "Release",
-                100.0,
-                FloatRange::Skewed { min: 1.0, max: 1_000.0, factor: FloatRange::skew_factor(-2.0) },
-            )
-            .with_unit(" ms")
-            .with_value_to_string(formatters::v2s_f32_rounded(1)),
-
-            makeup_db: FloatParam::new("Makeup", 0.0, FloatRange::Linear { min: -12.0, max: 24.0 })
-                .with_smoother(SmoothingStyle::Linear(20.0))
-                .with_unit(" dB")
-                .with_value_to_string(formatters::v2s_f32_rounded(1)),
-
-            bypass: BoolParam::new("Bypass", false),
-        }
-    }
-}
-
-/// Build the per-block compressor settings for one channel's params (makeup filled per sample).
-fn build_settings(p: &CompressorParams, lookahead: usize, sample_rate: f32) -> CompressorSettings {
-    CompressorSettings {
-        threshold_db: p.threshold_db.value(),
-        ratio: p.ratio.value(),
-        knee_db: p.knee_db.value(),
-        attack_coef: Compressor::time_to_coef(p.attack_ms.value(), sample_rate),
-        release_coef: Compressor::time_to_coef(p.release_ms.value(), sample_rate),
-        makeup_db: 0.0,
-        lookahead_samples: lookahead,
-        use_rms: p.detection.value() == DetectionMode::Rms,
-        bypass: p.bypass.value(),
-    }
-}
-
 impl Codename206 {
     fn lookahead_samples(&self) -> usize {
         (self.params.look_ahead_ms.value() * 0.001 * self.sample_rate).round() as usize
@@ -264,55 +116,7 @@ impl Plugin for Codename206 {
     }
 
     fn editor(&mut self, _async_executor: AsyncExecutor<Self>) -> Option<Box<dyn Editor>> {
-        let params = self.params.clone();
-        create_egui_editor(
-            self.params.editor_state.clone(),
-            (),
-            |_, _| {},
-            move |egui_ctx, setter, _state| {
-                // One column of controls for a single compressor channel.
-                let band_col = |ui: &mut egui::Ui, title: &str, p: &CompressorParams| {
-                    ui.strong(title);
-                    ui.add(widgets::ParamSlider::for_param(&p.detection, setter));
-                    ui.label("Threshold");
-                    ui.add(widgets::ParamSlider::for_param(&p.threshold_db, setter));
-                    ui.label("Ratio");
-                    ui.add(widgets::ParamSlider::for_param(&p.ratio, setter));
-                    ui.label("Knee");
-                    ui.add(widgets::ParamSlider::for_param(&p.knee_db, setter));
-                    ui.label("Attack");
-                    ui.add(widgets::ParamSlider::for_param(&p.attack_ms, setter));
-                    ui.label("Release");
-                    ui.add(widgets::ParamSlider::for_param(&p.release_ms, setter));
-                    ui.label("Makeup");
-                    ui.add(widgets::ParamSlider::for_param(&p.makeup_db, setter));
-                    ui.add(widgets::ParamSlider::for_param(&p.bypass, setter));
-                };
-
-                egui::CentralPanel::default().show(egui_ctx, |ui| {
-                    ui.heading(Self::NAME);
-                    ui.horizontal(|ui| {
-                        ui.label("Xover Lo/Mid");
-                        ui.add(widgets::ParamSlider::for_param(&params.crossover_low_hz, setter));
-                        ui.label("Xover Mid/Hi");
-                        ui.add(widgets::ParamSlider::for_param(&params.crossover_high_hz, setter));
-                        ui.label("Look-ahead");
-                        ui.add(widgets::ParamSlider::for_param(&params.look_ahead_ms, setter));
-                        ui.label("Ceiling");
-                        ui.add(widgets::ParamSlider::for_param(&params.output_ceiling_db, setter));
-                        ui.label("Lim Release");
-                        ui.add(widgets::ParamSlider::for_param(&params.limiter_release_ms, setter));
-                    });
-                    ui.separator();
-                    ui.columns(4, |cols| {
-                        band_col(&mut cols[0], "LOW", &params.low);
-                        band_col(&mut cols[1], "MID", &params.mid);
-                        band_col(&mut cols[2], "HIGH", &params.high);
-                        band_col(&mut cols[3], "ALL", &params.all);
-                    });
-                });
-            },
-        )
+        editor::create(self.params.clone(), self.meters.clone())
     }
 
     fn initialize(
@@ -327,6 +131,14 @@ impl Plugin for Codename206 {
             .map(NonZeroU32::get)
             .unwrap_or(2) as usize;
 
+        // Per-block decay so the meters fall ~12 dB over METER_DECAY_MS of silence.
+        self.meter_decay_weight =
+            0.25f64.powf((self.sample_rate as f64 * METER_DECAY_MS / 1000.0).recip()) as f32;
+
+        // Plot bucket length: emit a scope bucket every ~1/BUCKET_HZ seconds.
+        self.scope_bucket_len =
+            ((self.sample_rate / meters::BUCKET_HZ as f32).round() as usize).max(1);
+
         for comp in &mut self.comps {
             comp.prepare(self.sample_rate, channels, MAX_LOOKAHEAD_MS);
         }
@@ -352,13 +164,21 @@ impl Plugin for Codename206 {
             comp.reset();
         }
         self.limiter.reset();
+        // Transport restart / sample-rate change: drop stale meter values to silence and discard
+        // the in-flight plot bucket.
+        self.meters.clear();
+        self.scope_in = [0.0; 4];
+        self.scope_out = [0.0; 4];
+        self.scope_gr = [0.0; 4];
+        self.scope_hit = 0.0;
+        self.scope_samples = 0;
     }
 
     fn process(
         &mut self,
         buffer: &mut Buffer,
         _aux: &mut AuxiliaryBuffers,
-        _context: &mut impl ProcessContext<Self>,
+        context: &mut impl ProcessContext<Self>,
     ) -> ProcessStatus {
         let lookahead = self.lookahead_samples();
 
@@ -383,6 +203,18 @@ impl Plugin for Codename206 {
         let limiter_release =
             Compressor::time_to_coef(self.params.limiter_release_ms.value(), self.sample_rate);
 
+        // Only do the (cheap) metering work when the editor is actually open.
+        let metering = self.params.editor_state.is_open();
+        // The host keeps calling process() with silence while stopped/paused (FL does), so the
+        // scope is gated on the transport actually playing — otherwise it would scroll silence.
+        let playing = context.transport().playing;
+        let num_samples = buffer.samples();
+        let mut lvl_l = [0.0f32; meters::NUM_CHANNELS];
+        let mut lvl_r = [0.0f32; meters::NUM_CHANNELS];
+        let mut inp = [0.0f32; meters::NUM_CHANNELS]; // mono input level (detector / gain-curve x)
+        let mut gr = [0.0f32; meters::NUM_CHANNELS];
+        let mut lim_gr = 0.0f32;
+
         let mut in_frame = [0.0f32; 2];
         let mut band_in = [[0.0f32; 2]; 3];
         let mut band_out = [[0.0f32; 2]; 3];
@@ -392,6 +224,7 @@ impl Plugin for Codename206 {
 
         for mut frame in buffer.iter_samples() {
             let n = frame.len().min(2);
+            let r = (n - 1).min(1); // right-channel index (== left when mono)
             for ch in 0..n {
                 in_frame[ch] = *frame.get_mut(ch).unwrap();
             }
@@ -404,28 +237,105 @@ impl Plugin for Codename206 {
                 band_in[HIGH][ch] = hi;
             }
 
-            // Compress each band (per-sample smoothed makeup), then sum.
+            // Drive + compress each band (per-sample smoothed pre-gain & makeup), then sum.
             summed[..n].fill(0.0);
             for b in 0..3 {
+                let pre = util::db_to_gain(band_params[b].pre_gain_db.smoothed.next());
+                for ch in 0..n {
+                    band_in[b][ch] *= pre;
+                }
                 band_set[b].makeup_db = band_params[b].makeup_db.smoothed.next();
+                band_set[b].mix = band_params[b].mix.smoothed.next();
+                band_set[b].low_slope = band_params[b].low_slope.smoothed.next();
+                band_set[b].low_curve = band_params[b].low_curve.smoothed.next();
                 self.comps[b].process(&band_in[b][..n], &mut band_out[b][..n], &band_set[b]);
                 for ch in 0..n {
                     summed[ch] += band_out[b][ch];
                 }
+                if metering {
+                    let in_mono = band_in[b][0].abs().max(band_in[b][r].abs());
+                    let out_l = band_out[b][0].abs();
+                    let out_r = band_out[b][r].abs();
+                    // Wet gain reduction (what the comp computes), independent of the mix.
+                    let g = self.comps[b].gain_reduction_db();
+                    inp[b] = inp[b].max(in_mono);
+                    lvl_l[b] = lvl_l[b].max(out_l);
+                    lvl_r[b] = lvl_r[b].max(out_r);
+                    gr[b] = gr[b].max(g);
+                    if playing {
+                        self.scope_in[b] = self.scope_in[b].max(in_mono);
+                        self.scope_out[b] = self.scope_out[b].max(out_l.max(out_r));
+                        self.scope_gr[b] = self.scope_gr[b].max(g);
+                    }
+                }
             }
 
-            // 'All' aggregate channel over the summed bands.
+            // 'All' aggregate channel over the summed bands (driven before its compressor).
+            let all_pre = util::db_to_gain(self.params.all.pre_gain_db.smoothed.next());
+            for ch in 0..n {
+                summed[ch] *= all_pre;
+            }
             all_set.makeup_db = self.params.all.makeup_db.smoothed.next();
+            all_set.mix = self.params.all.mix.smoothed.next();
+            all_set.low_slope = self.params.all.low_slope.smoothed.next();
+            all_set.low_curve = self.params.all.low_curve.smoothed.next();
             self.comps[ALL].process(&summed[..n], &mut out_frame[..n], &all_set);
 
             // Output brickwall limiter.
             self.limiter.process(&out_frame[..n], &mut lim_frame[..n], ceiling, limiter_release);
 
+            if metering {
+                let in_mono = summed[0].abs().max(summed[r].abs());
+                let out_l = out_frame[0].abs();
+                let out_r = out_frame[r].abs();
+                let g = self.comps[ALL].gain_reduction_db();
+                inp[ALL] = inp[ALL].max(in_mono);
+                lvl_l[ALL] = lvl_l[ALL].max(out_l);
+                lvl_r[ALL] = lvl_r[ALL].max(out_r);
+                gr[ALL] = gr[ALL].max(g);
+                lim_gr = lim_gr.max(self.limiter.gain_reduction_db());
+
+                // Only advance the scope while the transport is playing, so it freezes (rather than
+                // scrolling silence) when the host is paused/stopped but still calling process().
+                if playing {
+                    self.scope_in[ALL] = self.scope_in[ALL].max(in_mono);
+                    self.scope_out[ALL] = self.scope_out[ALL].max(out_l.max(out_r));
+                    self.scope_gr[ALL] = self.scope_gr[ALL].max(g);
+                    self.scope_hit = self.scope_hit.max(self.limiter.gain_reduction_db());
+
+                    // Emit a plot bucket every scope_bucket_len samples (~BUCKET_HZ).
+                    self.scope_samples += 1;
+                    if self.scope_samples >= self.scope_bucket_len {
+                        let hit = if self.scope_hit > CEILING_HIT_GR_DB { 1.0 } else { 0.0 };
+                        self.meters.scope.push(&self.scope_in, &self.scope_out, &self.scope_gr, hit);
+                        self.scope_in = [0.0; meters::NUM_CHANNELS];
+                        self.scope_out = [0.0; meters::NUM_CHANNELS];
+                        self.scope_gr = [0.0; meters::NUM_CHANNELS];
+                        self.scope_hit = 0.0;
+                        self.scope_samples = 0;
+                    }
+                }
+            }
+
             for ch in 0..n {
                 *frame.get_mut(ch).unwrap() = lim_frame[ch];
             }
         }
 
+        // Publish one decimated value per meter for this block. The decay weight is per-sample,
+        // so raise it to the block length to keep the fall time constant independent of buffer size
+        // (we apply it once per block, not once per sample).
+        if metering {
+            let w = self.meter_decay_weight.powi(num_samples as i32);
+            for i in 0..meters::NUM_CHANNELS {
+                meters::decay_store(&self.meters.level_l[i], lvl_l[i], w);
+                meters::decay_store(&self.meters.level_r[i], lvl_r[i], w);
+                meters::decay_store(&self.meters.input_level[i], inp[i], w);
+                meters::decay_store(&self.meters.gain_reduction_db[i], gr[i], w);
+            }
+            meters::decay_store(&self.meters.limiter_gr_db, lim_gr, w);
+        }
+
         ProcessStatus::Normal
     }
 }

src/meters.rs

@@ -0,0 +1,173 @@
+//! Lock-free meter state shared from the audio thread to the editor.
+//!
+//! Two feeds, both written by `process()` (single producer) and read by the editor (single
+//! consumer), all wait-free:
+//!
+//! * **Bar meters** — decayed scalars per channel ([`Meters::level_l`] etc.), one store per block.
+//! * **Scrolling plot** — a [`ScopeRing`] of raw buckets clocked at [`BUCKET_HZ`] (independent of
+//!   the GUI frame rate), so the plot's horizontal resolution isn't capped by the ~60 fps repaint.
+
+use nih_plug::prelude::AtomicF32;
+use std::sync::atomic::{AtomicU64, Ordering};
+
+/// Metered channels: low, mid, high, then the 'All' aggregate — same order as the compressors.
+pub const NUM_CHANNELS: usize = 4;
+
+/// Rate the audio thread emits plot buckets at (Hz). Sets the plot's max horizontal resolution,
+/// decoupled from the editor frame rate. ~5 ms per bucket.
+pub const BUCKET_HZ: u32 = 200;
+
+/// Buckets buffered between GUI drains. At [`BUCKET_HZ`] this is ~2.5 s of slack — far more than
+/// the frame interval needs; if the GUI ever stalls longer, the oldest buckets are dropped.
+const RING_N: usize = 512;
+
+pub struct Meters {
+    /// Left output level per channel as a **linear** peak. Peak-with-decay.
+    pub level_l: [AtomicF32; NUM_CHANNELS],
+    /// Right output level per channel (== left for mono signals).
+    pub level_r: [AtomicF32; NUM_CHANNELS],
+    /// Mono **input** level per channel (post pre-gain = what the compressor detects). Drives the
+    /// gain-curve operating-point fill. Peak-with-decay.
+    pub input_level: [AtomicF32; NUM_CHANNELS],
+    /// Compressor gain reduction per channel in **dB (>= 0)**. Mono by design — detection is
+    /// stereo-linked, so the same gain applies to both channels.
+    pub gain_reduction_db: [AtomicF32; NUM_CHANNELS],
+    /// Output limiter gain reduction in **dB (>= 0)** — feeds the ALL channel's ceiling lamp.
+    pub limiter_gr_db: AtomicF32,
+    /// Bucket stream feeding the scrolling in/out/GR plot.
+    pub scope: ScopeRing,
+}
+
+impl Default for Meters {
+    fn default() -> Self {
+        Self {
+            level_l: std::array::from_fn(|_| AtomicF32::new(0.0)),
+            level_r: std::array::from_fn(|_| AtomicF32::new(0.0)),
+            input_level: std::array::from_fn(|_| AtomicF32::new(0.0)),
+            gain_reduction_db: std::array::from_fn(|_| AtomicF32::new(0.0)),
+            limiter_gr_db: AtomicF32::new(0.0),
+            scope: ScopeRing::default(),
+        }
+    }
+}
+
+impl Meters {
+    /// Zero the bar meters. Called from the plugin's `reset()` (transport restart / sample-rate
+    /// change) so the bars start from silence. The plot ring is left alone — it's continuous and
+    /// reflects the new (silent) buckets as they arrive. Real-time safe.
+    pub fn clear(&self) {
+        for i in 0..NUM_CHANNELS {
+            self.level_l[i].store(0.0, Ordering::Relaxed);
+            self.level_r[i].store(0.0, Ordering::Relaxed);
+            self.input_level[i].store(0.0, Ordering::Relaxed);
+            self.gain_reduction_db[i].store(0.0, Ordering::Relaxed);
+        }
+        self.limiter_gr_db.store(0.0, Ordering::Relaxed);
+    }
+}
+
+/// Update a meter atomic with a new block value using peak-hold-with-decay: jump instantly to a
+/// louder value, ease back down by `decay_weight` (0..1, closer to 1 = slower fall). Keeps meters
+/// from flickering while staying responsive to transients.
+pub fn decay_store(meter: &AtomicF32, block_value: f32, decay_weight: f32) {
+    let current = meter.load(Ordering::Relaxed);
+    let next = if block_value > current {
+        block_value
+    } else {
+        current * decay_weight + block_value * (1.0 - decay_weight)
+    };
+    meter.store(next, Ordering::Relaxed);
+}
+
+/// Lock-free single-producer/single-consumer ring of plot buckets. Each bucket holds a per-channel
+/// (input level, output level, gain reduction) triple. The producer (audio thread) appends with
+/// [`push`](ScopeRing::push); the consumer (GUI) reads new buckets with [`drain`](ScopeRing::drain),
+/// tracking its own cursor. Per-field atomics avoid tearing; the consumer leaves one slot of margin
+/// from the slot being written, so it never races the producer. If the consumer falls more than the
+/// ring behind, the oldest buckets are silently dropped (a visual gap at worst).
+pub struct ScopeRing {
+    /// `slot * NUM_CHANNELS + ch`, indexed by `bucket_index % RING_N`.
+    in_lin: Vec<AtomicF32>,
+    out_lin: Vec<AtomicF32>,
+    gr_db: Vec<AtomicF32>,
+    /// Per-bucket (not per-channel) flag: `1.0` if the output limiter hit the ceiling in this
+    /// bucket, else `0.0`. Indexed by `bucket_index % RING_N`.
+    hit: Vec<AtomicF32>,
+    /// Monotonic count of buckets ever written.
+    write: AtomicU64,
+}
+
+impl Default for ScopeRing {
+    fn default() -> Self {
+        let make = || (0..RING_N * NUM_CHANNELS).map(|_| AtomicF32::new(0.0)).collect();
+        Self {
+            in_lin: make(),
+            out_lin: make(),
+            gr_db: make(),
+            hit: (0..RING_N).map(|_| AtomicF32::new(0.0)).collect(),
+            write: AtomicU64::new(0),
+        }
+    }
+}
+
+impl ScopeRing {
+    /// Producer (audio thread): append one bucket of per-channel (in_lin, out_lin, gr_db).
+    pub fn push(
+        &self,
+        in_lin: &[f32; NUM_CHANNELS],
+        out_lin: &[f32; NUM_CHANNELS],
+        gr_db: &[f32; NUM_CHANNELS],
+        hit: f32,
+    ) {
+        let w = self.write.load(Ordering::Relaxed); // producer is the sole writer of `write`
+        let slot = w as usize % RING_N;
+        let base = slot * NUM_CHANNELS;
+        for ch in 0..NUM_CHANNELS {
+            self.in_lin[base + ch].store(in_lin[ch], Ordering::Relaxed);
+            self.out_lin[base + ch].store(out_lin[ch], Ordering::Relaxed);
+            self.gr_db[base + ch].store(gr_db[ch], Ordering::Relaxed);
+        }
+        self.hit[slot].store(hit, Ordering::Relaxed);
+        // Publish the bucket: the Release pairs with the consumer's Acquire so the stores above are
+        // visible before the new count.
+        self.write.store(w + 1, Ordering::Release);
+    }
+
+    /// Consumer (GUI): call `on_bucket` for each bucket in `*cursor..write`, advancing `cursor`.
+    /// Skips ahead (dropping oldest) if the consumer fell more than the ring behind.
+    pub fn drain(
+        &self,
+        cursor: &mut u64,
+        mut on_bucket: impl FnMut(&[f32; NUM_CHANNELS], &[f32; NUM_CHANNELS], &[f32; NUM_CHANNELS], f32),
+    ) {
+        let w = self.write.load(Ordering::Acquire);
+        if *cursor > w {
+            *cursor = w; // counter went backwards (shouldn't happen) — resync
+        }
+        // Stay one slot clear of the slot currently being written.
+        let oldest = w.saturating_sub((RING_N - 1) as u64);
+        if *cursor < oldest {
+            *cursor = oldest;
+        }
+        let mut in_buf = [0.0f32; NUM_CHANNELS];
+        let mut out_buf = [0.0f32; NUM_CHANNELS];
+        let mut gr_buf = [0.0f32; NUM_CHANNELS];
+        while *cursor < w {
+            let slot = *cursor as usize % RING_N;
+            let base = slot * NUM_CHANNELS;
+            for ch in 0..NUM_CHANNELS {
+                in_buf[ch] = self.in_lin[base + ch].load(Ordering::Relaxed);
+                out_buf[ch] = self.out_lin[base + ch].load(Ordering::Relaxed);
+                gr_buf[ch] = self.gr_db[base + ch].load(Ordering::Relaxed);
+            }
+            let hit = self.hit[slot].load(Ordering::Relaxed);
+            on_bucket(&in_buf, &out_buf, &gr_buf, hit);
+            *cursor += 1;
+        }
+    }
+
+    /// Current write high-water mark (for a fresh consumer to start from "now").
+    pub fn write_index(&self) -> u64 {
+        self.write.load(Ordering::Acquire)
+    }
+}

src/params.rs

@@ -0,0 +1,239 @@
+//! Plugin parameters and their layout.
+//!
+//! Holds the global controls plus four `CompressorParams` blocks (low/mid/high + the 'All'
+//! aggregate channel). `build_settings` translates a channel's params into the per-block
+//! [`CompressorSettings`] the DSP consumes.
+
+use nih_plug::prelude::*;
+use nih_plug_egui::EguiState;
+use std::sync::Arc;
+
+use crate::dsp::compressor::{Compressor, CompressorSettings, MAX_LOOKAHEAD_MS};
+
+/// Level-detection mode for a compressor's detector.
+#[derive(Enum, PartialEq, Clone, Copy)]
+pub enum DetectionMode {
+    #[id = "peak"]
+    #[name = "Peak"]
+    Peak,
+    #[id = "rms"]
+    #[name = "RMS"]
+    Rms,
+}
+
+#[derive(Params)]
+pub struct Codename206Params {
+    #[persist = "editor-state"]
+    pub editor_state: Arc<EguiState>,
+
+    /// Low/Mid crossover frequency.
+    #[id = "xover_lo"]
+    pub crossover_low_hz: FloatParam,
+    /// Mid/High crossover frequency.
+    #[id = "xover_hi"]
+    pub crossover_high_hz: FloatParam,
+    /// Global look-ahead time (constant reported latency — safe to adjust during playback).
+    #[id = "lookahead"]
+    pub look_ahead_ms: FloatParam,
+
+    /// Output brickwall ceiling (the limiter never lets output exceed this).
+    #[id = "ceiling"]
+    pub output_ceiling_db: FloatParam,
+    /// Output limiter release time.
+    #[id = "lim_rel"]
+    pub limiter_release_ms: FloatParam,
+
+    #[nested(id_prefix = "low", group = "Low")]
+    pub low: CompressorParams,
+    #[nested(id_prefix = "mid", group = "Mid")]
+    pub mid: CompressorParams,
+    #[nested(id_prefix = "high", group = "High")]
+    pub high: CompressorParams,
+    #[nested(id_prefix = "all", group = "All")]
+    pub all: CompressorParams,
+}
+
+#[derive(Params)]
+pub struct CompressorParams {
+    /// Drive into the compressor: scales the signal **before** detection, so it both pushes the
+    /// channel further into compression and feeds the downstream sum/limiter harder. Combined with
+    /// makeup (post-comp), this gives full per-channel input/output gain-staging.
+    #[id = "pregain"]
+    pub pre_gain_db: FloatParam,
+    #[id = "detect"]
+    pub detection: EnumParam<DetectionMode>,
+    #[id = "thresh"]
+    pub threshold_db: FloatParam,
+    #[id = "ratio"]
+    pub ratio: FloatParam,
+    #[id = "knee"]
+    pub knee_db: FloatParam,
+    /// Low shaper slope at the silence floor (1 = unity; >1 fans up/boost, <1 fans down/cut).
+    #[id = "lowslope"]
+    pub low_slope: FloatParam,
+    /// Low shaper curvature (−1..1): bipolar mid-bulge, 0 = straight. +bulges up (boost quiet
+    /// middle), − bulges down (suppress). Endpoints (silence + knee) stay fixed.
+    #[id = "lowcurve"]
+    pub low_curve: FloatParam,
+    #[id = "attack"]
+    pub attack_ms: FloatParam,
+    #[id = "release"]
+    pub release_ms: FloatParam,
+    #[id = "makeup"]
+    pub makeup_db: FloatParam,
+    /// Dry/wet mix (parallel compression). 100% = fully processed, 0% = dry (a clean bypass).
+    #[id = "mix"]
+    pub mix: FloatParam,
+}
+
+impl Default for Codename206Params {
+    fn default() -> Self {
+        Self {
+            editor_state: EguiState::from_size(760, 520),
+
+            crossover_low_hz: FloatParam::new(
+                "Crossover Lo/Mid",
+                200.0,
+                FloatRange::Skewed { min: 30.0, max: 1_000.0, factor: FloatRange::skew_factor(-1.0) },
+            )
+            .with_value_to_string(formatters::v2s_f32_hz_then_khz(0))
+            .with_string_to_value(formatters::s2v_f32_hz_then_khz()),
+
+            crossover_high_hz: FloatParam::new(
+                "Crossover Mid/Hi",
+                2_500.0,
+                FloatRange::Skewed { min: 500.0, max: 18_000.0, factor: FloatRange::skew_factor(-1.0) },
+            )
+            .with_value_to_string(formatters::v2s_f32_hz_then_khz(0))
+            .with_string_to_value(formatters::s2v_f32_hz_then_khz()),
+
+            look_ahead_ms: FloatParam::new(
+                "Look-ahead",
+                2.0,
+                FloatRange::Linear { min: 0.0, max: MAX_LOOKAHEAD_MS },
+            )
+            .with_unit(" ms")
+            .with_value_to_string(formatters::v2s_f32_rounded(2)),
+
+            output_ceiling_db: FloatParam::new(
+                "Ceiling",
+                0.0,
+                FloatRange::Linear { min: -24.0, max: 0.0 },
+            )
+            .with_unit(" dB")
+            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
+            limiter_release_ms: FloatParam::new(
+                "Limiter Release",
+                100.0,
+                FloatRange::Skewed { min: 1.0, max: 1_000.0, factor: FloatRange::skew_factor(-2.0) },
+            )
+            .with_unit(" ms")
+            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
+            low: CompressorParams::default(),
+            mid: CompressorParams::default(),
+            high: CompressorParams::default(),
+            all: CompressorParams::default(),
+        }
+    }
+}
+
+impl Default for CompressorParams {
+    fn default() -> Self {
+        Self {
+            pre_gain_db: FloatParam::new(
+                "Pre-gain",
+                0.0,
+                FloatRange::Linear { min: -24.0, max: 36.0 },
+            )
+            .with_smoother(SmoothingStyle::Linear(20.0))
+            .with_unit(" dB")
+            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
+            detection: EnumParam::new("Detection", DetectionMode::Peak),
+
+            threshold_db: FloatParam::new(
+                "Threshold",
+                -18.0,
+                FloatRange::Linear { min: -60.0, max: 0.0 },
+            )
+            .with_unit(" dB")
+            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
+            ratio: FloatParam::new(
+                "Ratio",
+                2.0,
+                FloatRange::Skewed { min: 1.0, max: 20.0, factor: FloatRange::skew_factor(-1.0) },
+            )
+            .with_value_to_string(Arc::new(|v| format!("{v:.2} : 1")))
+            .with_string_to_value(Arc::new(|s| {
+                s.split(':').next().and_then(|x| x.trim().parse::<f32>().ok())
+            })),
+
+            knee_db: FloatParam::new("Knee", 6.0, FloatRange::Linear { min: 0.0, max: 30.0 })
+                .with_unit(" dB")
+                .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
+            low_slope: FloatParam::new(
+                "Low Slope",
+                1.0,
+                FloatRange::Skewed { min: 0.5, max: 3.0, factor: FloatRange::skew_factor(-1.0) },
+            )
+            .with_smoother(SmoothingStyle::Linear(20.0))
+            .with_value_to_string(formatters::v2s_f32_rounded(2)),
+
+            low_curve: FloatParam::new("Low Curve", 0.0, FloatRange::Linear { min: -1.0, max: 1.0 })
+                .with_smoother(SmoothingStyle::Linear(20.0))
+                .with_value_to_string(formatters::v2s_f32_percentage(0))
+                .with_string_to_value(formatters::s2v_f32_percentage()),
+
+            attack_ms: FloatParam::new(
+                "Attack",
+                10.0,
+                FloatRange::Skewed { min: 0.0, max: 100.0, factor: FloatRange::skew_factor(-2.0) },
+            )
+            .with_unit(" ms")
+            .with_value_to_string(formatters::v2s_f32_rounded(2)),
+
+            release_ms: FloatParam::new(
+                "Release",
+                100.0,
+                FloatRange::Skewed { min: 1.0, max: 1_000.0, factor: FloatRange::skew_factor(-2.0) },
+            )
+            .with_unit(" ms")
+            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
+            makeup_db: FloatParam::new("Makeup", 0.0, FloatRange::Linear { min: -24.0, max: 24.0 })
+                .with_smoother(SmoothingStyle::Linear(20.0))
+                .with_unit(" dB")
+                .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
+            mix: FloatParam::new("Mix", 1.0, FloatRange::Linear { min: 0.0, max: 1.0 })
+                .with_smoother(SmoothingStyle::Linear(20.0))
+                .with_value_to_string(formatters::v2s_f32_percentage(0))
+                .with_string_to_value(formatters::s2v_f32_percentage()),
+        }
+    }
+}
+
+/// Build the per-block compressor settings for one channel's params (makeup filled per sample).
+pub fn build_settings(
+    p: &CompressorParams,
+    lookahead: usize,
+    sample_rate: f32,
+) -> CompressorSettings {
+    CompressorSettings {
+        threshold_db: p.threshold_db.value(),
+        ratio: p.ratio.value(),
+        knee_db: p.knee_db.value(),
+        low_slope: p.low_slope.value(),
+        low_curve: p.low_curve.value(),
+        attack_coef: Compressor::time_to_coef(p.attack_ms.value(), sample_rate),
+        release_coef: Compressor::time_to_coef(p.release_ms.value(), sample_rate),
+        makeup_db: 0.0,
+        lookahead_samples: lookahead,
+        use_rms: p.detection.value() == DetectionMode::Rms,
+        mix: p.mix.value(),
+    }
+}