feat: serial low-level shaper (Low Slope + Low Curve) before the compressor

Add a per-channel below-threshold shaper composed in series ahead of the comp:
gain = low_shape(level) + comp(level + low_shape(level)). The compressor's
threshold now sees the shaped level, so a Low Slope boost lifts quiet material
up into compression (and a cut pulls it out). Anchored at the -60 dB silence
floor. Low Curve bends the shaper toward a bounded saturation so the serial
composition doesn't blow up (0 = straight line).

gain_computer split into comp_gain_db + low_gain_db and composed; shared with
the editor gain-curve display. Slider order rearranged to read in signal order
(pre-gain -> low shaper -> compressor -> output). Defaults (slope 1, curve 0)
reproduce the plain compressor; 17 tests pass.

Known: the bipolar behaviour isn't final yet (milestone commit).

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

src/dsp/compressor.rs

@@ -26,6 +26,15 @@ 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;
+
+/// Exponential curvature (1/dB) for the low shaper at `low_curve` = 1. Bends the low gain toward a
+/// bounded saturation so the serial composition doesn't blow up. 0 = straight line.
+const LOW_CURVE_K_MAX: f32 = 0.1;
+
 /// 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 +49,11 @@ 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.
@@ -69,7 +83,8 @@ 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
 }
@@ -131,25 +146,54 @@ 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`. Also used by the editor's
-    /// gain-curve display, so it stays the single source of truth for the transfer shape.
-    pub 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 the silence floor ([`LOW_ANCHOR_DB`]): 0 at/below
+    /// the floor, rising with slope `low_slope - 1` and bending toward a bounded saturation set by
+    /// `low_curve` (0..1; 0 = straight line). This reshapes the level the compressor then sees.
+    fn low_gain_db(level_db: f32, low_slope: f32, low_curve: f32) -> f32 {
+        let d = level_db - LOW_ANCHOR_DB;
+        if d <= 0.0 {
+            return 0.0;
+        }
+        let m = low_slope - 1.0;
+        let k = low_curve * LOW_CURVE_K_MAX;
+        if k <= 1e-6 {
+            m * d // straight line (curvature off)
+        } else {
+            (m / k) * (1.0 - (-k * d).exp()) // saturates to m/k (bounded)
+        }
+    }
+
+    /// 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, 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
@@ -199,7 +243,14 @@ impl Compressor {
         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")
@@ -243,19 +294,39 @@ mod tests {
             lookahead_samples: 0,
             use_rms: 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_is_serial_into_threshold() {
+        // Serial: the low shaper reshapes the level, then the threshold sees the shaped level.
+        // 30 dB above the -60 floor, 12 dB below threshold (hard knee, no curve).
+        let (lvl, thr) = (-30.0, -18.0);
+        // Unity slope -> just the compressor (below threshold here -> 0).
+        assert_close(Compressor::gain_computer(lvl, thr, 4.0, 0.0, 1.0, 0.0), 0.0, 1e-6);
+        // Boost (slope 2, straight) lifts -30 by 30 dB to 0 dB -> 18 dB over threshold, comp pulls
+        // back (1/4 - 1) * 18 = -13.5 -> net 30 - 13.5 = 16.5.
+        assert_close(Compressor::gain_computer(lvl, 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(lvl, thr, 4.0, 0.0, 0.5, 0.0), -15.0, 1e-3);
+        // Curvature bounds the low gain: slope 2 + full curve saturates the boost (~9.5 dB) so it
+        // no longer crosses the threshold.
+        let g = Compressor::gain_computer(lvl, thr, 4.0, 0.0, 2.0, 1.0);
+        assert!((8.0..11.0).contains(&g), "expected bounded low boost ~9.5, got {g}");
     }
 
     #[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);
     }
 
@@ -264,11 +335,11 @@ 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]

src/editor/gain_curve.rs

@@ -26,6 +26,8 @@ pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, selected: usiz
     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]));
@@ -53,7 +55,7 @@ pub(super) fn draw(ui: &mut egui::Ui, params: &Codename206Params, selected: usiz
     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); // <= 0 dB
+        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)));
     }

src/editor/mod.rs

@@ -47,10 +47,15 @@ pub(crate) fn create(params: Arc<Codename206Params>, meters: Arc<Meters>) -> Opt
 
             // 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");

src/lib.rs

@@ -245,6 +245,8 @@ impl Plugin for Codename206 {
                 }
                 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];
@@ -273,6 +275,8 @@ impl Plugin for Codename206 {
             }
             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.

src/params.rs

@@ -68,6 +68,12 @@ pub struct CompressorParams {
     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 (0 = straight line, 1 = max bend toward a bounded saturation).
+    #[id = "lowcurve"]
+    pub low_curve: FloatParam,
     #[id = "attack"]
     pub attack_ms: FloatParam,
     #[id = "release"]
@@ -168,6 +174,19 @@ impl Default for CompressorParams {
                 .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: 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()),
+
             attack_ms: FloatParam::new(
                 "Attack",
                 10.0,
@@ -207,6 +226,8 @@ pub fn build_settings(
         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,