feat(274): name the モトツ pair with a slash in the transect subtitles

モトツ橋にモトツシェルター -> モトツ橋 / モトツシェルター, matching the two nodes the
visualiser lights side by side on the road. The rest of the diff is Aegisub's
saved cursor position.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PLt25nCVMsoroKALYUBu6u

song-274/274-transect.ass

@@ -15,9 +15,9 @@ Video File: 274_transect.mp4
 Video AR Mode: 4
 Video AR Value: 1.777778
 Video Zoom Percent: 0.500000
-Scroll Position: 107
-Active Line: 124
-Video Position: 18574
+Scroll Position: 74
+Active Line: 82
+Video Position: 8587
 
 [V4+ Styles]
 Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
@@ -123,7 +123,7 @@ Dialogue: 0,0:04:38.70,0:04:41.02,274-place,,0,0,0,,福山大橋
 Dialogue: 0,0:04:41.02,0:04:43.29,274-place,,0,0,0,,千坂橋
 Dialogue: 0,0:04:43.29,0:04:45.12,274-place,,0,0,0,,オコタン橋
 Dialogue: 0,0:04:45.12,0:04:46.22,274-place,,0,0,0,,モトツトンネル
-Dialogue: 0,0:04:46.22,0:04:48.98,274-place,,0,0,0,,モトツ橋にモトツシェルター
+Dialogue: 0,0:04:46.22,0:04:48.98,274-place,,0,0,0,,モトツ橋 / モトツシェルター
 Dialogue: 0,0:04:48.98,0:04:51.39,274-place,,0,0,0,,福山トンネル
 Dialogue: 0,0:04:51.39,0:04:53.61,274-place,,0,0,0,,福穂橋
 Dialogue: 0,0:04:53.61,0:04:55.52,274-place,,0,0,0,,穂高トンネル

src/song-274/Composition.tsx

@@ -4,7 +4,7 @@ import { loadTransectData } from "./data";
 import { darkPalette } from "./palette";
 import { Transect, type TransectProps } from "./Transect";
 
-const FPS = 30;
+const FPS = 60;
 /** Length of song-274/audio/274_release2.wav (see BOM.md). */
 const AUDIO_DURATION_S = 619.270083;
 

src/song-274/camera.ts

@@ -27,6 +27,11 @@ export const ZOOM_IN_S = 3;
  * This is the standard deviation, in seconds; the window spans ±3 of these.
  */
 export const FOLLOW_SMOOTH_S = 0.6;
+/**
+ * How often that window is sampled, in seconds. Independent of fps: finer
+ * sampling costs more per frame and changes the average only slightly.
+ */
+export const FOLLOW_SAMPLE_S = 1 / 30;
 /**
  * An overview frames the covered road plus some road ahead of the head, so the
  * head does not sit against the frame edge: this fraction of the distance
@@ -95,7 +100,7 @@ export const overviewWeight = (parts: Span[], t: number): number => {
 
 /** The head's position averaged over a Gaussian window around `t`. */
 export const smoothedFocus = (focusAt: (t: number) => Point, t: number): Point => {
-  const step = 1 / 30;
+  const step = FOLLOW_SAMPLE_S;
   const reach = 3 * FOLLOW_SMOOTH_S;
   let sumX = 0;
   let sumY = 0;

src/song-274/fade.ts

@@ -5,6 +5,8 @@
 
 /** Seconds of complete black at start */
 export const FADE_IN_PRE = 0.4;
+/** Seconds of complete black at end */
+export const FADE_OUT_POST = 0.4;
 /** Seconds from black to the picture at the start. 0 disables the fade-in. */
 export const FADE_IN_S = 0.3;
 /** Seconds from the picture to black at the end. 0 disables the fade-out. */
@@ -19,6 +21,8 @@ const clamp01 = (v: number) => Math.min(1, Math.max(0, v));
  */
 export const blackOpacity = (t: number, lastFrameS: number): number => {
   const fadeIn = (t < FADE_IN_PRE) ? 1 : (FADE_IN_S > 0 ? 1 - clamp01((t - FADE_IN_PRE) / FADE_IN_S) : 0);
-  const fadeOut = FADE_OUT_S > 0 ? clamp01((t - (lastFrameS - FADE_OUT_S)) / FADE_OUT_S) : 0;
+  // Black from here to the last frame; the fade-out ramp ends exactly here.
+  const blackFromS = lastFrameS - FADE_OUT_POST;
+  const fadeOut = (t >= blackFromS) ? 1 : (FADE_OUT_S > 0 ? clamp01((t - (blackFromS - FADE_OUT_S)) / FADE_OUT_S) : 0);
   return Math.max(fadeIn, fadeOut);
 };

src/song-274/spectrum.ts

@@ -1,5 +1,6 @@
 import { visualizeAudio, type MediaUtilsAudioData } from "@remotion/media-utils";
 import { random } from "remotion";
+import { secondsToFrames } from "./time";
 
 /**
  * The outro spectrum: from SPECTRUM_START_S the nodes stop following the lyric
@@ -43,11 +44,16 @@ export const TREBLE_TILT = 0.5;
 export const CURVE = 0.6;
 
 /**
- * Release: a node's level is the loudest of its last RELEASE_FRAMES frames, each
- * frame back multiplied by RELEASE_DECAY once more. Hits jump up at once and
- * fall back smoothly, without any state carried between frames.
+ * Release: a node's level is the loudest reading over the RELEASE_WINDOW_S
+ * before it, sampled every RELEASE_STEP_S and multiplied by RELEASE_DECAY once
+ * more per step back. Hits jump up at once and fall back smoothly, without any
+ * state carried between frames.
+ *
+ * Sampling by time rather than by frame keeps the tail the same length, and the
+ * work per frame the same, whatever the fps.
  */
-export const RELEASE_FRAMES = 6;
+export const RELEASE_WINDOW_S = 0.2;
+export const RELEASE_STEP_S = 1 / 30;
 export const RELEASE_DECAY = 0.75;
 
 /** FFT size for visualizeAudio: bins are (sample rate / 2) / FFT_SAMPLES wide (23.4 Hz). */
@@ -69,7 +75,7 @@ export const spectrumWeight = (t: number): number =>
  * otherwise the spectrum's start, so nothing is fetched for the rest of the song.
  */
 export const audioWindowFrame = (frame: number, fps: number): number =>
-  Math.max(frame, Math.floor((SPECTRUM_START_S - SPECTRUM_FADE_S) * fps));
+  Math.max(frame, secondsToFrames(SPECTRUM_START_S - SPECTRUM_FADE_S, fps));
 
 /** `count` log-spaced band centres from FREQ_LOW_HZ to FREQ_HIGH_HZ. */
 export const bandFrequencies = (count: number): number[] =>
@@ -131,7 +137,12 @@ export const bandLevels = (args: {
     frequencies.length > 1 ? Math.pow(FREQ_HIGH_HZ / FREQ_LOW_HZ, 1 / (frequencies.length - 1)) : 2;
   const levels = frequencies.map(() => 0);
 
-  for (let back = 0; back < RELEASE_FRAMES && frame - back >= 0; back++) {
+  const steps = Math.max(1, Math.round(RELEASE_WINDOW_S / RELEASE_STEP_S));
+  for (let step = 0; step < steps; step++) {
+    const back = secondsToFrames(step * RELEASE_STEP_S, fps);
+    if (frame - back < 0) {
+      break;
+    }
     const spectrum = visualizeAudio({
       audioData,
       frame: frame - back,
@@ -140,7 +151,7 @@ export const bandLevels = (args: {
       dataOffsetInSeconds,
       optimizeFor: "accuracy",
     });
-    const decay = Math.pow(RELEASE_DECAY, back);
+    const decay = Math.pow(RELEASE_DECAY, step);
     frequencies.forEach((hz, i) => {
       const raw = bandValue(spectrum, hz, ratio, binHz);
       const level =

src/song-274/time.ts

@@ -0,0 +1,14 @@
+/**
+ * Frame-rate conversions.
+ *
+ * Every timing constant in this composition is in seconds, so the animation is
+ * identical at any fps. Only Remotion's own API counts frames, so convert here
+ * rather than writing frame counts into the constants.
+ */
+
+/** Seconds to a whole number of frames, rounded to the nearest. */
+export const secondsToFrames = (seconds: number, fps: number): number =>
+  Math.round(seconds * fps);
+
+/** A frame number back to its time in seconds. */
+export const framesToSeconds = (frames: number, fps: number): number => frames / fps;