Update server.js

server.js

@@ -783,10 +783,19 @@ async function buildVideo(questions, audioFiles, framesDir, videosDir, onProgres
   );
 
   // ------------------------------------------------------------------
-  // 3. Render frames and build the concat list using real durations.
+  // 3. Render frames and build a sequential image list at 30fps.
+  //    The concat demuxer ignores 'duration' lines for PNG inputs on
+  //    this ffmpeg build (Duration: N/A, treats them as raw 25fps stream).
+  //    Solution: use the image2 demuxer at a fixed 30fps and write each
+  //    frame as a symlink repeated the correct number of times.
+  //    Frame count = round(durationSeconds * FPS) — no ffmpeg duration
+  //    parsing needed, timing is mathematically exact.
   // ------------------------------------------------------------------
-  const framesList = path.join(framesDir, "frames_list.txt");
-  const concatList = [];
+  const FPS = 30;
+  const seqDir = path.join(framesDir, "seq");
+  fs.mkdirSync(seqDir, { recursive: true });
+
+  let seqIndex = 0; // global counter across all frames
 
   for (let i = 0; i < usedQuestions.length; i++) {
     const q = usedQuestions[i];
@@ -798,20 +807,29 @@ async function buildVideo(questions, audioFiles, framesDir, videosDir, onProgres
     const rFrame = path.join(framesDir, `q${i + 1}_result.png`);
     await renderFrame(q, "result", rFrame);
 
-    concatList.push(`file '${qFrame}'\nduration ${questionDuration}`);
-    concatList.push(`file '${rFrame}'\nduration ${resultDuration}`);
+    // Write symlinks for question phase
+    const qFrameCount = Math.max(1, Math.round(questionDuration * FPS));
+    for (let f = 0; f < qFrameCount; f++) {
+      const linkPath = path.join(seqDir, `frame${String(seqIndex).padStart(6, "0")}.png`);
+      fs.symlinkSync(qFrame, linkPath);
+      seqIndex++;
+    }
+
+    // Write symlinks for result phase
+    const rFrameCount = Math.max(1, Math.round(resultDuration * FPS));
+    for (let f = 0; f < rFrameCount; f++) {
+      const linkPath = path.join(seqDir, `frame${String(seqIndex).padStart(6, "0")}.png`);
+      fs.symlinkSync(rFrame, linkPath);
+      seqIndex++;
+    }
 
     if (onProgress) onProgress(i + 1, usedQuestions.length);
   }
 
-  // ffmpeg concat requires the last entry repeated without a duration
-  const lastResult = path.join(framesDir, `q${usedQuestions.length}_result.png`);
-  concatList.push(`file '${lastResult}'`);
-  fs.writeFileSync(framesList, concatList.join("\n"));
+  console.log(`🖼  Total frames written: ${seqIndex} @ ${FPS}fps = ${(seqIndex / FPS).toFixed(1)}s`);
 
   // ------------------------------------------------------------------
-  // 4. Concatenate only the audio files we're actually using.
-  //    audioDir is derived from the files themselves (parent directory).
+  // 4. Concatenate audio files.
   // ------------------------------------------------------------------
   const audioDir = path.dirname(usedAudio[0]);
   const combinedAudio = path.join(audioDir, "combined.mp3");
@@ -819,9 +837,8 @@ async function buildVideo(questions, audioFiles, framesDir, videosDir, onProgres
 
   const videoPath = path.join(videosDir, `wyr_${Date.now()}.mp4`);
 
-  // Probe the combined audio duration so we can set an explicit output duration.
-  // This is more reliable than -shortest, which cuts at whichever stream ends first —
-  // and with the concat demuxer the video stream duration is often reported wrong.
+  // Probe audio so we can set -t explicitly (guards against any off-by-one
+  // between frame count and audio length).
   const audioDurationSecs = await new Promise((resolve, reject) => {
     ffmpeg.ffprobe(combinedAudio, (err, meta) => {
       if (err) return reject(err);
@@ -834,18 +851,13 @@ async function buildVideo(questions, audioFiles, framesDir, videosDir, onProgres
 
   await new Promise((resolve, reject) => {
     ffmpeg()
-      .input(framesList)
-      // No -r on input: the concat demuxer ignores it and uses its own 25fps default.
-      // Duration is controlled by the duration lines in frames_list.txt.
-      .inputOptions(["-f concat", "-safe 0"])
+      // image2 demuxer at fixed 30fps — reads frame000000.png, frame000001.png, ...
+      // Each file appears exactly the right number of times so timing is exact.
+      .input(path.join(seqDir, "frame%06d.png"))
+      .inputOptions(["-f image2", `-r ${FPS}`])
       .input(combinedAudio)
-      // No -r on input: the concat demuxer ignores it for image inputs.
-      // Duration is driven purely by the 'duration' lines in frames_list.txt.
-      // -filter:v fps=30 resamples the variable-rate concat output to constant 30fps,
-      // which libx264 requires. -t pins the output to the real audio length so we
-      // don't get cut short or run long regardless of what concat reports.
       .outputOptions([
-        "-filter:v fps=30",
+        `-r ${FPS}`,
         "-c:v libx264",
         "-pix_fmt yuv420p",
         "-preset fast",