Fix: Premultiply alpha for WebM VP9 export to prevent black halos

public/app.js

@@ -1820,6 +1820,20 @@
     return getTopBottomCanvasTb;
   }
 
+  function preMultiplyAlpha(ctx, width, height) {
+    const imgData = ctx.getImageData(0, 0, width, height);
+    const d = imgData.data;
+    for (let i = 0; i < d.length; i += 4) {
+      const a = d[i + 3];
+      if (a === 255) continue;
+      if (a === 0) { d[i] = d[i+1] = d[i+2] = 0; continue; }
+      d[i] = (d[i] * a + 127) >> 8;
+      d[i + 1] = (d[i + 1] * a + 127) >> 8;
+      d[i + 2] = (d[i + 2] * a + 127) >> 8;
+    }
+    ctx.putImageData(imgData, 0, 0);
+  }
+
   /**
    * Hardware-Accelerated In-Browser Video Export Engine (WebCodecs + Muxer)
    * Supports: WebM VP9 Alpha & MP4 H.264 with robust codec probing & lossless audio
@@ -2083,6 +2097,14 @@
         finalFrameCanvas = getTopBottomCanvas(renderCanvas);
       }
 
+      if (supportsAlpha && !isTopBottom) {
+        preMultiplyAlpha(
+          finalFrameCanvas.getContext('2d'),
+          finalFrameCanvas.width,
+          finalFrameCanvas.height
+        );
+      }
+
       // Create VideoFrame for WebCodecs
       const timestampUs = Math.round((f / fps) * 1_000_000);
       const durationUs = Math.round((1 / fps) * 1_000_000);