From 1bb3c0e262997f4ef70fff277abb29d1a8ff7607 Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Tue, 16 Sep 2025 17:42:23 +0200
Subject: [PATCH] Avoid opaque canvases in case of Firefox (fixes #117)

---
 src/media-sink.ts   | 13 ++++++++++---
 src/media-source.ts | 23 +++++++++++++++++++----
 src/misc.ts         |  9 +++++++++
 src/sample.ts       |  8 ++++++--
 4 files changed, 44 insertions(+), 9 deletions(-)

diff --git a/src/media-sink.ts b/src/media-sink.ts
index 989d9b2..335dfcc 100644
--- a/src/media-sink.ts
+++ b/src/media-sink.ts
@@ -18,6 +18,7 @@ import {
 	getInt24,
 	getUint24,
 	insertSorted,
+	isFirefox,
 	isSafari,
 	last,
 	mapAsyncGenerator,
@@ -1199,14 +1200,20 @@ export class CanvasSink {
 			this._nextCanvasIndex = (this._nextCanvasIndex + 1) % this._canvasPool.length;
 		}
 
-		const context
-			= canvas.getContext('2d', { alpha: false }) as CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D;
+		const context = canvas.getContext('2d', {
+			alpha: isFirefox(), // Firefox has VideoFrame glitches with opaque canvases
+		}) as CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D;
 		assert(context);
 
 		context.resetTransform();
 
 		if (!canvasIsNew) {
-			context.clearRect(0, 0, this._width, this._height);
+			if (isFirefox()) {
+				context.fillStyle = 'black';
+				context.fillRect(0, 0, this._width, this._height);
+			} else {
+				context.clearRect(0, 0, this._width, this._height);
+			}
 		}
 
 		sample.drawWithFit(context, {
diff --git a/src/media-source.ts b/src/media-source.ts
index b72c791..44a7d42 100644
--- a/src/media-source.ts
+++ b/src/media-source.ts
@@ -18,7 +18,16 @@ import {
 	VideoCodec,
 } from './codec';
 import { OutputAudioTrack, OutputSubtitleTrack, OutputTrack, OutputVideoTrack } from './output';
-import { assert, assertNever, CallSerializer, clamp, promiseWithResolvers, setInt24, setUint24 } from './misc';
+import {
+	assert,
+	assertNever,
+	CallSerializer,
+	clamp,
+	isFirefox,
+	promiseWithResolvers,
+	setInt24,
+	setUint24,
+} from './misc';
 import { Muxer } from './muxer';
 import { SubtitleParser } from './subtitles';
 import { toAlaw, toUlaw } from './pcm';
@@ -248,12 +257,18 @@ class VideoEncoderWrapper {
 							canvasIsNew = true;
 						}
 
-						const context = this.resizeCanvas.getContext('2d', { alpha: false }) as
-							CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D;
+						const context = this.resizeCanvas.getContext('2d', {
+							alpha: isFirefox(), // Firefox has VideoFrame glitches with opaque canvases
+						}) as CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D;
 						assert(context);
 
 						if (!canvasIsNew) {
-							context.clearRect(0, 0, this.codedWidth, this.codedHeight);
+							if (isFirefox()) {
+								context.fillStyle = 'black';
+								context.fillRect(0, 0, this.codedWidth, this.codedHeight);
+							} else {
+								context.clearRect(0, 0, this.codedWidth, this.codedHeight);
+							}
 						}
 
 						videoSample.drawWithFit(context, { fit: sizeChangeBehavior });
diff --git a/src/misc.ts b/src/misc.ts
index 3126f03..1c842f2 100644
--- a/src/misc.ts
+++ b/src/misc.ts
@@ -642,6 +642,15 @@ export const isSafari = () => {
 	return result;
 };
 
+let isFirefoxCache: boolean | null = null;
+export const isFirefox = () => {
+	if (isFirefoxCache !== null) {
+		return isFirefoxCache;
+	}
+
+	return isFirefoxCache = typeof navigator !== 'undefined' && navigator.userAgent?.includes('Firefox');
+};
+
 /**
  * T or a promise that resolves to T.
  * @group Miscellaneous
diff --git a/src/sample.ts b/src/sample.ts
index 1f2e074..c713aa8 100644
--- a/src/sample.ts
+++ b/src/sample.ts
@@ -15,6 +15,7 @@ import {
 	toDataView,
 	toUint8Array,
 	SetRequired,
+	isFirefox,
 } from './misc';
 
 /**
@@ -228,7 +229,10 @@ export class VideoSample {
 			}
 
 			const canvas = new OffscreenCanvas(width, height);
-			const context = canvas.getContext('2d', { alpha: false, willReadFrequently: true });
+			const context = canvas.getContext('2d', {
+				alpha: isFirefox(), // Firefox has VideoFrame glitches with opaque canvases
+				willReadFrequently: true,
+			});
 			assert(context);
 
 			// Draw it to a canvas
@@ -343,7 +347,7 @@ export class VideoSample {
 			dest.set(this._data);
 		} else {
 			const canvas = this._data;
-			const context = canvas.getContext('2d', { alpha: false });
+			const context = canvas.getContext('2d');
 			assert(context);
 
 			const imageData = context.getImageData(0, 0, this.codedWidth, this.codedHeight);