From 831e838f74424f68592d4ae5f57f82c4304efd0b Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Fri, 19 Dec 2025 13:06:11 +0100
Subject: [PATCH] Implement workaround for
 https://issues.chromium.org/issues/470109459

---
 src/codec-data.ts | 93 ++++++++++++++++++++++++++++++++++-------------
 src/media-sink.ts | 20 +++++++++-
 2 files changed, 86 insertions(+), 27 deletions(-)

diff --git a/src/codec-data.ts b/src/codec-data.ts
index 2fd8ca7..26fa41e 100644
--- a/src/codec-data.ts
+++ b/src/codec-data.ts
@@ -24,6 +24,7 @@ import {
 	toUint8Array,
 	getChromiumVersion,
 	isChromium,
+	setUint24,
 } from './misc';
 import { PacketType } from './packet';
 import { MetadataTags } from './metadata';
@@ -161,38 +162,65 @@ const removeEmulationPreventionBytes = (data: Uint8Array) => {
 	return new Uint8Array(result);
 };
 
-/** Converts an AVC packet in Annex B format to length-prefixed format. */
-export const transformAnnexBToLengthPrefixed = (packetData: Uint8Array) => {
-	const NAL_UNIT_LENGTH_SIZE = 4;
+const ANNEX_B_START_CODE = new Uint8Array([0, 0, 0, 1]);
 
-	const nalUnits = findNalUnitsInAnnexB(packetData);
-
-	if (nalUnits.length === 0) {
-		// If no NAL units were found, it's not valid Annex B data
-		return null;
-	}
-
-	let totalSize = 0;
-	for (const nalUnit of nalUnits) {
-		totalSize += NAL_UNIT_LENGTH_SIZE + nalUnit.byteLength;
-	}
-
-	const avccData = new Uint8Array(totalSize);
-	const dataView = new DataView(avccData.buffer);
+export const concatNalUnitsInAnnexB = (nalUnits: Uint8Array[]) => {
+	const totalLength = nalUnits.reduce((a, b) => a + ANNEX_B_START_CODE.byteLength + b.byteLength, 0);
+	const result = new Uint8Array(totalLength);
 	let offset = 0;
 
-	// Write each NAL unit with its length prefix
 	for (const nalUnit of nalUnits) {
-		const length = nalUnit.byteLength;
+		result.set(ANNEX_B_START_CODE, offset);
+		offset += ANNEX_B_START_CODE.byteLength;
 
-		dataView.setUint32(offset, length, false);
-		offset += 4;
-
-		avccData.set(nalUnit, offset);
+		result.set(nalUnit, offset);
 		offset += nalUnit.byteLength;
 	}
 
-	return avccData;
+	return result;
+};
+
+export const concatNalUnitsInLengthPrefixed = (nalUnits: Uint8Array[], lengthSize: 1 | 2 | 3 | 4) => {
+	const totalLength = nalUnits.reduce((a, b) => a + lengthSize + b.byteLength, 0);
+	const result = new Uint8Array(totalLength);
+	let offset = 0;
+
+	for (const nalUnit of nalUnits) {
+		const dataView = new DataView(result.buffer, result.byteOffset, result.byteLength);
+
+		switch (lengthSize) {
+			case 1:
+				dataView.setUint8(offset, nalUnit.byteLength);
+				break;
+			case 2:
+				dataView.setUint16(offset, nalUnit.byteLength, false);
+				break;
+			case 3:
+				setUint24(dataView, offset, nalUnit.byteLength, false);
+				break;
+			case 4:
+				dataView.setUint32(offset, nalUnit.byteLength, false);
+				break;
+		}
+
+		offset += lengthSize;
+
+		result.set(nalUnit, offset);
+		offset += nalUnit.byteLength;
+	}
+
+	return result;
+};
+
+/** Converts an AVC packet in Annex B format to length-prefixed format. */
+export const transformAnnexBToLengthPrefixed = (packetData: Uint8Array) => {
+	const nalUnits = findNalUnitsInAnnexB(packetData);
+	if (nalUnits.length === 0) {
+		// It's not valid Annex B data
+		return null;
+	}
+
+	return concatNalUnitsInLengthPrefixed(nalUnits, 4);
 };
 
 // Data specified in ISO 14496-15
@@ -227,7 +255,22 @@ export const extractAvcNalUnits = (packetData: Uint8Array, decoderConfig: VideoD
 	}
 };
 
-const extractNalUnitTypeForAvc = (data: Uint8Array) => {
+export const concatAvcNalUnits = (nalUnits: Uint8Array[], decoderConfig: VideoDecoderConfig) => {
+	if (decoderConfig.description) {
+		// Stream is length-prefixed. Let's extract the size of the length prefix from the decoder config
+
+		const bytes = toUint8Array(decoderConfig.description);
+		const lengthSizeMinusOne = bytes[4]! & 0b11;
+		const lengthSize = (lengthSizeMinusOne + 1) as 1 | 2 | 3 | 4;
+
+		return concatNalUnitsInLengthPrefixed(nalUnits, lengthSize);
+	} else {
+		// Stream is in Annex B format
+		return concatNalUnitsInAnnexB(nalUnits);
+	}
+};
+
+export const extractNalUnitTypeForAvc = (data: Uint8Array) => {
 	return data[0]! & 0x1F;
 };
 
diff --git a/src/media-sink.ts b/src/media-sink.ts
index 711d0c4..539606e 100644
--- a/src/media-sink.ts
+++ b/src/media-sink.ts
@@ -8,9 +8,12 @@
 
 import { parsePcmCodec, PCM_AUDIO_CODECS, PcmAudioCodec, VideoCodec, AudioCodec } from './codec';
 import {
+	concatAvcNalUnits,
 	deserializeAvcDecoderConfigurationRecord,
 	determineVideoPacketType,
+	extractAvcNalUnits,
 	extractHevcNalUnits,
+	extractNalUnitTypeForAvc,
 	extractNalUnitTypeForHevc,
 	HevcNalUnitType,
 	parseAvcSps,
@@ -928,8 +931,6 @@ class VideoDecoderWrapper extends DecoderWrapper<VideoSample> {
 			this.raslSkipped = true;
 		}
 
-		this.currentPacketIndex++;
-
 		if (this.customDecoder) {
 			this.customDecoderQueueSize++;
 			void this.customDecoderCallSerializer
@@ -942,9 +943,24 @@ class VideoDecoderWrapper extends DecoderWrapper<VideoSample> {
 				insertSorted(this.inputTimestamps, packet.timestamp, x => x);
 			}
 
+			// Workaround for https://issues.chromium.org/issues/470109459
+			if (isChromium() && this.currentPacketIndex === 0 && this.codec === 'avc') {
+				const nalUnits = extractAvcNalUnits(packet.data, this.decoderConfig);
+				const filteredNalUnits = nalUnits.filter((x) => {
+					const type = extractNalUnitTypeForAvc(x);
+					// These trip up Chromium's key frame detection, so let's strip them
+					return !(type >= 20 && type <= 31);
+				});
+
+				const newData = concatAvcNalUnits(filteredNalUnits, this.decoderConfig);
+				packet = new EncodedPacket(newData, packet.type, packet.timestamp, packet.duration);
+			}
+
 			this.decoder.decode(packet.toEncodedVideoChunk());
 			this.decodeAlphaData(packet);
 		}
+
+		this.currentPacketIndex++;
 	}
 
 	decodeAlphaData(packet: EncodedPacket) {