From f424f6e84000d3a01925a178a0dba44791704875 Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Sun, 7 Dec 2025 19:24:08 +0100
Subject: [PATCH] Count AVC SEI recovery points as keyframes (except on older
 Chromium versions)

---
 src/codec-data.ts | 66 ++++++++++++++++++++++++++++++++++++++++++++++-
 src/misc.ts       | 18 +++++++++++++
 2 files changed, 83 insertions(+), 1 deletion(-)

diff --git a/src/codec-data.ts b/src/codec-data.ts
index 03e1b24..2fd8ca7 100644
--- a/src/codec-data.ts
+++ b/src/codec-data.ts
@@ -22,6 +22,8 @@ import {
 	textEncoder,
 	toDataView,
 	toUint8Array,
+	getChromiumVersion,
+	isChromium,
 } from './misc';
 import { PacketType } from './packet';
 import { MetadataTags } from './metadata';
@@ -34,6 +36,7 @@ import { MetadataTags } from './metadata';
 
 export enum AvcNalUnitType {
 	IDR = 5,
+	SEI = 6,
 	SPS = 7,
 	PPS = 8,
 	SPS_EXT = 13,
@@ -1714,7 +1717,68 @@ export const determineVideoPacketType = (
 	switch (codec) {
 		case 'avc': {
 			const nalUnits = extractAvcNalUnits(packetData, decoderConfig);
-			const isKeyframe = nalUnits.some(x => extractNalUnitTypeForAvc(x) === AvcNalUnitType.IDR);
+			let isKeyframe = nalUnits.some(x => extractNalUnitTypeForAvc(x) === AvcNalUnitType.IDR);
+
+			if (!isKeyframe && (!isChromium() || getChromiumVersion()! >= 144)) {
+				// In addition to IDR, Recovery Point SEI also counts as a valid H.264 keyframe by current consensus.
+				// See https://github.com/w3c/webcodecs/issues/650 for the relevant discussion. WebKit and Firefox have
+				// always supported them, but Chromium hasn't, therefore the (admittedly dirty) version check.
+
+				for (const nalUnit of nalUnits) {
+					const type = extractNalUnitTypeForAvc(nalUnit);
+					if (type !== AvcNalUnitType.SEI) {
+						continue;
+					}
+
+					const bytes = removeEmulationPreventionBytes(nalUnit);
+					let pos = 1; // Skip NALU header
+
+					// sei_rbsp()
+					do {
+						// sei_message()
+						let payloadType = 0;
+						while (true) {
+							const nextByte = bytes[pos++];
+							if (nextByte === undefined) break;
+							payloadType += nextByte;
+
+							if (nextByte < 255) {
+								break;
+							}
+						}
+
+						let payloadSize = 0;
+						while (true) {
+							const nextByte = bytes[pos++];
+							if (nextByte === undefined) break;
+							payloadSize += nextByte;
+
+							if (nextByte < 255) {
+								break;
+							}
+						}
+
+						// sei_payload()
+						const PAYLOAD_TYPE_RECOVERY_POINT = 6;
+						if (payloadType === PAYLOAD_TYPE_RECOVERY_POINT) {
+							const bitstream = new Bitstream(bytes);
+							bitstream.pos = 8 * pos;
+
+							const recoveryFrameCount = readExpGolomb(bitstream);
+							const exactMatchFlag = bitstream.readBits(1);
+
+							if (recoveryFrameCount === 0 && exactMatchFlag === 1) {
+								// https://github.com/w3c/webcodecs/pull/910
+								// "recovery_frame_cnt == 0 and exact_match_flag=1 in the SEI recovery payload"
+								isKeyframe = true;
+								break;
+							}
+						}
+
+						pos += payloadSize;
+					} while (pos < bytes.length - 1);
+				}
+			}
 
 			return isKeyframe ? 'key' : 'delta';
 		};
diff --git a/src/misc.ts b/src/misc.ts
index 399d62d..d612db1 100644
--- a/src/misc.ts
+++ b/src/misc.ts
@@ -700,6 +700,24 @@ export const isChromium = () => {
 	return isChromiumCache = !!(typeof navigator !== 'undefined' && navigator.vendor?.includes('Google Inc'));
 };
 
+let chromiumVersionCache: number | null = null;
+export const getChromiumVersion = () => {
+	if (chromiumVersionCache !== null) {
+		return chromiumVersionCache;
+	}
+
+	if (typeof navigator === 'undefined') {
+		return null;
+	}
+
+	const match = /\bChrome\/(\d+)/.exec(navigator.userAgent);
+	if (!match) {
+		return null;
+	}
+
+	return chromiumVersionCache = Number(match[1]!);
+};
+
 /**
  * T or a promise that resolves to T.
  * @group Miscellaneous