Add WebVTT subtitle support to MP4 & a bunch of misc changes

src/muxer.ts

@@ -1,5 +1,5 @@
 import { Output, OutputAudioTrack, OutputSubtitleTrack, OutputTrack, OutputVideoTrack } from "./output";
-import { EncodedSubtitleChunk, EncodedSubtitleChunkMetadata } from "./subtitles";
+import { SubtitleCue, SubtitleMetadata } from "./subtitles";
 
 export abstract class Muxer {
 	output: Output;
@@ -11,9 +11,62 @@ export abstract class Muxer {
 	abstract start(): void;
 	abstract addEncodedVideoChunk(track: OutputVideoTrack, chunk: EncodedVideoChunk, meta?: EncodedVideoChunkMetadata): void;
 	abstract addEncodedAudioChunk(track: OutputAudioTrack, chunk: EncodedAudioChunk, meta?: EncodedAudioChunkMetadata): void;
-	abstract addEncodedSubtitleChunk(track: OutputSubtitleTrack, chunk: EncodedSubtitleChunk, meta?: EncodedSubtitleChunkMetadata): void;
+	abstract addSubtitleCue(track: OutputSubtitleTrack, cue: SubtitleCue, meta?: SubtitleMetadata): void;
 	abstract finalize(): void;
 
 	beforeTrackAdd(track: OutputTrack) {}
 	onTrackClose(track: OutputTrack) {}
+
+	private trackTimestampInfo = new WeakMap<OutputTrack, {
+		timestampOffset: number,
+		maxTimestamp: number,
+		lastKeyFrameTimestamp: number
+	}>();
+
+	abstract timestampsMustStartAtZero: boolean;
+	protected validateAndNormalizeTimestamp(track: OutputTrack, rawTimestampInUs: number, isKeyFrame: boolean) {
+		let timestampInSeconds = rawTimestampInUs / 1e6;
+
+		let timestampInfo = this.trackTimestampInfo.get(track);
+		if (!timestampInfo) {
+			if (!isKeyFrame) {
+				throw new Error('First frame must be a key frame.');
+			}
+
+			if (this.timestampsMustStartAtZero && timestampInSeconds > 0) {
+				throw new Error(`Timestamps must start at zero (got ${timestampInSeconds}s).`);
+			}
+
+			timestampInfo = {
+				timestampOffset: timestampInSeconds,
+				maxTimestamp: track.source.offsetTimestamps ? 0 : timestampInSeconds,
+				lastKeyFrameTimestamp: track.source.offsetTimestamps ? 0 : timestampInSeconds
+			};
+			this.trackTimestampInfo.set(track, timestampInfo);
+		}
+
+		if (track.source.offsetTimestamps) {
+			timestampInSeconds -= timestampInfo.timestampOffset;
+		}
+
+		if (timestampInSeconds < 0) {
+			throw new Error(`Timestamps must be non-negative (got ${timestampInSeconds}s).`);
+		}
+
+		if (timestampInSeconds < timestampInfo.lastKeyFrameTimestamp) {
+			throw new Error(`Timestamp cannot be smaller than last key frame's timestamp (got ${timestampInSeconds}s, last key frame at ${timestampInfo.lastKeyFrameTimestamp}s).`);
+		}
+
+		if (isKeyFrame) {
+			if (timestampInSeconds < timestampInfo.maxTimestamp) {
+				throw new Error(`Key frame timestamps cannot be smaller than any timestamp that came before (got ${timestampInSeconds}s, max timestamp was ${timestampInfo.maxTimestamp}s).`);
+			}
+
+			timestampInfo.lastKeyFrameTimestamp = timestampInSeconds;
+		}
+		
+		timestampInfo.maxTimestamp = Math.max(timestampInfo.maxTimestamp, timestampInSeconds);
+
+		return timestampInSeconds;
+	}
 }