Minor touchups

README.md

@@ -243,15 +243,16 @@ set explicitly:
 - Use `'offset'` to offset the timestamp of each track by that track's first chunk's timestamp. This way, it
 starts at 0.
 - Use `'cross-track-offset'` to offset the timestamp of each track by the _minimum of all tracks' first chunk timestamp_.
-This works like `'offset'`, but should be used when the all tracks use the same timebase.
+This works like `'offset'`, but should be used when the all tracks use the same clock.
 
 ### Muxing media chunks
 Then, with VideoEncoder and AudioEncoder set up, send encoded chunks to the muxer using the following methods:
 ```ts
 addVideoChunk(
     chunk: EncodedVideoChunk,
     meta?: EncodedVideoChunkMetadata,
-    timestamp?: number
+    timestamp?: number,
+	compositionTimeOffset?: number
 ): void;
 
 addAudioChunk(
@@ -274,6 +275,11 @@ let videoEncoder = new VideoEncoder({
 videoEncoder.configure(/* ... */);
 ```
 
+The optional field `compositionTimeOffset` can be used when the decode time of the chunk doesn't equal its presentation
+time; this is the case when [B-frames](https://en.wikipedia.org/wiki/Video_compression_picture_types) are present.
+B-frames don't occur when using the WebCodecs API for encoding. The decode time is calculated by subtracting
+`compositionTimeOffset` from `timestamp`, meaning `timestamp` dictates the presentation time.
+
 Should you have obtained your encoded media data from a source other than the WebCodecs API, you can use these following
 methods to directly send your raw data to the muxer:
 ```ts
@@ -283,7 +289,7 @@ addVideoChunkRaw(
     timestamp: number, // in microseconds
     duration: number, // in microseconds
     meta?: EncodedVideoChunkMetadata,
-    compositionTimeOffset: number // in microseconds
+    compositionTimeOffset?: number // in microseconds
 ): void;
 
 addAudioChunkRaw(

src/box.ts

@@ -277,16 +277,17 @@ export const url = () => fullBox('url ', 0, 1); // Self-reference flag enabled
  * also indicates how to interpret the sample (for example, whether to decompress the video data and, if so, how).
  */
 export const stbl = (track: Track) => {
-	const needsCTTS = track.compositionTimeOffsetTable.length > 1 ||
+	const needsCtts = track.compositionTimeOffsetTable.length > 1 ||
 		track.compositionTimeOffsetTable.some((x) => x.sampleCompositionTimeOffset !== 0);
+
 	return box('stbl', null, [
 		stsd(track),
 		stts(track),
 		stss(track),
 		stsc(track),
 		stsz(track),
 		stco(track),
-		needsCTTS ? ctts(track) : null
+		needsCtts ? ctts(track) : null
 	]);
 };
 

src/misc.ts

@@ -65,11 +65,13 @@ export const last = <T>(arr: T[]) => {
 
 export const lastPresentedSample = (samples: Sample[]): Sample | undefined => {
 	let result: Sample | undefined = undefined;
-	for (const s of samples) {
-		if (!result || s.presentationTimestamp > result.presentationTimestamp) {
-			result = s;
+
+	for (let sample of samples) {
+		if (!result || sample.presentationTimestamp > result.presentationTimestamp) {
+			result = sample;
 		}
 	}
+
 	return result;
 };