From 84482cb5eabfc368db2b70392552efe3ceefed0a Mon Sep 17 00:00:00 2001
From: Vanilagy <1696106+Vanilagy@users.noreply.github.com>
Date: Wed, 12 Jun 2024 19:24:15 +0200
Subject: [PATCH] Minor touchups

---
 README.md            | 12 +++++++++---
 build/mp4-muxer.d.ts |  2 +-
 src/box.ts           |  5 +++--
 src/misc.ts          |  8 +++++---
 4 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/README.md b/README.md
index 7b2edca..5a6b975 100644
--- a/README.md
+++ b/README.md
@@ -243,7 +243,7 @@ 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:
@@ -251,7 +251,8 @@ Then, with VideoEncoder and AudioEncoder set up, send encoded chunks to the muxe
 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(
diff --git a/build/mp4-muxer.d.ts b/build/mp4-muxer.d.ts
index b2bf2e1..303f545 100644
--- a/build/mp4-muxer.d.ts
+++ b/build/mp4-muxer.d.ts
@@ -173,7 +173,7 @@ declare class Muxer<T extends Target> {
 	 * @param data The raw data of the video chunk.
 	 * @param type Whether the video chunk is a keyframe or delta frame.
 	 * @param timestamp The timestamp of the video chunk.
-	 * @param duration The duration of the video chunk (typically 0).
+	 * @param duration The duration of the video chunk.
 	 * @param meta Optionally, any encoder metadata.
 	 * @param compositionTimeOffset The composition time offset (i.e. presentation timestamp minus decode timestamp) of
 	 * the video chunk.
diff --git a/src/box.ts b/src/box.ts
index 1804329..e6dba05 100644
--- a/src/box.ts
+++ b/src/box.ts
@@ -277,8 +277,9 @@ 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),
@@ -286,7 +287,7 @@ export const stbl = (track: Track) => {
 		stsc(track),
 		stsz(track),
 		stco(track),
-		needsCTTS ? ctts(track) : null
+		needsCtts ? ctts(track) : null
 	]);
 };
 
diff --git a/src/misc.ts b/src/misc.ts
index f4d4875..f08fa96 100644
--- a/src/misc.ts
+++ b/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;
 };