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+import MP4Box from 'https://cdn.jsdelivr.net/npm/mp4box@0.5.3/+esm';
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+
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+// From: https://w3c.github.io/webcodecs/samples/video-decode-display/
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+
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+// Wraps an MP4Box File as a WritableStream underlying sink.
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+class MP4FileSink {
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+ #setStatus = null;
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+ #file = null;
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+ #offset = 0;
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+
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+ constructor(file, setStatus) {
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+ this.#file = file;
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+ this.#setStatus = setStatus;
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+ }
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+
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+ write(chunk) {
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+ // MP4Box.js requires buffers to be ArrayBuffers, but we have a Uint8Array.
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+ const buffer = new ArrayBuffer(chunk.byteLength);
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+ new Uint8Array(buffer).set(chunk);
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+
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+ // Inform MP4Box where in the file this chunk is from.
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+ buffer.fileStart = this.#offset;
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+ this.#offset += buffer.byteLength;
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+
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+ // Append chunk.
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+ this.#setStatus("fetch", (this.#offset / (1024 ** 2)).toFixed(1) + " MiB");
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+ this.#file.appendBuffer(buffer);
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+ }
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+
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+ close() {
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+ this.#setStatus("fetch", "Done");
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+ this.#file.flush();
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+ }
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+}
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+
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+// Demuxes the first video track of an MP4 file using MP4Box, calling
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+// `onConfig()` and `onChunk()` with appropriate WebCodecs objects.
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+export class MP4Demuxer {
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+ #onConfig = null;
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+ #onChunk = null;
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+ #setStatus = null;
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+ #file = null;
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+
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+ constructor(uri, {onConfig, onChunk, setStatus}) {
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+ this.#onConfig = onConfig;
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+ this.#onChunk = onChunk;
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+ this.#setStatus = setStatus;
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+
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+ // Configure an MP4Box File for demuxing.
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+ this.#file = MP4Box.createFile();
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+ this.#file.onError = error => setStatus("demux", error);
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+ this.#file.onReady = this.#onReady.bind(this);
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+ this.#file.onSamples = this.#onSamples.bind(this);
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+
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+ // Fetch the file and pipe the data through.
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+ const fileSink = new MP4FileSink(this.#file, setStatus);
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+ fetch(uri).then(response => {
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+ // highWaterMark should be large enough for smooth streaming, but lower is
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+ // better for memory usage.
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+ response.body.pipeTo(new WritableStream(fileSink, {highWaterMark: 2}));
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+ });
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+ }
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+
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+ // Get the appropriate `description` for a specific track. Assumes that the
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+ // track is H.264, H.265, VP8, VP9, or AV1.
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+ #description(track) {
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+ const trak = this.#file.getTrackById(track.id);
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+ for (const entry of trak.mdia.minf.stbl.stsd.entries) {
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+ const box = entry.avcC || entry.hvcC || entry.vpcC || entry.av1C;
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+ if (box) {
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+ const stream = new MP4Box.DataStream(undefined, 0, MP4Box.DataStream.BIG_ENDIAN);
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+ box.write(stream);
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+ return new Uint8Array(stream.buffer, 8); // Remove the box header.
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+ }
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+ }
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+ throw new Error("avcC, hvcC, vpcC, or av1C box not found");
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+ }
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+
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+ #onReady(info) {
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+ this.#setStatus("demux", "Ready");
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+ const track = info.videoTracks[0];
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+
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+ // Generate and emit an appropriate VideoDecoderConfig.
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+ this.#onConfig({
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+ // Browser doesn't support parsing full vp8 codec (eg: `vp08.00.41.08`),
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+ // they only support `vp8`.
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+ codec: track.codec.startsWith('vp08') ? 'vp8' : track.codec,
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+ codedHeight: track.video.height,
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+ codedWidth: track.video.width,
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+ description: this.#description(track),
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+ });
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+
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+ // Start demuxing.
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+ this.#file.setExtractionOptions(track.id);
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+ this.#file.start();
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+ }
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+
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+ #onSamples(track_id, ref, samples) {
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+ // Generate and emit an EncodedVideoChunk for each demuxed sample.
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+ for (const sample of samples) {
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+ this.#onChunk(new EncodedVideoChunk({
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+ type: sample.is_sync ? "key" : "delta",
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+ timestamp: 1e6 * sample.cts / sample.timescale,
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+ duration: 1e6 * sample.duration / sample.timescale,
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+ data: sample.data
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+ }));
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+ }
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+ }
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+}
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