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367 lines
11 KiB
367 lines
11 KiB
/**
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* mux.js
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*
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* Copyright (c) Brightcove
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* Licensed Apache-2.0 https://github.com/videojs/mux.js/blob/master/LICENSE
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*
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* Utilities to detect basic properties and metadata about MP4s.
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*/
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'use strict';
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var toUnsigned = require('../utils/bin').toUnsigned;
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var toHexString = require('../utils/bin').toHexString;
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var findBox = require('../mp4/find-box.js');
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var parseType = require('../mp4/parse-type.js');
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var parseTfhd = require('../tools/parse-tfhd.js');
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var parseTrun = require('../tools/parse-trun.js');
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var parseTfdt = require('../tools/parse-tfdt.js');
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var timescale, startTime, compositionStartTime, getVideoTrackIds, getTracks,
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getTimescaleFromMediaHeader;
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/**
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* Parses an MP4 initialization segment and extracts the timescale
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* values for any declared tracks. Timescale values indicate the
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* number of clock ticks per second to assume for time-based values
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* elsewhere in the MP4.
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*
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* To determine the start time of an MP4, you need two pieces of
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* information: the timescale unit and the earliest base media decode
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* time. Multiple timescales can be specified within an MP4 but the
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* base media decode time is always expressed in the timescale from
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* the media header box for the track:
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* ```
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* moov > trak > mdia > mdhd.timescale
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* ```
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* @param init {Uint8Array} the bytes of the init segment
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* @return {object} a hash of track ids to timescale values or null if
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* the init segment is malformed.
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*/
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timescale = function(init) {
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var
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result = {},
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traks = findBox(init, ['moov', 'trak']);
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// mdhd timescale
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return traks.reduce(function(result, trak) {
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var tkhd, version, index, id, mdhd;
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tkhd = findBox(trak, ['tkhd'])[0];
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if (!tkhd) {
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return null;
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}
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version = tkhd[0];
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index = version === 0 ? 12 : 20;
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id = toUnsigned(tkhd[index] << 24 |
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tkhd[index + 1] << 16 |
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tkhd[index + 2] << 8 |
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tkhd[index + 3]);
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mdhd = findBox(trak, ['mdia', 'mdhd'])[0];
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if (!mdhd) {
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return null;
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}
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version = mdhd[0];
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index = version === 0 ? 12 : 20;
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result[id] = toUnsigned(mdhd[index] << 24 |
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mdhd[index + 1] << 16 |
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mdhd[index + 2] << 8 |
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mdhd[index + 3]);
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return result;
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}, result);
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};
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/**
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* Determine the base media decode start time, in seconds, for an MP4
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* fragment. If multiple fragments are specified, the earliest time is
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* returned.
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*
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* The base media decode time can be parsed from track fragment
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* metadata:
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* ```
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* moof > traf > tfdt.baseMediaDecodeTime
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* ```
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* It requires the timescale value from the mdhd to interpret.
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*
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* @param timescale {object} a hash of track ids to timescale values.
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* @return {number} the earliest base media decode start time for the
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* fragment, in seconds
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*/
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startTime = function(timescale, fragment) {
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var trafs, baseTimes, result;
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// we need info from two childrend of each track fragment box
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trafs = findBox(fragment, ['moof', 'traf']);
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// determine the start times for each track
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baseTimes = [].concat.apply([], trafs.map(function(traf) {
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return findBox(traf, ['tfhd']).map(function(tfhd) {
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var id, scale, baseTime;
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// get the track id from the tfhd
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id = toUnsigned(tfhd[4] << 24 |
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tfhd[5] << 16 |
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tfhd[6] << 8 |
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tfhd[7]);
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// assume a 90kHz clock if no timescale was specified
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scale = timescale[id] || 90e3;
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// get the base media decode time from the tfdt
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baseTime = findBox(traf, ['tfdt']).map(function(tfdt) {
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var version, result;
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version = tfdt[0];
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result = toUnsigned(tfdt[4] << 24 |
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tfdt[5] << 16 |
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tfdt[6] << 8 |
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tfdt[7]);
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if (version === 1) {
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result *= Math.pow(2, 32);
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result += toUnsigned(tfdt[8] << 24 |
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tfdt[9] << 16 |
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tfdt[10] << 8 |
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tfdt[11]);
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}
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return result;
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})[0];
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baseTime = typeof baseTime === 'number' && !isNaN(baseTime) ? baseTime : Infinity;
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// convert base time to seconds
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return baseTime / scale;
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});
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}));
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// return the minimum
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result = Math.min.apply(null, baseTimes);
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return isFinite(result) ? result : 0;
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};
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/**
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* Determine the composition start, in seconds, for an MP4
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* fragment.
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*
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* The composition start time of a fragment can be calculated using the base
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* media decode time, composition time offset, and timescale, as follows:
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*
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* compositionStartTime = (baseMediaDecodeTime + compositionTimeOffset) / timescale
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*
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* All of the aforementioned information is contained within a media fragment's
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* `traf` box, except for timescale info, which comes from the initialization
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* segment, so a track id (also contained within a `traf`) is also necessary to
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* associate it with a timescale
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*
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*
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* @param timescales {object} - a hash of track ids to timescale values.
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* @param fragment {Unit8Array} - the bytes of a media segment
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* @return {number} the composition start time for the fragment, in seconds
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**/
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compositionStartTime = function(timescales, fragment) {
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var trafBoxes = findBox(fragment, ['moof', 'traf']);
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var baseMediaDecodeTime = 0;
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var compositionTimeOffset = 0;
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var trackId;
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if (trafBoxes && trafBoxes.length) {
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// The spec states that track run samples contained within a `traf` box are contiguous, but
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// it does not explicitly state whether the `traf` boxes themselves are contiguous.
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// We will assume that they are, so we only need the first to calculate start time.
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var tfhd = findBox(trafBoxes[0], ['tfhd'])[0];
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var trun = findBox(trafBoxes[0], ['trun'])[0];
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var tfdt = findBox(trafBoxes[0], ['tfdt'])[0];
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if (tfhd) {
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var parsedTfhd = parseTfhd(tfhd);
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trackId = parsedTfhd.trackId;
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}
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if (tfdt) {
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var parsedTfdt = parseTfdt(tfdt);
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baseMediaDecodeTime = parsedTfdt.baseMediaDecodeTime;
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}
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if (trun) {
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var parsedTrun = parseTrun(trun);
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if (parsedTrun.samples && parsedTrun.samples.length) {
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compositionTimeOffset = parsedTrun.samples[0].compositionTimeOffset || 0;
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}
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}
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}
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// Get timescale for this specific track. Assume a 90kHz clock if no timescale was
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// specified.
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var timescale = timescales[trackId] || 90e3;
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// return the composition start time, in seconds
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return (baseMediaDecodeTime + compositionTimeOffset) / timescale;
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};
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/**
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* Find the trackIds of the video tracks in this source.
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* Found by parsing the Handler Reference and Track Header Boxes:
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* moov > trak > mdia > hdlr
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* moov > trak > tkhd
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*
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* @param {Uint8Array} init - The bytes of the init segment for this source
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* @return {Number[]} A list of trackIds
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*
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* @see ISO-BMFF-12/2015, Section 8.4.3
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**/
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getVideoTrackIds = function(init) {
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var traks = findBox(init, ['moov', 'trak']);
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var videoTrackIds = [];
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traks.forEach(function(trak) {
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var hdlrs = findBox(trak, ['mdia', 'hdlr']);
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var tkhds = findBox(trak, ['tkhd']);
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hdlrs.forEach(function(hdlr, index) {
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var handlerType = parseType(hdlr.subarray(8, 12));
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var tkhd = tkhds[index];
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var view;
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var version;
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var trackId;
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if (handlerType === 'vide') {
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view = new DataView(tkhd.buffer, tkhd.byteOffset, tkhd.byteLength);
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version = view.getUint8(0);
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trackId = (version === 0) ? view.getUint32(12) : view.getUint32(20);
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videoTrackIds.push(trackId);
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}
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});
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});
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return videoTrackIds;
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};
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getTimescaleFromMediaHeader = function(mdhd) {
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// mdhd is a FullBox, meaning it will have its own version as the first byte
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var version = mdhd[0];
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var index = version === 0 ? 12 : 20;
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return toUnsigned(
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mdhd[index] << 24 |
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mdhd[index + 1] << 16 |
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mdhd[index + 2] << 8 |
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mdhd[index + 3]
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);
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};
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/**
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* Get all the video, audio, and hint tracks from a non fragmented
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* mp4 segment
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*/
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getTracks = function(init) {
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var traks = findBox(init, ['moov', 'trak']);
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var tracks = [];
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traks.forEach(function(trak) {
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var track = {};
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var tkhd = findBox(trak, ['tkhd'])[0];
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var view, tkhdVersion;
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// id
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if (tkhd) {
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view = new DataView(tkhd.buffer, tkhd.byteOffset, tkhd.byteLength);
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tkhdVersion = view.getUint8(0);
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track.id = (tkhdVersion === 0) ? view.getUint32(12) : view.getUint32(20);
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}
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var hdlr = findBox(trak, ['mdia', 'hdlr'])[0];
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// type
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if (hdlr) {
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var type = parseType(hdlr.subarray(8, 12));
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if (type === 'vide') {
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track.type = 'video';
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} else if (type === 'soun') {
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track.type = 'audio';
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} else {
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track.type = type;
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}
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}
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// codec
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var stsd = findBox(trak, ['mdia', 'minf', 'stbl', 'stsd'])[0];
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if (stsd) {
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var sampleDescriptions = stsd.subarray(8);
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// gives the codec type string
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track.codec = parseType(sampleDescriptions.subarray(4, 8));
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var codecBox = findBox(sampleDescriptions, [track.codec])[0];
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var codecConfig, codecConfigType;
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if (codecBox) {
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// https://tools.ietf.org/html/rfc6381#section-3.3
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if ((/^[a-z]vc[1-9]$/i).test(track.codec)) {
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// we don't need anything but the "config" parameter of the
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// avc1 codecBox
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codecConfig = codecBox.subarray(78);
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codecConfigType = parseType(codecConfig.subarray(4, 8));
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if (codecConfigType === 'avcC' && codecConfig.length > 11) {
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track.codec += '.';
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// left padded with zeroes for single digit hex
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// profile idc
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track.codec += toHexString(codecConfig[9]);
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// the byte containing the constraint_set flags
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track.codec += toHexString(codecConfig[10]);
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// level idc
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track.codec += toHexString(codecConfig[11]);
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} else {
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// TODO: show a warning that we couldn't parse the codec
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// and are using the default
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track.codec = 'avc1.4d400d';
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}
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} else if ((/^mp4[a,v]$/i).test(track.codec)) {
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// we do not need anything but the streamDescriptor of the mp4a codecBox
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codecConfig = codecBox.subarray(28);
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codecConfigType = parseType(codecConfig.subarray(4, 8));
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if (codecConfigType === 'esds' && codecConfig.length > 20 && codecConfig[19] !== 0) {
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track.codec += '.' + toHexString(codecConfig[19]);
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// this value is only a single digit
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track.codec += '.' + toHexString((codecConfig[20] >>> 2) & 0x3f).replace(/^0/, '');
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} else {
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// TODO: show a warning that we couldn't parse the codec
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// and are using the default
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track.codec = 'mp4a.40.2';
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}
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} else {
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// flac, opus, etc
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track.codec = track.codec.toLowerCase();
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}
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}
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}
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var mdhd = findBox(trak, ['mdia', 'mdhd'])[0];
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if (mdhd) {
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track.timescale = getTimescaleFromMediaHeader(mdhd);
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}
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tracks.push(track);
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});
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return tracks;
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};
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module.exports = {
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// export mp4 inspector's findBox and parseType for backwards compatibility
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findBox: findBox,
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parseType: parseType,
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timescale: timescale,
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startTime: startTime,
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compositionStartTime: compositionStartTime,
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videoTrackIds: getVideoTrackIds,
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tracks: getTracks,
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getTimescaleFromMediaHeader: getTimescaleFromMediaHeader
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};
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