probe.js
5.16 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
/**
* mux.js
*
* Copyright (c) 2015 Brightcove
* All rights reserved.
*
* Utilities to detect basic properties and metadata about MP4s.
*/
'use strict';
var findBox, parseType, timescale, startTime;
// Find the data for a box specified by its path
findBox = function(data, path) {
var results = [],
i, size, type, end, subresults;
if (!path.length) {
// short-circuit the search for empty paths
return null;
}
for (i = 0; i < data.byteLength;) {
size = data[i] << 24;
size |= data[i + 1] << 16;
size |= data[i + 2] << 8;
size |= data[i + 3];
type = parseType(data.subarray(i + 4, i + 8));
end = size > 1 ? i + size : data.byteLength;
if (type === path[0]) {
if (path.length === 1) {
// this is the end of the path and we've found the box we were
// looking for
results.push(data.subarray(i + 8, end));
} else {
// recursively search for the next box along the path
subresults = findBox(data.subarray(i + 8, end), path.slice(1));
if (subresults.length) {
results = results.concat(subresults);
}
}
}
i = end;
}
// we've finished searching all of data
return results;
};
/**
* Returns the string representation of an ASCII encoded four byte buffer.
* @param buffer {Uint8Array} a four-byte buffer to translate
* @return {string} the corresponding string
*/
parseType = function(buffer) {
var result = '';
result += String.fromCharCode(buffer[0]);
result += String.fromCharCode(buffer[1]);
result += String.fromCharCode(buffer[2]);
result += String.fromCharCode(buffer[3]);
return result;
};
/**
* Parses an MP4 initialization segment and extracts the timescale
* values for any declared tracks. Timescale values indicate the
* number of clock ticks per second to assume for time-based values
* elsewhere in the MP4.
*
* To determine the start time of an MP4, you need two pieces of
* information: the timescale unit and the earliest base media decode
* time. Multiple timescales can be specified within an MP4 but the
* base media decode time is always expressed in the timescale from
* the media header box for the track:
* ```
* moov > trak > mdia > mdhd.timescale
* ```
* @param init {Uint8Array} the bytes of the init segment
* @return {object} a hash of track ids to timescale values or null if
* the init segment is malformed.
*/
timescale = function(init) {
var
result = {},
traks = findBox(init, ['moov', 'trak']);
// mdhd timescale
return traks.reduce(function(result, trak) {
var tkhd, version, index, id, mdhd;
tkhd = findBox(trak, ['tkhd'])[0];
if (!tkhd) {
return null;
}
version = tkhd[0];
index = version === 0 ? 12 : 20;
id = tkhd[index] << 24 |
tkhd[index + 1] << 16 |
tkhd[index + 2] << 8 |
tkhd[index + 3];
mdhd = findBox(trak, ['mdia', 'mdhd'])[0];
if (!mdhd) {
return null;
}
version = mdhd[0];
index = version === 0 ? 12 : 20;
result[id] = mdhd[index] << 24 |
mdhd[index + 1] << 16 |
mdhd[index + 2] << 8 |
mdhd[index + 3];
return result;
}, result);
};
/**
* Determine the base media decode start time, in seconds, for an MP4
* fragment. If multiple fragments are specified, the earliest time is
* returned.
*
* The base media decode time can be parsed from track fragment
* metadata:
* ```
* moof > traf > tfdt.baseMediaDecodeTime
* ```
* It requires the timescale value from the mdhd to interpret.
*
* @param timescale {object} a hash of track ids to timescale values.
* @return {number} the earliest base media decode start time for the
* fragment, in seconds
*/
startTime = function(timescale, fragment) {
var trafs, baseTimes, result;
// we need info from two childrend of each track fragment box
trafs = findBox(fragment, ['moof', 'traf']);
// determine the start times for each track
baseTimes = [].concat.apply([], trafs.map(function(traf) {
return findBox(traf, ['tfhd']).map(function(tfhd) {
var id, scale, baseTime;
// get the track id from the tfhd
id = tfhd[4] << 24 |
tfhd[5] << 16 |
tfhd[6] << 8 |
tfhd[7];
// assume a 90kHz clock if no timescale was specified
scale = timescale[id] || 90e3;
// get the base media decode time from the tfdt
baseTime = findBox(traf, ['tfdt']).map(function(tfdt) {
var version, result;
version = tfdt[0];
result = tfdt[4] << 24 |
tfdt[5] << 16 |
tfdt[6] << 8 |
tfdt[7];
if (version === 1) {
result *= Math.pow(2, 32);
result += tfdt[8] << 24 |
tfdt[9] << 16 |
tfdt[10] << 8 |
tfdt[11];
}
return result;
})[0];
baseTime = baseTime || Infinity;
// convert base time to seconds
return baseTime / scale;
});
}));
// return the minimum
result = Math.min.apply(null, baseTimes);
return isFinite(result) ? result : 0;
};
module.exports = {
parseType: parseType,
timescale: timescale,
startTime: startTime
};