src/demux/exp-golomb.js
/**
* Parser for exponential Golomb codes, a variable-bitwidth number encoding scheme used by h264.
*/
import { logger } from '../utils/logger';
class ExpGolomb {
constructor(data) {
this.data = data;
// the number of bytes left to examine in this.data
this.bytesAvailable = data.byteLength;
// the current word being examined
this.word = 0; // :uint
// the number of bits left to examine in the current word
this.bitsAvailable = 0; // :uint
}
// ():void
loadWord() {
const data = this.data;
const bytesAvailable = this.bytesAvailable;
const position = data.byteLength - bytesAvailable;
const workingBytes = new Uint8Array(4);
const availableBytes = Math.min(4, bytesAvailable);
if (availableBytes === 0) {
throw new Error('no bytes available');
}
workingBytes.set(data.subarray(position, position + availableBytes));
this.word = new DataView(workingBytes.buffer).getUint32(0);
// track the amount of this.data that has been processed
this.bitsAvailable = availableBytes * 8;
this.bytesAvailable -= availableBytes;
}
// (count:int):void
skipBits(count) {
let skipBytes; // :int
if (this.bitsAvailable > count) {
this.word <<= count;
this.bitsAvailable -= count;
} else {
count -= this.bitsAvailable;
skipBytes = count >> 3;
count -= skipBytes >> 3;
this.bytesAvailable -= skipBytes;
this.loadWord();
this.word <<= count;
this.bitsAvailable -= count;
}
}
// (size:int):uint
readBits(size) {
let bits = Math.min(this.bitsAvailable, size); // :uint
const valu = this.word >>> (32 - bits); // :uint
if (size > 32) {
logger.error('Cannot read more than 32 bits at a time');
}
this.bitsAvailable -= bits;
if (this.bitsAvailable > 0) {
this.word <<= bits;
} else if (this.bytesAvailable > 0) {
this.loadWord();
}
bits = size - bits;
if (bits > 0 && this.bitsAvailable) {
return (valu << bits) | this.readBits(bits);
} else {
return valu;
}
}
// ():uint
skipLZ() {
let leadingZeroCount; // :uint
for (
leadingZeroCount = 0;
leadingZeroCount < this.bitsAvailable;
++leadingZeroCount
) {
if ((this.word & (0x80000000 >>> leadingZeroCount)) !== 0) {
// the first bit of working word is 1
this.word <<= leadingZeroCount;
this.bitsAvailable -= leadingZeroCount;
return leadingZeroCount;
}
}
// we exhausted word and still have not found a 1
this.loadWord();
return leadingZeroCount + this.skipLZ();
}
// ():void
skipUEG() {
this.skipBits(1 + this.skipLZ());
}
// ():void
skipEG() {
this.skipBits(1 + this.skipLZ());
}
// ():uint
readUEG() {
const clz = this.skipLZ(); // :uint
return this.readBits(clz + 1) - 1;
}
// ():int
readEG() {
const valu = this.readUEG(); // :int
if (0x01 & valu) {
// the number is odd if the low order bit is set
return (1 + valu) >>> 1; // add 1 to make it even, and divide by 2
} else {
return -1 * (valu >>> 1); // divide by two then make it negative
}
}
// Some convenience functions
// :Boolean
readBoolean() {
return this.readBits(1) === 1;
}
// ():int
readUByte() {
return this.readBits(8);
}
// ():int
readUShort() {
return this.readBits(16);
}
// ():int
readUInt() {
return this.readBits(32);
}
/**
* Advance the ExpGolomb decoder past a scaling list. The scaling
* list is optionally transmitted as part of a sequence parameter
* set and is not relevant to transmuxing.
* @param count {number} the number of entries in this scaling list
* @see Recommendation ITU-T H.264, Section 7.3.2.1.1.1
*/
skipScalingList(count) {
let lastScale = 8;
let nextScale = 8;
let deltaScale;
for (let j = 0; j < count; j++) {
if (nextScale !== 0) {
deltaScale = this.readEG();
nextScale = (lastScale + deltaScale + 256) % 256;
}
lastScale = nextScale === 0 ? lastScale : nextScale;
}
}
/**
* Read a sequence parameter set and return some interesting video
* properties. A sequence parameter set is the H264 metadata that
* describes the properties of upcoming video frames.
* @param data {Uint8Array} the bytes of a sequence parameter set
* @return {object} an object with configuration parsed from the
* sequence parameter set, including the dimensions of the
* associated video frames.
*/
readSPS() {
let frameCropLeftOffset = 0;
let frameCropRightOffset = 0;
let frameCropTopOffset = 0;
let frameCropBottomOffset = 0;
let numRefFramesInPicOrderCntCycle;
let scalingListCount;
let i;
const readUByte = this.readUByte.bind(this);
const readBits = this.readBits.bind(this);
const readUEG = this.readUEG.bind(this);
const readBoolean = this.readBoolean.bind(this);
const skipBits = this.skipBits.bind(this);
const skipEG = this.skipEG.bind(this);
const skipUEG = this.skipUEG.bind(this);
const skipScalingList = this.skipScalingList.bind(this);
readUByte();
const profileIdc = readUByte(); // profile_idc
readBits(5); // profileCompat constraint_set[0-4]_flag, u(5)
skipBits(3); // reserved_zero_3bits u(3),
readUByte(); // level_idc u(8)
skipUEG(); // seq_parameter_set_id
// some profiles have more optional data we don't need
if (
profileIdc === 100 ||
profileIdc === 110 ||
profileIdc === 122 ||
profileIdc === 244 ||
profileIdc === 44 ||
profileIdc === 83 ||
profileIdc === 86 ||
profileIdc === 118 ||
profileIdc === 128
) {
const chromaFormatIdc = readUEG();
if (chromaFormatIdc === 3) {
skipBits(1);
} // separate_colour_plane_flag
skipUEG(); // bit_depth_luma_minus8
skipUEG(); // bit_depth_chroma_minus8
skipBits(1); // qpprime_y_zero_transform_bypass_flag
if (readBoolean()) {
// seq_scaling_matrix_present_flag
scalingListCount = chromaFormatIdc !== 3 ? 8 : 12;
for (i = 0; i < scalingListCount; i++) {
if (readBoolean()) {
// seq_scaling_list_present_flag[ i ]
if (i < 6) {
skipScalingList(16);
} else {
skipScalingList(64);
}
}
}
}
}
skipUEG(); // log2_max_frame_num_minus4
const picOrderCntType = readUEG();
if (picOrderCntType === 0) {
readUEG(); // log2_max_pic_order_cnt_lsb_minus4
} else if (picOrderCntType === 1) {
skipBits(1); // delta_pic_order_always_zero_flag
skipEG(); // offset_for_non_ref_pic
skipEG(); // offset_for_top_to_bottom_field
numRefFramesInPicOrderCntCycle = readUEG();
for (i = 0; i < numRefFramesInPicOrderCntCycle; i++) {
skipEG();
} // offset_for_ref_frame[ i ]
}
skipUEG(); // max_num_ref_frames
skipBits(1); // gaps_in_frame_num_value_allowed_flag
const picWidthInMbsMinus1 = readUEG();
const picHeightInMapUnitsMinus1 = readUEG();
const frameMbsOnlyFlag = readBits(1);
if (frameMbsOnlyFlag === 0) {
skipBits(1);
} // mb_adaptive_frame_field_flag
skipBits(1); // direct_8x8_inference_flag
if (readBoolean()) {
// frame_cropping_flag
frameCropLeftOffset = readUEG();
frameCropRightOffset = readUEG();
frameCropTopOffset = readUEG();
frameCropBottomOffset = readUEG();
}
let pixelRatio = [1, 1];
if (readBoolean()) {
// vui_parameters_present_flag
if (readBoolean()) {
// aspect_ratio_info_present_flag
const aspectRatioIdc = readUByte();
switch (aspectRatioIdc) {
case 1:
pixelRatio = [1, 1];
break;
case 2:
pixelRatio = [12, 11];
break;
case 3:
pixelRatio = [10, 11];
break;
case 4:
pixelRatio = [16, 11];
break;
case 5:
pixelRatio = [40, 33];
break;
case 6:
pixelRatio = [24, 11];
break;
case 7:
pixelRatio = [20, 11];
break;
case 8:
pixelRatio = [32, 11];
break;
case 9:
pixelRatio = [80, 33];
break;
case 10:
pixelRatio = [18, 11];
break;
case 11:
pixelRatio = [15, 11];
break;
case 12:
pixelRatio = [64, 33];
break;
case 13:
pixelRatio = [160, 99];
break;
case 14:
pixelRatio = [4, 3];
break;
case 15:
pixelRatio = [3, 2];
break;
case 16:
pixelRatio = [2, 1];
break;
case 255: {
pixelRatio = [
(readUByte() << 8) | readUByte(),
(readUByte() << 8) | readUByte(),
];
break;
}
}
}
}
return {
width: Math.ceil(
(picWidthInMbsMinus1 + 1) * 16 -
frameCropLeftOffset * 2 -
frameCropRightOffset * 2
),
height:
(2 - frameMbsOnlyFlag) * (picHeightInMapUnitsMinus1 + 1) * 16 -
(frameMbsOnlyFlag ? 2 : 4) *
(frameCropTopOffset + frameCropBottomOffset),
pixelRatio: pixelRatio,
};
}
readSliceType() {
// skip NALu type
this.readUByte();
// discard first_mb_in_slice
this.readUEG();
// return slice_type
return this.readUEG();
}
}
export default ExpGolomb;
