Function parseAvcSps

src/codec_data.ts:124–221

(sps: Uint8Array)

/** Parses an AVC SPS (Sequence Parameter Set) to extract basic information. */
export const parseAvcSps = (sps: Uint8Array): AvcSpsInfo | null => {
	try {
		const bitstream = new Bitstream(removeEmulationPreventionBytes(sps));

		bitstream.skipBits(1); // forbidden_zero_bit
		bitstream.skipBits(2); // nal_ref_idc
		const nalUnitType = bitstream.readBits(5);

		if (nalUnitType !== 7) { // SPS NAL unit type is 7
			return null;
		}

		const profileIdc = bitstream.readAlignedByte();
		const constraintFlags = bitstream.readAlignedByte();
		const levelIdc = bitstream.readAlignedByte();

		readExpGolomb(bitstream); // seq_parameter_set_id

		let chromaFormatIdc: number | null = null;
		let bitDepthLumaMinus8: number | null = null;
		let bitDepthChromaMinus8: number | null = null;

		// Handle high profile chroma_format_idc
		if (
			profileIdc === 100
			|| profileIdc === 110
			|| profileIdc === 122
			|| profileIdc === 244
			|| profileIdc === 44
			|| profileIdc === 83
			|| profileIdc === 86
			|| profileIdc === 118
			|| profileIdc === 128
		) {
			chromaFormatIdc = readExpGolomb(bitstream);
			if (chromaFormatIdc === 3) {
				bitstream.skipBits(1); // separate_colour_plane_flag
			}
			bitDepthLumaMinus8 = readExpGolomb(bitstream);
			bitDepthChromaMinus8 = readExpGolomb(bitstream);
			bitstream.skipBits(1); // qpprime_y_zero_transform_bypass_flag
			const seqScalingMatrixPresentFlag = bitstream.readBits(1);
			if (seqScalingMatrixPresentFlag) {
				for (let i = 0; i < (chromaFormatIdc !== 3 ? 8 : 12); i++) {
					const seqScalingListPresentFlag = bitstream.readBits(1);
					if (seqScalingListPresentFlag) {
						const sizeOfScalingList = i < 6 ? 16 : 64;
						let lastScale = 8;
						let nextScale = 8;
						for (let j = 0; j < sizeOfScalingList; j++) {
							if (nextScale !== 0) {
								const deltaScale = readSignedExpGolomb(bitstream);
								nextScale = (lastScale + deltaScale + 256) % 256;
							}
							lastScale = nextScale === 0 ? lastScale : nextScale;
						}
					}
				}