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// Copyright 2015 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "media/cast/encoding/vpx_quantizer_parser.h"
#include "base/logging.h"
namespace media {
namespace cast {
namespace {
// VpxBitReader is a re-implementation of a subset of the VP8 entropy decoder.
// It is used to decompress the VP8 bitstream for the purposes of quickly
// parsing the VP8 frame headers. It is mostly the exact same implementation
// found in third_party/libvpx/.../vp8/decoder/dboolhuff.h except that only
// the portion of the implementation needed to parse the frame headers is
// present. As of this writing, the implementation in libvpx could not be
// re-used because of the way that the code is structured, and lack of the
// necessary parts being exported.
class VpxBitReader {
public:
VpxBitReader(const uint8_t* data, size_t size)
: encoded_data_(data), encoded_data_end_(data + size) {
VpxDecoderReadBytes();
}
~VpxBitReader() = default;
VpxBitReader(const VpxBitReader&) = delete;
VpxBitReader& operator=(const VpxBitReader&) = delete;
VpxBitReader(VpxBitReader&&) = delete;
VpxBitReader& operator=(VpxBitReader&&) = delete;
// Decode one bit. The output is 0 or 1.
unsigned int DecodeBit();
// Decode a value with |num_bits|. The decoding order is MSB first.
unsigned int DecodeValue(unsigned int num_bits);
private:
// Read new bytes frome the encoded data buffer until |bit_count_| > 0.
void VpxDecoderReadBytes();
const uint8_t* encoded_data_; // Current byte to decode.
const uint8_t* const encoded_data_end_; // The end of the byte to decode.
// The following two variables are maintained by the decoder.
// General decoding rule:
// If |value_| is in the range of 0 to half of |range_|, output 0.
// Otherwise output 1.
// |range_| and |value_| need to be shifted when necessary to avoid underflow.
unsigned int range_ = 255;
unsigned int value_ = 0;
// Number of valid bits left to decode. Initializing it to -8 to let the
// decoder load two bytes at the beginning. The lower byte is used as
// a buffer byte. During the decoding, decoder needs to call
// VpxDecoderReadBytes() to load new bytes when it becomes negative.
int bit_count_ = -8;
};
// The number of bits to be left-shifted to make the variable range_ over 128.
const uint8_t vpx_shift[128] = {
0, 7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3,
3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
// Mapping from the q_index(0-127) to the quantizer value(0-63).
const uint8_t vpx_quantizer_lookup[128] = {
0, 1, 2, 3, 4, 5, 6, 6, 7, 8, 9, 10, 10, 11, 12, 12, 13, 13, 14,
15, 16, 17, 18, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 27, 28, 28, 29, 29,
30, 30, 31, 31, 32, 32, 33, 33, 34, 34, 35, 35, 36, 36, 37, 37, 38, 38, 39,
39, 40, 40, 41, 41, 42, 42, 42, 43, 43, 43, 44, 44, 44, 45, 45, 45, 46, 46,
46, 47, 47, 47, 48, 48, 48, 49, 49, 49, 50, 50, 50, 51, 51, 51, 52, 52, 52,
53, 53, 53, 54, 54, 54, 55, 55, 55, 56, 56, 56, 57, 57, 57, 58, 58, 58, 59,
59, 59, 60, 60, 60, 61, 61, 61, 62, 62, 62, 63, 63, 63};
void VpxBitReader::VpxDecoderReadBytes() {
int shift = -bit_count_;
while ((shift >= 0) && (encoded_data_ < encoded_data_end_)) {
bit_count_ += 8;
value_ |= static_cast<unsigned int>(*encoded_data_) << shift;
++encoded_data_;
shift -= 8;
}
}
unsigned int VpxBitReader::DecodeBit() {
unsigned int decoded_bit = 0;
unsigned int split = 1 + (((range_ - 1) * 128) >> 8);
if (bit_count_ < 0) {
VpxDecoderReadBytes();
}
DCHECK_GE(bit_count_, 0);
unsigned int shifted_split = split << 8;
if (value_ >= shifted_split) {
range_ -= split;
value_ -= shifted_split;
decoded_bit = 1;
} else {
range_ = split;
}
if (range_ < 128) {
int shift = vpx_shift[range_];
range_ <<= shift;
value_ <<= shift;
bit_count_ -= shift;
}
return decoded_bit;
}
unsigned int VpxBitReader::DecodeValue(unsigned int num_bits) {
unsigned int decoded_value = 0;
for (int i = static_cast<int>(num_bits) - 1; i >= 0; i--) {
decoded_value |= (DecodeBit() << i);
}
return decoded_value;
}
// Parse the Segment Header part in the first partition.
void ParseSegmentHeader(VpxBitReader* bit_reader) {
const bool segmentation_enabled = (bit_reader->DecodeBit() != 0);
DVLOG(2) << "segmentation_enabled:" << segmentation_enabled;
if (segmentation_enabled) {
const bool update_mb_segmentation_map = (bit_reader->DecodeBit() != 0);
const bool update_mb_segmentation_data = (bit_reader->DecodeBit() != 0);
DVLOG(2) << "update_mb_segmentation_data:" << update_mb_segmentation_data;
if (update_mb_segmentation_data) {
bit_reader->DecodeBit();
for (int i = 0; i < 4; ++i) {
if (bit_reader->DecodeBit()) {
bit_reader->DecodeValue(7 + 1); // Parse 7 bits value + 1 sign bit.
}
}
for (int i = 0; i < 4; ++i) {
if (bit_reader->DecodeBit()) {
bit_reader->DecodeValue(6 + 1); // Parse 6 bits value + 1 sign bit.
}
}
}
if (update_mb_segmentation_map) {
for (int i = 0; i < 3; ++i) {
if (bit_reader->DecodeBit()) {
bit_reader->DecodeValue(8);
}
}
}
}
}
// Parse the Filter Header in the first partition.
void ParseFilterHeader(VpxBitReader* bit_reader) {
// Parse 1 bit filter_type + 6 bits loop_filter_level + 3 bits
// sharpness_level.
bit_reader->DecodeValue(1 + 6 + 3);
if (bit_reader->DecodeBit()) {
if (bit_reader->DecodeBit()) {
for (int i = 0; i < 4; ++i) {
if (bit_reader->DecodeBit()) {
bit_reader->DecodeValue(6 + 1); // Parse 6 bits value + 1 sign bit.
}
}
for (int i = 0; i < 4; ++i) {
if (bit_reader->DecodeBit()) {
bit_reader->DecodeValue(6 + 1); // Parse 6 bits value + 1 sign bit.
}
}
}
}
}
} // unnamed namespace
int ParseVpxHeaderQuantizer(const uint8_t* encoded_data, size_t size) {
DCHECK(encoded_data);
if (size <= 3) {
return -1;
}
const bool is_key = !(encoded_data[0] & 1);
const unsigned int header_3bytes =
encoded_data[0] | (encoded_data[1] << 8) | (encoded_data[2] << 16);
// Parse the size of the first partition.
unsigned int partition_size = (header_3bytes >> 5);
encoded_data += 3; // Skip 3 bytes.
size -= 3;
if (is_key) {
if (size <= 7) {
return -1;
}
encoded_data += 7; // Skip 7 bytes.
size -= 7;
}
if (size < partition_size) {
return -1;
}
VpxBitReader bit_reader(encoded_data, partition_size);
if (is_key) {
bit_reader.DecodeValue(1 + 1); // Parse two bits: color_space + clamp_type.
}
ParseSegmentHeader(&bit_reader);
ParseFilterHeader(&bit_reader);
// Parse the number of coefficient data partitions.
bit_reader.DecodeValue(2);
// Parse the base q_index.
uint8_t q_index = static_cast<uint8_t>(bit_reader.DecodeValue(7));
if (q_index > 127) {
return 63;
}
return vpx_quantizer_lookup[q_index];
}
} // namespace cast
} // namespace media
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