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/*
* Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "modules/video_coding/utility/vp8_header_parser.h"
#include "rtc_base/logging.h"
#include "rtc_base/system/arch.h"
namespace webrtc {
namespace vp8 {
namespace {
const size_t kCommonPayloadHeaderLength = 3;
const size_t kKeyPayloadHeaderLength = 10;
const int kMbFeatureTreeProbs = 3;
const int kNumMbSegments = 4;
const int kNumRefLfDeltas = 4;
const int kNumModeLfDeltas = 4;
} // namespace
// Bitstream parser according to
// https://tools.ietf.org/html/rfc6386#section-7.3
void VP8InitBitReader(VP8BitReader* const br,
const uint8_t* start,
const uint8_t* end) {
br->range_ = 255;
br->buf_ = start;
br->buf_end_ = end;
br->value_ = 0;
br->bits_ = 0;
// Read 2 bytes.
int i = 0;
while (++i <= 2) {
if (br->buf_ != br->buf_end_) {
br->value_ = br->value_ << 8 | *br->buf_++;
} else {
br->value_ = br->value_ << 8;
}
}
}
// Bit decoder according to https://tools.ietf.org/html/rfc6386#section-7.3
// Reads one bit from the bitstream, given that it has probability prob/256 to
// be 1.
int Vp8BitReaderGetBool(VP8BitReader* br, int prob) {
uint32_t split = 1 + (((br->range_ - 1) * prob) >> 8);
uint32_t split_hi = split << 8;
int retval = 0;
if (br->value_ >= split_hi) {
retval = 1;
br->range_ -= split;
br->value_ -= split_hi;
} else {
retval = 0;
br->range_ = split;
}
while (br->range_ < 128) {
br->value_ <<= 1;
br->range_ <<= 1;
if (++br->bits_ == 8) {
br->bits_ = 0;
if (br->buf_ != br->buf_end_) {
br->value_ |= *br->buf_++;
}
}
}
return retval;
}
uint32_t VP8GetValue(VP8BitReader* br, int num_bits) {
uint32_t v = 0;
while (num_bits--) {
// According to https://tools.ietf.org/html/rfc6386
// Probability 128/256 is used to encode header fields.
v = (v << 1) | Vp8BitReaderGetBool(br, 128);
}
return v;
}
// Not a read_signed_literal() from RFC 6386!
// This one is used to read e.g. quantizer_update, which is written as:
// L(num_bits), sign-bit.
int32_t VP8GetSignedValue(VP8BitReader* br, int num_bits) {
int v = VP8GetValue(br, num_bits);
int sign = VP8GetValue(br, 1);
return sign ? -v : v;
}
static void ParseSegmentHeader(VP8BitReader* br) {
int use_segment = VP8GetValue(br, 1);
if (use_segment) {
int update_map = VP8GetValue(br, 1);
if (VP8GetValue(br, 1)) { // update_segment_feature_data.
VP8GetValue(br, 1); // segment_feature_mode.
int s;
for (s = 0; s < kNumMbSegments; ++s) {
bool quantizer_update = VP8GetValue(br, 1);
if (quantizer_update) {
VP8GetSignedValue(br, 7);
}
}
for (s = 0; s < kNumMbSegments; ++s) {
bool loop_filter_update = VP8GetValue(br, 1);
if (loop_filter_update) {
VP8GetSignedValue(br, 6);
}
}
}
if (update_map) {
int s;
for (s = 0; s < kMbFeatureTreeProbs; ++s) {
bool segment_prob_update = VP8GetValue(br, 1);
if (segment_prob_update) {
VP8GetValue(br, 8);
}
}
}
}
}
static void ParseFilterHeader(VP8BitReader* br) {
VP8GetValue(br, 1); // filter_type.
VP8GetValue(br, 6); // loop_filter_level.
VP8GetValue(br, 3); // sharpness_level.
// mb_lf_adjustments.
int loop_filter_adj_enable = VP8GetValue(br, 1);
if (loop_filter_adj_enable) {
int mode_ref_lf_delta_update = VP8GetValue(br, 1);
if (mode_ref_lf_delta_update) {
int i;
for (i = 0; i < kNumRefLfDeltas; ++i) {
int ref_frame_delta_update_flag = VP8GetValue(br, 1);
if (ref_frame_delta_update_flag) {
VP8GetSignedValue(br, 6); // delta_magnitude.
}
}
for (i = 0; i < kNumModeLfDeltas; ++i) {
int mb_mode_delta_update_flag = VP8GetValue(br, 1);
if (mb_mode_delta_update_flag) {
VP8GetSignedValue(br, 6); // delta_magnitude.
}
}
}
}
}
bool GetQp(const uint8_t* buf, size_t length, int* qp) {
if (length < kCommonPayloadHeaderLength) {
RTC_LOG(LS_WARNING) << "Failed to get QP, invalid length.";
return false;
}
VP8BitReader br;
const uint32_t bits = buf[0] | (buf[1] << 8) | (buf[2] << 16);
int key_frame = !(bits & 1);
// Size of first partition in bytes.
uint32_t partition_length = (bits >> 5);
size_t header_length = kCommonPayloadHeaderLength;
if (key_frame) {
header_length = kKeyPayloadHeaderLength;
}
if (header_length + partition_length > length) {
RTC_LOG(LS_WARNING) << "Failed to get QP, invalid length: " << length;
return false;
}
buf += header_length;
VP8InitBitReader(&br, buf, buf + partition_length);
if (key_frame) {
// Color space and pixel type.
VP8GetValue(&br, 1);
VP8GetValue(&br, 1);
}
ParseSegmentHeader(&br);
ParseFilterHeader(&br);
// Parse log2_nbr_of_dct_partitions value.
VP8GetValue(&br, 2);
// Base QP.
const int base_q0 = VP8GetValue(&br, 7);
if (br.buf_ == br.buf_end_) {
RTC_LOG(LS_WARNING) << "Failed to get QP, bitstream is truncated or"
" corrupted.";
return false;
}
*qp = base_q0;
return true;
}
} // namespace vp8
} // namespace webrtc
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