1 /*
2 * Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
3 *
4 * Use of this source code is governed by a BSD-style license
5 * that can be found in the LICENSE file in the root of the source
6 * tree. An additional intellectual property rights grant can be found
7 * in the file PATENTS. All contributing project authors may
8 * be found in the AUTHORS file in the root of the source tree.
9 */
10 #include "modules/video_coding/utility/vp8_header_parser.h"
11
12 #include "rtc_base/logging.h"
13 #include "rtc_base/system/arch.h"
14
15 namespace webrtc {
16
17 namespace vp8 {
18 namespace {
19 const size_t kCommonPayloadHeaderLength = 3;
20 const size_t kKeyPayloadHeaderLength = 10;
21 const int kMbFeatureTreeProbs = 3;
22 const int kNumMbSegments = 4;
23 const int kNumRefLfDeltas = 4;
24 const int kNumModeLfDeltas = 4;
25
26 } // namespace
27
28 // Bitstream parser according to
29 // https://tools.ietf.org/html/rfc6386#section-7.3
VP8InitBitReader(VP8BitReader * const br,const uint8_t * start,const uint8_t * end)30 void VP8InitBitReader(VP8BitReader* const br,
31 const uint8_t* start,
32 const uint8_t* end) {
33 br->range_ = 255;
34 br->buf_ = start;
35 br->buf_end_ = end;
36 br->value_ = 0;
37 br->bits_ = 0;
38
39 // Read 2 bytes.
40 int i = 0;
41 while (++i <= 2) {
42 if (br->buf_ != br->buf_end_) {
43 br->value_ = br->value_ << 8 | *br->buf_++;
44 } else {
45 br->value_ = br->value_ << 8;
46 }
47 }
48 }
49
50 // Bit decoder according to https://tools.ietf.org/html/rfc6386#section-7.3
51 // Reads one bit from the bitstream, given that it has probability prob/256 to
52 // be 1.
Vp8BitReaderGetBool(VP8BitReader * br,int prob)53 int Vp8BitReaderGetBool(VP8BitReader* br, int prob) {
54 uint32_t split = 1 + (((br->range_ - 1) * prob) >> 8);
55 uint32_t split_hi = split << 8;
56 int retval = 0;
57 if (br->value_ >= split_hi) {
58 retval = 1;
59 br->range_ -= split;
60 br->value_ -= split_hi;
61 } else {
62 retval = 0;
63 br->range_ = split;
64 }
65
66 while (br->range_ < 128) {
67 br->value_ <<= 1;
68 br->range_ <<= 1;
69 if (++br->bits_ == 8) {
70 br->bits_ = 0;
71 if (br->buf_ != br->buf_end_) {
72 br->value_ |= *br->buf_++;
73 }
74 }
75 }
76 return retval;
77 }
78
VP8GetValue(VP8BitReader * br,int num_bits)79 uint32_t VP8GetValue(VP8BitReader* br, int num_bits) {
80 uint32_t v = 0;
81 while (num_bits--) {
82 // According to https://tools.ietf.org/html/rfc6386
83 // Probability 128/256 is used to encode header fields.
84 v = (v << 1) | Vp8BitReaderGetBool(br, 128);
85 }
86 return v;
87 }
88
89 // Not a read_signed_literal() from RFC 6386!
90 // This one is used to read e.g. quantizer_update, which is written as:
91 // L(num_bits), sign-bit.
VP8GetSignedValue(VP8BitReader * br,int num_bits)92 int32_t VP8GetSignedValue(VP8BitReader* br, int num_bits) {
93 int v = VP8GetValue(br, num_bits);
94 int sign = VP8GetValue(br, 1);
95 return sign ? -v : v;
96 }
97
ParseSegmentHeader(VP8BitReader * br)98 static void ParseSegmentHeader(VP8BitReader* br) {
99 int use_segment = VP8GetValue(br, 1);
100 if (use_segment) {
101 int update_map = VP8GetValue(br, 1);
102 if (VP8GetValue(br, 1)) { // update_segment_feature_data.
103 VP8GetValue(br, 1); // segment_feature_mode.
104 int s;
105 for (s = 0; s < kNumMbSegments; ++s) {
106 bool quantizer_update = VP8GetValue(br, 1);
107 if (quantizer_update) {
108 VP8GetSignedValue(br, 7);
109 }
110 }
111 for (s = 0; s < kNumMbSegments; ++s) {
112 bool loop_filter_update = VP8GetValue(br, 1);
113 if (loop_filter_update) {
114 VP8GetSignedValue(br, 6);
115 }
116 }
117 }
118 if (update_map) {
119 int s;
120 for (s = 0; s < kMbFeatureTreeProbs; ++s) {
121 bool segment_prob_update = VP8GetValue(br, 1);
122 if (segment_prob_update) {
123 VP8GetValue(br, 8);
124 }
125 }
126 }
127 }
128 }
129
ParseFilterHeader(VP8BitReader * br)130 static void ParseFilterHeader(VP8BitReader* br) {
131 VP8GetValue(br, 1); // filter_type.
132 VP8GetValue(br, 6); // loop_filter_level.
133 VP8GetValue(br, 3); // sharpness_level.
134
135 // mb_lf_adjustments.
136 int loop_filter_adj_enable = VP8GetValue(br, 1);
137 if (loop_filter_adj_enable) {
138 int mode_ref_lf_delta_update = VP8GetValue(br, 1);
139 if (mode_ref_lf_delta_update) {
140 int i;
141 for (i = 0; i < kNumRefLfDeltas; ++i) {
142 int ref_frame_delta_update_flag = VP8GetValue(br, 1);
143 if (ref_frame_delta_update_flag) {
144 VP8GetSignedValue(br, 6); // delta_magnitude.
145 }
146 }
147 for (i = 0; i < kNumModeLfDeltas; ++i) {
148 int mb_mode_delta_update_flag = VP8GetValue(br, 1);
149 if (mb_mode_delta_update_flag) {
150 VP8GetSignedValue(br, 6); // delta_magnitude.
151 }
152 }
153 }
154 }
155 }
156
GetQp(const uint8_t * buf,size_t length,int * qp)157 bool GetQp(const uint8_t* buf, size_t length, int* qp) {
158 if (length < kCommonPayloadHeaderLength) {
159 RTC_LOG(LS_WARNING) << "Failed to get QP, invalid length.";
160 return false;
161 }
162 VP8BitReader br;
163 const uint32_t bits = buf[0] | (buf[1] << 8) | (buf[2] << 16);
164 int key_frame = !(bits & 1);
165 // Size of first partition in bytes.
166 uint32_t partition_length = (bits >> 5);
167 size_t header_length = kCommonPayloadHeaderLength;
168 if (key_frame) {
169 header_length = kKeyPayloadHeaderLength;
170 }
171 if (header_length + partition_length > length) {
172 RTC_LOG(LS_WARNING) << "Failed to get QP, invalid length: " << length;
173 return false;
174 }
175 buf += header_length;
176
177 VP8InitBitReader(&br, buf, buf + partition_length);
178 if (key_frame) {
179 // Color space and pixel type.
180 VP8GetValue(&br, 1);
181 VP8GetValue(&br, 1);
182 }
183 ParseSegmentHeader(&br);
184 ParseFilterHeader(&br);
185 // Parse log2_nbr_of_dct_partitions value.
186 VP8GetValue(&br, 2);
187 // Base QP.
188 const int base_q0 = VP8GetValue(&br, 7);
189 if (br.buf_ == br.buf_end_) {
190 RTC_LOG(LS_WARNING) << "Failed to get QP, bitstream is truncated or"
191 " corrupted.";
192 return false;
193 }
194 *qp = base_q0;
195 return true;
196 }
197
198 } // namespace vp8
199
200 } // namespace webrtc
201