1 // Copyright 2014 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 // 5 // This file contains an implementation of an H264 Annex-B video stream parser. 6 // Note: ported from Chromium commit head: 77be7ae 7 8 #ifndef H264_BIT_READER_H_ 9 #define H264_BIT_READER_H_ 10 11 #include <stddef.h> 12 #include <stdint.h> 13 #include <sys/types.h> 14 15 #include "base/macros.h" 16 17 namespace media { 18 19 // A class to provide bit-granularity reading of H.264 streams. 20 // This is not a generic bit reader class, as it takes into account 21 // H.264 stream-specific constraints, such as skipping emulation-prevention 22 // bytes and stop bits. See spec for more details. 23 class H264BitReader { 24 public: 25 H264BitReader(); 26 ~H264BitReader(); 27 28 // Initialize the reader to start reading at |data|, |size| being size 29 // of |data| in bytes. 30 // Return false on insufficient size of stream.. 31 // TODO(posciak,fischman): consider replacing Initialize() with 32 // heap-allocating and creating bit readers on demand instead. 33 bool Initialize(const uint8_t* data, off_t size); 34 35 // Read |num_bits| next bits from stream and return in |*out|, first bit 36 // from the stream starting at |num_bits| position in |*out|. 37 // |num_bits| may be 1-32, inclusive. 38 // Return false if the given number of bits cannot be read (not enough 39 // bits in the stream), true otherwise. 40 bool ReadBits(int num_bits, int* out); 41 42 // Return the number of bits left in the stream. 43 off_t NumBitsLeft(); 44 45 // See the definition of more_rbsp_data() in spec. 46 bool HasMoreRBSPData(); 47 48 // Return the number of emulation prevention bytes already read. 49 size_t NumEmulationPreventionBytesRead(); 50 51 private: 52 // Advance to the next byte, loading it into curr_byte_. 53 // Return false on end of stream. 54 bool UpdateCurrByte(); 55 56 // Pointer to the next unread (not in curr_byte_) byte in the stream. 57 const uint8_t* data_; 58 59 // Bytes left in the stream (without the curr_byte_). 60 off_t bytes_left_; 61 62 // Contents of the current byte; first unread bit starting at position 63 // 8 - num_remaining_bits_in_curr_byte_ from MSB. 64 int curr_byte_; 65 66 // Number of bits remaining in curr_byte_ 67 int num_remaining_bits_in_curr_byte_; 68 69 // Used in emulation prevention three byte detection (see spec). 70 // Initially set to 0xffff to accept all initial two-byte sequences. 71 int prev_two_bytes_; 72 73 // Number of emulation preventation bytes (0x000003) we met. 74 size_t emulation_prevention_bytes_; 75 76 DISALLOW_COPY_AND_ASSIGN(H264BitReader); 77 }; 78 79 } // namespace media 80 81 #endif // H264_BIT_READER_H_ 82
