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 7 #ifndef MEDIA_FILTERS_H264_BIT_READER_H_ 8 #define MEDIA_FILTERS_H264_BIT_READER_H_ 9 10 #include <sys/types.h> 11 12 #include "base/basictypes.h" 13 #include "media/base/media_export.h" 14 15 namespace media { 16 17 // A class to provide bit-granularity reading of H.264 streams. 18 // This is not a generic bit reader class, as it takes into account 19 // H.264 stream-specific constraints, such as skipping emulation-prevention 20 // bytes and stop bits. See spec for more details. 21 class MEDIA_EXPORT H264BitReader { 22 public: 23 H264BitReader(); 24 ~H264BitReader(); 25 26 // Initialize the reader to start reading at |data|, |size| being size 27 // of |data| in bytes. 28 // Return false on insufficient size of stream.. 29 // TODO(posciak,fischman): consider replacing Initialize() with 30 // heap-allocating and creating bit readers on demand instead. 31 bool Initialize(const uint8* data, off_t size); 32 33 // Read |num_bits| next bits from stream and return in |*out|, first bit 34 // from the stream starting at |num_bits| position in |*out|. 35 // |num_bits| may be 1-32, inclusive. 36 // Return false if the given number of bits cannot be read (not enough 37 // bits in the stream), true otherwise. 38 bool ReadBits(int num_bits, int* out); 39 40 // Return the number of bits left in the stream. 41 off_t NumBitsLeft(); 42 43 // See the definition of more_rbsp_data() in spec. 44 bool HasMoreRBSPData(); 45 46 // Return the number of emulation prevention bytes already read. 47 size_t NumEmulationPreventionBytesRead(); 48 49 private: 50 // Advance to the next byte, loading it into curr_byte_. 51 // Return false on end of stream. 52 bool UpdateCurrByte(); 53 54 // Pointer to the next unread (not in curr_byte_) byte in the stream. 55 const uint8* data_; 56 57 // Bytes left in the stream (without the curr_byte_). 58 off_t bytes_left_; 59 60 // Contents of the current byte; first unread bit starting at position 61 // 8 - num_remaining_bits_in_curr_byte_ from MSB. 62 int curr_byte_; 63 64 // Number of bits remaining in curr_byte_ 65 int num_remaining_bits_in_curr_byte_; 66 67 // Used in emulation prevention three byte detection (see spec). 68 // Initially set to 0xffff to accept all initial two-byte sequences. 69 int prev_two_bytes_; 70 71 // Number of emulation preventation bytes (0x000003) we met. 72 size_t emulation_prevention_bytes_; 73 74 DISALLOW_COPY_AND_ASSIGN(H264BitReader); 75 }; 76 77 } // namespace media 78 79 #endif // MEDIA_FILTERS_H264_BIT_READER_H_ 80
