media/filters/h264_bit_reader.h

   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_