content/common/gpu/media/h264_bit_reader.h

   1 // Copyright (c) 2012 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 CONTENT_COMMON_GPU_MEDIA_H264_BIT_READER_H_
   8 #define CONTENT_COMMON_GPU_MEDIA_H264_BIT_READER_H_
   9
  10 #include <sys/types.h>
  11
  12 #include "base/basictypes.h"
  13 #include "content/common/content_export.h"
  14
  15 namespace content {
  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 CONTENT_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  private:
  47   // Advance to the next byte, loading it into curr_byte_.
  48   // Return false on end of stream.
  49   bool UpdateCurrByte();
  50
  51   // Pointer to the next unread (not in curr_byte_) byte in the stream.
  52   const uint8* data_;
  53
  54   // Bytes left in the stream (without the curr_byte_).
  55   off_t bytes_left_;
  56
  57   // Contents of the current byte; first unread bit starting at position
  58   // 8 - num_remaining_bits_in_curr_byte_ from MSB.
  59   int curr_byte_;
  60
  61   // Number of bits remaining in curr_byte_
  62   int num_remaining_bits_in_curr_byte_;
  63
  64   // Used in emulation prevention three byte detection (see spec).
  65   // Initially set to 0xffff to accept all initial two-byte sequences.
  66   int prev_two_bytes_;
  67
  68   DISALLOW_COPY_AND_ASSIGN(H264BitReader);
  69 };
  70
  71 }  // namespace content
  72
  73 #endif  // CONTENT_COMMON_GPU_MEDIA_H264_BIT_READER_H_