content/common/gpu/media/h264_bit_reader.cc

   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 #include "content/common/gpu/media/h264_bit_reader.h"
   6 #include "base/logging.h"
   7
   8 namespace content {
   9
  10 H264BitReader::H264BitReader()
  11     : data_(NULL), bytes_left_(0), curr_byte_(0),
  12       num_remaining_bits_in_curr_byte_(0), prev_two_bytes_(0) {
  13 }
  14
  15 H264BitReader::~H264BitReader() {}
  16
  17 bool H264BitReader::Initialize(const uint8* data, off_t size) {
  18   DCHECK(data);
  19
  20   if (size < 1)
  21     return false;
  22
  23   data_ = data;
  24   bytes_left_ = size;
  25   num_remaining_bits_in_curr_byte_ = 0;
  26   // Initially set to 0xffff to accept all initial two-byte sequences.
  27   prev_two_bytes_ = 0xffff;
  28
  29   return true;
  30 }
  31
  32 bool H264BitReader::UpdateCurrByte() {
  33   if (bytes_left_ < 1)
  34     return false;
  35
  36   // Emulation prevention three-byte detection.
  37   // If a sequence of 0x000003 is found, skip (ignore) the last byte (0x03).
  38   if (*data_ == 0x03 && (prev_two_bytes_ & 0xffff) == 0) {
  39     // Detected 0x000003, skip last byte.
  40     ++data_;
  41     --bytes_left_;
  42     // Need another full three bytes before we can detect the sequence again.
  43     prev_two_bytes_ = 0xffff;
  44
  45     if (bytes_left_ < 1)
  46       return false;
  47   }
  48
  49   // Load a new byte and advance pointers.
  50   curr_byte_ = *data_++ & 0xff;
  51   --bytes_left_;
  52   num_remaining_bits_in_curr_byte_ = 8;
  53
  54   prev_two_bytes_ = (prev_two_bytes_ << 8) | curr_byte_;
  55
  56   return true;
  57 }
  58
  59 // Read |num_bits| (1 to 31 inclusive) from the stream and return them
  60 // in |out|, with first bit in the stream as MSB in |out| at position
  61 // (|num_bits| - 1).
  62 bool H264BitReader::ReadBits(int num_bits, int *out) {
  63   int bits_left = num_bits;
  64   *out = 0;
  65   DCHECK(num_bits <= 31);
  66
  67   while (num_remaining_bits_in_curr_byte_ < bits_left) {
  68     // Take all that's left in current byte, shift to make space for the rest.
  69     *out |= (curr_byte_ << (bits_left - num_remaining_bits_in_curr_byte_));
  70     bits_left -= num_remaining_bits_in_curr_byte_;
  71
  72     if (!UpdateCurrByte())
  73       return false;
  74   }
  75
  76   *out |= (curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_left));
  77   *out &= ((1 << num_bits) - 1);
  78   num_remaining_bits_in_curr_byte_ -= bits_left;
  79
  80   return true;
  81 }
  82
  83 off_t H264BitReader::NumBitsLeft() {
  84   return (num_remaining_bits_in_curr_byte_ + bytes_left_ * 8);
  85 }
  86
  87 bool H264BitReader::HasMoreRBSPData() {
  88   // Make sure we have more bits, if we are at 0 bits in current byte
  89   // and updating current byte fails, we don't have more data anyway.
  90   if (num_remaining_bits_in_curr_byte_ == 0 && !UpdateCurrByte())
  91       return false;
  92
  93   // On last byte?
  94   if (bytes_left_)
  95     return true;
  96
  97   // Last byte, look for stop bit;
  98   // We have more RBSP data if the last non-zero bit we find is not the
  99   // first available bit.
 100   return (curr_byte_ &
 101           ((1 << (num_remaining_bits_in_curr_byte_ - 1)) - 1)) != 0;
 102 }
 103
 104 }  // namespace content