黄一山有钱吗:stagefright框架（四）-Video Buffer传输流程

这篇文章将介紹Stagefright中是如何和OMX video decoder传递buffer。

(1) OMXCodec会在一开始的时候透过read函数来传送未解码的data给decoder，并且要求decoder将解码后的data传回来

status_t OMXCodec::read(...)
{
  if (mInitialBufferSubmit)
  {
    mInitialBufferSubmit = false;

    drainInputBuffers(); <----- OMX_EmptyThisBuffer
    fillOutputBuffers(); <----- OMX_FillThisBuffer
  }

  ...
}

void OMXCodec::drainInputBuffers()
{
  Vector *buffers = &mPortBuffers[kPortIndexInput];

  for (i = 0; i < buffers->size(); ++i)
  {
    drainInputBuffer(&buffers->editItemAt(i));
  }
}

void OMXCodec::drainInputBuffer(BufferInfo *info)
{
  mOMX->emptyBuffer(...);
}

void OMXCodec::fillOutputBuffers()
{
  Vector *buffers = &mPortBuffers[kPortIndexOutput];

  for (i = 0; i < buffers->size(); ++i)
  {
    fillOutputBuffer(&buffers->editItemAt(i));
  }
}

void OMXCodec::fillOutputBuffer(BufferInfo *info)
{
  mOMX->fillBuffer(...);
}


(2) Decoder从input port读取资料后，开始进行解码，并且回传EmptyBufferDone通知OMXCodec

void OMXCodec::on_message(const omx_message &msg)
{
  switch (msg.type)
  {
    case omx_message::EMPTY_BUFFER_DONE:
    {
      IOMX::buffer_id buffer = msg.u.extended_buffer_data.buffer;
      drainInputBuffer(&buffers->editItemAt(i));
    }
  }
}


OMXCodec收到EMPTY_BUFFER_DONE之后，继续传送下一个未解码的资料给decoder。

(3) Decoder将解码完的资料送到output port，并回传FillBufferDone通知OMXCodec

void OMXCodec::on_message(const omx_message &msg)
{
  switch (msg.type)
  {
    case omx_message::FILL_BUFFER_DONE:
    {
      IOMX::buffer_id buffer = msg.u.extended_buffer_data.buffer;
      fillOutputBuffer(info);

      mFilledBuffers.push_back(i);
      mBufferFilled.signal();
    }
  }
}


OMXCodec收到FILL_BUFFER_DONE之后，将解码后的资料放入mFilledBuffers，发出mBufferFilled信号，并且要求decoder继续送出资料。

(4) read函数在后段等待mBufferFilled信号。当mFilledBuffers被填入资料后，read函数将其指定给buffer指标，并回传给AwesomePlayer

status_t OMXCodec::read(MediaBuffer **buffer, ...)
{
  ...

  while (mFilledBuffers.empty())
  {
    mBufferFilled.wait(mLock);
  }

  BufferInfo *info = &mPortBuffers[kPortIndexOutput].editItemAt(index);
  info->mMediaBuffer->add_ref();
  *buffer = info->mMediaBuffer;
}