Stagefright (4) - Video Buffer傳輸流程

這篇文章將介紹Stagefright中是如何和OMX video decoder傳遞buffer。(1) OMXCodec會在一開始的時候透過read函式來傳送未解碼的 data給decoder，並且要求decoder將解碼後的data傳回來status_t OMXCodec::read(...){  if (mInitialBufferSubmit)  {    mInitialBufferSubmit = false;    drainInputBuffers(); <----- OMX_EmptyThisBuffer    fillOutputBuffers(); <----- OMX_FillThisBuffer  }  ...}void OMXCodec::drainInputBuffers(){  Vector<BufferInfo> *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<BufferInfo> *buffers = &mPortBuffers[kPortIndexOutput];  for (i = 0; i < buffers->size(); ++i)  {    fillOutputBuffer(&buffers->editItemAt(i));  }}void OMXCodec::fillOutputBuffer(BufferInfo *info){  mOMX->fillBuffer(...);}(2) Decoder從input port讀取資料後，開始進行解碼，並且回傳EmptyBufferDone通知OMXCodecvoid 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之後，繼續傳送下一個未解碼的資料給decoder。(3) Decoder將解碼完的資料送到output port，並回傳FillBufferDone通知OMXCodecvoid 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之後，將解碼後的資料放入mFilledBuffers，發出mBufferFilled信號，並且要求 decoder繼續送出資料。(4) read函式在後段等待mBufferFilled信號。當mFilledBuffers被填入資料後，read函式將其指定給buffer指標，並回傳 給AwesomePlayerstatus_t OMXCodec::read(MediaBuffer **buffer, ...){  ...  while (mFilledBuffers.empty())  {    mBufferFilled.wait(mLock);  }  BufferInfo *info = &mPortBuffers[kPortIndexOutput].editItemAt(index);  info->mMediaBuffer->add_ref();  *buffer = info->mMediaBuffer;}