stagefright框架（四）-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; }