這篇文章將介紹Stagefright中是如何和OMX video decoder传送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
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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(...);
}
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(2) Decoder從input port讀取資料後，開始進行解碼，並且回傳EmptyBufferDone通知OMXCodec
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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));
}
}
}
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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
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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();
}
}
}
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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
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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;
}
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