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;
}