stagefright框架(三)-選擇Video Decoder
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在《Stagefright (1) – Video Playback的流程》中,我們並沒有詳述Stagefright是如何根據影片檔的類型來選擇適合的video decoder,現在,就讓我們來看一看。
(1) Video decoder是在onPrepareAsyncEvent中的initVideoDecoder被決定的OMXCodec::Create()會回傳video decoder給mVideoSource。
status_t AwesomePlayer::initVideoDecoder()
{
mVideoSource = OMXCodec::Create(mClient.interface(),
mVideoTrack->getFormat(),
false,
mVideoTrack);
}
sp<MediaSource> OMXCodec::Create(&omx,&meta, createEncoder,&source, matchComponentName)
{
meta->findCString(kKeyMIMEType,&mime);
findMatchingCodecs(mime,...,&matchingCodecs);........(2)
for (size_t i= 0; i < matchingCodecs.size();++i)
{
componentName = matchingCodecs[i].string();
softwareCodec =
InstantiateSoftwareCodec(componentName,...);.....(3)
if (softwareCodec!= NULL) return softwareCodec;
err = omx->allocateNode(componentName,...,&node);...(4)
if (err== OK)
{
codec = new OMXCodec(..., componentName, ...);......(5)
return codec;
}
}
}
(2) 根據mVideoTrack的MIME從kDecoderInfo挑出合適的components
void OMXCodec::findMatchingCodecs(mime,..., matchingCodecs)
{
for (int index= 0;;++index)
{
componentName = GetCodec(
kDecoderInfo,
sizeof(kDecoderInfo)/sizeof(kDecoderInfo[0]),
mime,
index);
matchingCodecs->push(String8(componentName));
}
}
static const CodecInfo kDecoderInfo[]=
{
...
{ MEDIA_MIMETYPE_VIDEO_MPEG4,"OMX.qcom.video.decoder.mpeg4" },
{ MEDIA_MIMETYPE_VIDEO_MPEG4,"OMX.TI.Video.Decoder" },
{ MEDIA_MIMETYPE_VIDEO_MPEG4,"M4vH263Decoder" },
...
}
(3) 根據matchingCodecs中component的順序,我們會先去檢查其是否為software decoder
static sp<MediaSource> InstantiateSoftwareCodec(name,...)
{
FactoryInfo kFactoryInfo[]=
{
...
FACTORY_REF(M4vH263Decoder)
...
};
for (i = 0; i <sizeof(kFactoryInfo)/sizeof(kFactoryInfo[0]);++i)
{
if (!strcmp(name, kFactoryInfo[i].name))
return (*kFactoryInfo[i].CreateFunc)(source);
}
}
所有的software decoder都會被列在kFactoryInfo中,我們藉由傳進來的name來對應到適合的decoder。
(4) 如果該component不是software decoder,則試著去配置對應的OMX component
status_t OMX::allocateNode(name,..., node)
{
mMaster->makeComponentInstance(
name,
&OMXNodeInstance::kCallbacks,
instance,
handle);
}
OMX_ERRORTYPE OMXMaster::makeComponentInstance(name,...)
{
plugin->makeComponentInstance(name,...);
}
OMX_ERRORTYPE OMXPVCodecsPlugin::makeComponentInstance(name,...)
{
return OMX_MasterGetHandle(..., name,...);
}
OMX_ERRORTYPE OMX_MasterGetHandle(...)
{
return OMX_GetHandle(...);
}
(5) 若該component為OMX deocder,則回傳;否則繼續檢查下一個component
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