stagefright框架(三)-選擇Video Decoder

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 decodermVideoSource

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mVideoTrackMIMEkDecoderInfo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" },
...
}

GetCodec會依據mimekDecoderInfo挑出所有的component name,然後存到matchingCodecs中。

(3) 根據matchingCodecscomponent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componentOMX deocder,則回傳;否則繼續檢查下一個component


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