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" },
...
}
GetCodec會依據mime從kDecoderInfo挑出所有的
component name，然後存到matchingCodecs中。
(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omponent。