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 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component
