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