Stagefright (3) - 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componentsvoid 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 decoderstatic 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 componentstatus_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。