Stagefright (1) - Video Playback的流程

在Android上，預設的多媒體框架 (multimedia framework)是OpenCORE。OpenCORE的優點是兼顧了跨平台的移植性，而且已經過多方驗證，所以相當穩定；但是其缺點是過於龐大複 雜，需要耗費相當多的時間去維護。從Android 2.0開始，Google引進了架構稍為簡潔的Stagefright，並且有逐漸取代OpenCORE的趨勢。以下我們就先來看看 Stagefright是如何播放一個影片檔。

Stagefright在Android中是以shared library的形式存在(libstagefright.so)，其中的module -- AwesomePlayer可用來播放video/audio。AwesomePlayer提供許多API，可以讓上層的應用程式(Java/JNI)來 呼叫，我們以一個簡單的程式來說明video playback的流程。

在Java中，若要播放一個影片檔，我們會這樣寫：

MediaPlayer mp = new MediaPlayer();

mp.setDataSource(PATH_TO_FILE); ...... (1)

mp.prepare(); ........................ (2)、(3)

mp.start(); .......................... (4)

在Stagefright中，則會看到相對應的處理；

(1) 將檔案的絕對路徑指定給mUri

status_t AwesomePlayer::setDataSource(const char* uri, ...)

{

  return setDataSource_l(uri, ...);

}

status_t AwesomePlayer::setDataSource_l(const char* uri, ...)

{

  mUri = uri;

}

(2) 啟 動mQueue，作為event handler

status_t AwesomePlayer::prepare()

{

  return prepare_l();

}

status_t AwesomePlayer::prepare_l()

{

  prepareAsync_l();

  while (mFlags & PREPARING)

  {

    mPreparedCondition.wait(mLock);

  }

}

status_t AwesomePlayer::prepareAsync_l()

{

  mQueue.start();

  mFlags |= PREPARING;

  mAsyncPrepareEvent = new AwesomeEvent(

                             this

                             &AwesomePlayer::onPrepareAsyncEvent);

  mQueue.postEvent(mAsyncPrepareEvent);

}

(3) onPrepareAsyncEvent被觸發

void AwesomePlayer::onPrepareAsyncEvent()

{

  finishSetDataSource_l();

  initVideoDecoder(); ...... (3.3)

  initAudioDecoder();

}

status_t AwesomePlayer::finishSetDataSource_l()

{

  dataSource = DataSource::CreateFromURI(mUri.string(), ...);

  sp<MediaExtractor> extractor =

                     MediaExtractor::Create(dataSource); ..... (3.1)

  return setDataSource_l(extractor); ......................... (3.2)

}

(3.1) 解 析mUri所指定的檔案，並且根據其header來選擇對應的extractor

sp<MediaExtractor> MediaExtractor::Create(const sp<DataSource> &source, ...)

{

  source->sniff(&tmp, ...);

  mime = tmp.string();

  if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG4)

  {

    return new MPEG4Extractor(source);

  }

  else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_MPEG))

  {

    return new MP3Extractor(source);

  }

  else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AMR_NB)

  {

    return new AMRExtractor(source);

  }

}

(3.2) 使用extractor對檔案做A/V的分離 (mVideoTrack/mAudioTrack)

status_t AwesomePlayer::setDataSource_l(const sp<MediaExtractor> &extractor)

{

  for (size_t i = 0; i < extractor->countTracks(); ++i)

  {

    sp<MetaData> meta = extractor->getTrackMetaData(i);

    CHECK(meta->findCString(kKeyMIMEType, &mime));

    if (!haveVideo && !strncasecmp(mime, "video/", 6))

    {

      setVideoSource(extractor->getTrack(i));

      haveVideo = true;

    }

    else if (!haveAudio && !strncasecmp(mime, "audio/", 6))

    {

      setAudioSource(extractor->getTrack(i));

      haveAudio = true;

    }

  }

}

void AwesomePlayer::setVideoSource(sp<MediaSource> source)

{

  mVideoTrack = source;

}

(3.3) 根 據mVideoTrack中的編碼類型來選擇video decoder (mVideoSource)

status_t AwesomePlayer::initVideoDecoder()

{

  mVideoSource = OMXCodec::Create(mClient.interface(),

                                  mVideoTrack->getFormat(),

                                  false,

                                  mVideoTrack);

}

(4) 將mVideoEvent放入mQueue中，開始解碼播放，並交由mVideoRenderer來畫出

status_t AwesomePlayer::play()

{

  return play_l();

}

status_t AwesomePlayer::play_l()

{

  postVideoEvent_l();

}

void AwesomePlayer::postVideoEvent_l(int64_t delayUs)

{

  mQueue.postEventWithDelay(mVideoEvent, delayUs);

}

void AwesomePlayer::onVideoEvent()

{

  mVideoSource->read(&mVideoBuffer, &options);

  [Check Timestamp]

  mVideoRenderer->render(mVideoBuffer);

  postVideoEvent_l();

}