android stagefright系列 ---- stagefright框架(一)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,所以相對來說較為穩定;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 (註1)

[圖1] Stagefright在Android多媒體架構中的位置。

[圖2] Stagefright所涵蓋的模組 (註2)
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 (註3)。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();
}



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