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; }
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(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); }
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(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) }
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(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); } }
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(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; }
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(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); }
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(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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时间: 2024-10-07 13:46:39