我们就先来看看Stagefright是如何播放一个影片档。
Stagefright在Android中是以shared library的形式存在(libstagefright.so),其中的module -- AwesomePlayer可用来播放video/audio (註3)。AwesomePlayer提供许多API,可以让上层的应用程式(Java/JNI)来呼叫,我们以一个简单的程式来说明video playback的流程。
在Java中,若要播放一个影片档,我们会这样写:
MediaPlayer mp = new MediaPlayer();
mp.setDataSource(PATH_TO_FILE); ...... (1)
mp.prepare(); ........................ (2)、(3)
mp.start(); .......................... (4)
MediaPlayer decode 通过binder调用远程MediaPlayerService decode
/*static*/ sp<IMemory> MediaPlayer::decode(int fd, int64_t offset, int64_t length, uint32_t *pSampleRate, int* pNumChannels, int* pFormat)
{
LOGV("decode(%d, %lld, %lld)", fd, offset, length);
sp<IMemory> p;
const sp<IMediaPlayerService>& service = getMediaPlayerService();
if (service != 0) {
p = service->decode(fd, offset, length, pSampleRate, pNumChannels, pFormat);
} else {
LOGE("Unable to locate media service");
}
return p;
}
在main_mediaserver.cpp 中有启动服务:
AudioFlinger::instantiate();
MediaPlayerService::instantiate();
//CameraService::instantiate();
AudioPolicyService::instantiate();
sp<IMemory> MediaPlayerService::decode(const char* url, uint32_t *pSampleRate, int* pNumChannels, int* pFormat)
中根据不同的url,调用createPlayer
在MediaPlayerService 中根据不同的类型创建不同的player
static sp<MediaPlayerBase> createPlayer(player_type playerType, void* cookie,
notify_callback_f notifyFunc)
{
sp<MediaPlayerBase> p;
switch (playerType) {
case HI_PLAYER:
#ifdef SKYWORTH_STB_SUPPORT
LOGV("Create StbPlayer");
p = new StbPlayer();
#else
LOGV("Create HiPlayer");
p = new HiMediaPlayer();
#endif
break;
#ifdef BUILD_DTV
case HI_DTVPLAYER:
p = new HiDtvPlayer();
break;
#endif
case SONIVOX_PLAYER:
LOGV(" create MidiFile");
p = new MidiFile();
break;
case STAGEFRIGHT_PLAYER:
LOGV(" create StagefrightPlayer");
p = new StagefrightPlayer;
break;
StagefrightPlayer 包装 AwesomePlayer ,继承MediaPlayerInterface 。
StagefrightPlayer 使用 OMXCodec 解码。
在Stagefright中,则会看到相对应的处理;
(1) 将档案的绝对路径指定给mUri
status_t AwesomePlayer::setDataSource(constchar* uri, ...)
{
return setDataSource_l(uri, ...);
}
status_t AwesomePlayer::setDataSource_l(constchar* uri, ...)
{
mUri = uri;
}
(2)启动mQueue,作为eventhandler
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被触发
voidAwesomePlayer::onPrepareAsyncEvent()
{
finishSetDataSource_l();
initVideoDecoder(); ...... (3.3)
initAudioDecoder();
}
status_tAwesomePlayer::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所指定的档案,并且根据其header来选择对应的extractor
sp<MediaExtractor>MediaExtractor::Create(constsp<DataSource> &source, ...)
{
source->sniff(&tmp, ...);
mime = tmp.string();
if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG4)
{
return new MPEG4Extractor(source);
}
elseif (!strcasecmp(mime,MEDIA_MIMETYPE_AUDIO_MPEG))
{
return new MP3Extractor(source);
}
elseif (!strcasecmp(mime,MEDIA_MIMETYPE_AUDIO_AMR_NB)
{
return new AMRExtractor(source);
}
}
(3.2) 使用extractor对档案做A/V(Audio/Video)的分离 (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;
}
}
}
voidAwesomePlayer::setVideoSource(sp<MediaSource> source)
{
mVideoTrack = source;
}
(3.3) 根据mVideoTrack中的编码类型来选择videodecoder (mVideoSource)
status_t AwesomePlayer::initVideoDecoder()
{
mVideoSource =OMXCodec::Create(mClient.interface(),mVideoTrack->getFormat(),false,mVideoTrack);
}
(4) 将mVideoEvent放入mQueue中,开始解码播放,并交由mVideoRenderer来画出
status_t AwesomePlayer::play()
{
return play_l();
}
status_t AwesomePlayer::play_l()
{
postVideoEvent_l();
}
voidAwesomePlayer::postVideoEvent_l(int64_tdelayUs)
{
mQueue.postEventWithDelay(mVideoEvent, delayUs);
}
voidAwesomePlayer::onVideoEvent()
{
mVideoSource->read(&mVideoBuffer, &options);
[Check Timestamp]
mVideoRenderer->render(mVideoBuffer);
postVideoEvent_l();
}
选择Video Decoder
我们来看一看Stagefright是如何根据影片档的类型来选择适合的Video Decoder
(1) VideoDecoder是在onPrepareAsyncEvent中的initVideoDecoder被決定的。OMXCodec::Create()会回传Video Decoder给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) 根据mVideoTrack的MIME从kDecoderInfo挑出合适的components
voidOMXCodec::findMatchingCodecs(mime, ..., matchingCodecs)
{
for (int index = 0;;++index)
{
componentName = GetCodec(kDecoderInfo,sizeof(kDecoderInfo)/sizeof(kDecoderInfo[0]),mime,index);
matchingCodecs->push(String8(componentName));
}
}
staticconst 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会依据MIME从kDecoderInfo挑出所有的component name,然后存到matchingCodecs中。
(3) 根据matchingCodecs中component的顺序,我们会先去检查其是否为softwaredecoder
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都会被列在kFactoryInfo中,我们藉由传进来的name来对应到适合的decoder。
(4) 如果該component不是software decoder,则试著去配置对应的OMX component
status_t OMX::allocateNode(name, ..., node)
{
mMaster->makeComponentInstance(name,&OMXNodeInstance::kCallbacks,instance,handle);
}
OMX_ERRORTYPEOMXMaster::makeComponentInstance(name, ...)
{
plugin->makeComponentInstance(name, ...);
}
OMX_ERRORTYPEOMXPVCodecsPlugin::makeComponentInstance(name, ...)
{
return OMX_MasterGetHandle(..., name, ...);
}
OMX_ERRORTYPE OMX_MasterGetHandle(...)
{
return OMX_GetHandle(...);
}
(5) 若该component為OMX deocder,则回传;否则继续检查下一个component
3.VideoBuffer传输流程
下面介绍Stagefright中是如何和OMX Video decoder传递buffer
(1)OMXCodec会在一开始的时候透过read函式来传送未解码的data给decoder,并且要求decoder将解码后的data传回來
status_t OMXCodec::read(...)
{
if(mInitialBufferSubmit)
{
mInitialBufferSubmit = false;
drainInputBuffers(); //----- OMX_EmptyThisBuffer(清空InputBuffer)
fillOutputBuffers(); //----- OMX_FillThisBuffer(填充OutputBuffer)
}
...
}
voidOMXCodec::drainInputBuffers()
{
Vector<BufferInfo> *buffers = &mPortBuffers[kPortIndexInput];
for (i = 0; i < buffers->size(); ++i)
{
drainInputBuffer(&buffers->editItemAt(i));
}
}
voidOMXCodec::drainInputBuffer(BufferInfo *info)
{
mOMX->emptyBuffer(...);
}
voidOMXCodec::fillOutputBuffers()
{
Vector<BufferInfo> *buffers = &mPortBuffers[kPortIndexOutput];
for (i = 0; i < buffers->size(); ++i)
{
fillOutputBuffer(&buffers->editItemAt(i));
}
}
voidOMXCodec::fillOutputBuffer(BufferInfo *info)
{
mOMX->fillBuffer(...);
}
(2)Decoder从input port读取资料后,开始进行解码,并且回传EmptyBufferDone通知OMXCodec
voidOMXCodec::on_message(const omx_message&msg)
{
switch (msg.type)
{
caseomx_message::EMPTY_BUFFER_DONE:
{
IOMX::buffer_id buffer = msg.u.extended_buffer_data.buffer;
drainInputBuffer(&buffers->editItemAt(i));
}
}
}
OMXCodec收到EMPTY_BUFFER_DONE之后,继续传送下一个未解码的资料给decoder。
(3) Decoder将解码完的资料送到output port,并回传FillBufferDone通知OMXCodec
voidOMXCodec::on_message(const omx_message&msg)
{
switch (msg.type)
{
caseomx_message::FILL_BUFFER_DONE:
{
IOMX::buffer_id buffer = msg.u.extended_buffer_data.buffer;
fillOutputBuffer(info);
mFilledBuffers.push_back(i);
mBufferFilled.signal();
}
}
}
OMXCodec收到FILL_BUFFER_DONE之后,将解码后的资料放入mFilledBuffers,发出mBufferFilled信号,并且要求decoder继续送出资料。
(4) read函式在后段等待mBufferFilled信号。当mFilledBuffers被填入资料后,read函式将其指定给buffer指标,并回传给AwesomePlayer
status_t OMXCodec::read(MediaBuffer**buffer, ...)
{
...
while (mFilledBuffers.empty())
{
mBufferFilled.wait(mLock);
}
BufferInfo *info =&mPortBuffers[kPortIndexOutput].editItemAt(index);
info->mMediaBuffer->add_ref();
*buffer = info->mMediaBuffer;
}
4.VideoRendering
AwesomePlayer::onVideoEvent除了透过OMXCodec::read取得解码后的资料外,还必须将这些资料(mVideoBuffer)传给Video renderer,以便画到UIScreen上去。
(1) 要将mVideoBuffer中的资料绘制出来之前,必须先建立mVideoRenderer
voidAwesomePlayer::onVideoEvent()
{
...
if(mVideoRenderer == NULL)
{
initRenderer_l();
}
...
}
voidAwesomePlayer::initRenderer_l()
{
if (!strncmp("OMX.",component, 4))
{
mVideoRenderer = newAwesomeRemoteRenderer(mClient.interface()->createRenderer(mISurface,component,...));.......... (2)
}
else
{
mVideoRenderer = newAwesomeLocalRenderer(...,component,mISurface); ............................ (3)
}
}
(2) 如果video decoder是OMX component,則建立一個AwesomeRemoteRenderer作為mVideoRenderer
从上段的程式码(1)来看,AwesomeRemoteRenderer的本质是由OMX::createRenderer所创建的。createRenderer会先建立一个hardware renderer -- SharedVideoRenderer (libstagefrighthw.so);若失败,则建立software renderer -- SoftwareRenderer (surface)。
sp<IOMXRenderer>OMX::createRenderer(...)
{
VideoRenderer *impl = NULL;
libHandle = dlopen("libstagefrighthw.so",RTLD_NOW);
if (libHandle)
{
CreateRendererFunc func = dlsym(libHandle, ...);
impl = (*func)(...); <----------------- Hardware Renderer
}
if (!impl)
{
impl = newSoftwareRenderer(...); <---- Software Renderer
}
}
(3) 如果video decoder是software component,則建立一个AwesomeLocalRenderer作為mVideoRenderer
AwesomeLocalRenderer的constructor会呼叫本身的init函式,其所做的事和OMX::createRenderer一模一样。
voidAwesomeLocalRenderer::init(...)
{
mLibHandle= dlopen("libstagefrighthw.so",RTLD_NOW);
if(mLibHandle)
{
CreateRendererFunc func = dlsym(...);
mTarget = (*func)(...); <---------------- Hardware Renderer
}
if(mTarget == NULL)
{
mTarget = new SoftwareRenderer(...);<--- Software Renderer
}
}
(4)mVideoRenderer一经建立就可以开始将解码后的资料传給它
voidAwesomePlayer::onVideoEvent()
{
if (!mVideoBuffer)
{
mVideoSource->read(&mVideoBuffer, ...);
}
[Check Timestamp]
if (mVideoRenderer == NULL)
{
initRenderer_l();
}
mVideoRenderer->render(mVideoBuffer); <----- Render Data
}
.AudioPlayback的流程
这篇文章将会开始audio处理的流程。Stagefright中关於audio的部分是交由AudioPlayer来处理,它是在AwesomePlayer::play_l中被建立的。
(1) 当上层应用程式要求播放影音时,AudioPlayer同时被建立出来,并且被啟动
status_t AwesomePlayer::play_l()
{
...
mAudioPlayer = new AudioPlayer(mAudioSink, ...);
mAudioPlayer->start(...);
...
}
(2)AudioPlayer在啟动的过程中会先去读取第一笔解码后的资料,并且开啟audio output
status_t AudioPlayer::start(...)
{
mSource->read(&mFirstBuffer);
if(mAudioSink.get() != NULL)
{
mAudioSink->open(...,&AudioPlayer::AudioSinkCallback, ...);
mAudioSink->start();
}
else
{
mAudioTrack = new AudioTrack(...,&AudioPlayer::AudioCallback, ...);
mAudioTrack->start();
}
}
从AudioPlayer::start的程式码来看,AudioPlayer似乎并没有将mFirstBuffer传给audio output。
(3) 开启audio output的同时,AudioPlayer会将callback函式設給它,之后每当callback函式被呼叫,AudioPlayer便去audio decoder读取解码后的资料
size_tAudioPlayer::AudioSinkCallback(audioSink, buffer, size, ...)
{
return fillBuffer(buffer, size);
}
void AudioPlayer::AudioCallback(..., info)
{
buffer = info;
fillBuffer(buffer->raw, buffer->size);
}
size_t AudioPlayer::fillBuffer(data, size)
{
mSource->read(&mInputBuffer, ...);
memcpy(data, mInputBuffer->data(), ...);
}
解码后audio资料的读取就是由callback函式所驱动,但是callback函式又是怎麼由audio output去驱动的,目前从程式码上还看不出来。另外一方面,从上面的程式片段可以看出,fillBuffer将资料(mInputBuffer)复製到data之后,audio output应该会去取用data。
至于audio decoder的工作流程则和video decoder相同,看参见上述第三部分Video Buffer传输流程
6.Audio和Video的同步
讲完了audio和video的处理流程,接下来要看的是audio和video同步化(synchronization)的问题。OpenCORE的做法是设置一个主clock,而audio和video就分别以此作為输出的依据。而在Stagefright中,audio的输出是透过callback函式来驱动,video则根据audio的timestamp来做同步。以下是详细的说明:
(1) 当callback函式驱动AudioPlayer读取解码后的资料时,AudioPlayer会取得两个时间戳 --mPositionTimeMediaUs和mPositionTimeRealUs
size_t AudioPlayer::fillBuffer(data, size)
{
...
mSource->read(&mInputBuffer, ...);
mInputBuffer->meta_data()->findInt64(kKeyTime,&mPositionTimeMediaUs);
mPositionTimeRealUs = ((mNumFramesPlayed + size_done / mFrameSize) *1000000) /mSampleRate;
...
}
mPositionTimeMediaUs是资料裡面所载明的时间戳(timestamp);mPositionTimeRealUs则是播放此资料的实际时间(依据frame number及sample rate得出)。
(2) Stagefright中的video便依据从AudioPlayer得出来之两个时间戳的差值,作為播放的依据
void AwesomePlayer::onVideoEvent()
{
...
mVideoSource->read(&mVideoBuffer, ...);
mVideoBuffer->meta_data()->findInt64(kKeyTime, &timeUs);
mAudioPlayer->getMediaTimeMapping(&realTimeUs, &mediaTimeUs);
mTimeSourceDeltaUs = realTimeUs - mediaTimeUs;
nowUs = ts->getRealTimeUs() - mTimeSourceDeltaUs;
latenessUs = nowUs - timeUs;
...
}
AwesomePlayer從AudioPlayer取得realTimeUs(即mPositionTimeRealUs)和mediaTimeUs(即mPositionTimeMediaUs),并算出其差值mTimeSourceDeltaUs。
(3) 最后我们将该video资料做排程
void AwesomePlayer::onVideoEvent()
{
...
if (latenessUs > 40000)
{
mVideoBuffer->release();
mVideoBuffer = NULL;
postVideoEvent_l();
return;
}
if (latenessUs < -10000)
{
postVideoEvent_l(10000);
return;
}
mVideoRenderer->render(mVideoBuffer);
...
}