android中多媒体解码openmax的实现

每个AwesomePlayer 只有一个OMX服务的入口，但是AwesomePlayer不一定就只需要1种解码器。有可能音视频都有，或者有很多种。这个时候这些解码器都需要OMX的服务，也就是OMX那头需要建立不同的解码器的组件来对应着AwesomePlayer中不同的code。OMX中非常重要的2个成员就是 OMXMaster
和 OMXNodeInstance。OMX通过这俩个成员来创建和维护不同的openmax 解码器组件，为AwesomePlayer中不同解码提供服务。让我们看看他们是怎么实现这些工作的。



1. OMX中 OMXNodeInstance
负责创建并维护不同的实例，这些实例是根据上面需求创建的，以node作为唯一标识。这样播放器中每个OMXCodec在OMX服务端都对应有了自己的OMXNodeInstance实例。

2.OMXMaster
维护底层软硬件解码库，根据OMXNodeInstance中想要的解码器来创建解码实体组件。

接下来我们假设视频解码器需要的是AVC，来看看解码器创建的流程。

(默认走软解码)

1.准备工作初始化OMXMaster

OMX构造函数中会进行初始化。

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OMXMaster *mMaster;

OMXMaster *mMaster;

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OMX::OMX() : mMaster(new OMXMaster), mNodeCounter(0) { }

OMX::OMX()
: mMaster(new OMXMaster),
mNodeCounter(0) {
}

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OMXMaster::OMXMaster() : mVendorLibHandle(NULL) { addVendorPlugin(); addPlugin(new SoftOMXPlugin); }

OMXMaster::OMXMaster()
: mVendorLibHandle(NULL) {
addVendorPlugin();
addPlugin(new SoftOMXPlugin);
}

OMXMaster 负责OMX中编解码器插件管理，软件解码和硬件解码都是使用OMX标准，挂载plugins的方式来进行管理。

软解通过 addPlugin(new SoftOMXPlugin);会把这些编解码器的名字都放在mPluginByComponentName中。
android 默认会提供一系列的软件解码器。目前支持这些格式的软编解码。

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kComponents[] = { { "OMX.google.aac.decoder", "aacdec", "audio_decoder.aac" }, { "OMX.google.aac.encoder", "aacenc", "audio_encoder.aac" }, { "OMX.google.amrnb.decoder", "amrdec", "audio_decoder.amrnb" }, { "OMX.google.amrnb.encoder", "amrnbenc", "audio_encoder.amrnb" }, { "OMX.google.amrwb.decoder", "amrdec", "audio_decoder.amrwb" }, { "OMX.google.amrwb.encoder", "amrwbenc", "audio_encoder.amrwb" }, { "OMX.google.h264.decoder", "h264dec", "video_decoder.avc" }, { "OMX.google.h264.encoder", "h264enc", "video_encoder.avc" }, { "OMX.google.g711.alaw.decoder", "g711dec", "audio_decoder.g711alaw" }, { "OMX.google.g711.mlaw.decoder", "g711dec", "audio_decoder.g711mlaw" }, { "OMX.google.h263.decoder", "mpeg4dec", "video_decoder.h263" }, { "OMX.google.h263.encoder", "mpeg4enc", "video_encoder.h263" }, { "OMX.google.mpeg4.decoder", "mpeg4dec", "video_decoder.mpeg4" }, { "OMX.google.mpeg4.encoder", "mpeg4enc", "video_encoder.mpeg4" }, { "OMX.google.mp3.decoder", "mp3dec", "audio_decoder.mp3" }, { "OMX.google.vorbis.decoder", "vorbisdec", "audio_decoder.vorbis" }, { "OMX.google.vpx.decoder", "vpxdec", "video_decoder.vpx" }, { "OMX.google.raw.decoder", "rawdec", "audio_decoder.raw" }, { "OMX.google.flac.encoder", "flacenc", "audio_encoder.flac" }, };

kComponents[] = {
{ "OMX.google.aac.decoder", "aacdec", "audio_decoder.aac" },
{ "OMX.google.aac.encoder", "aacenc", "audio_encoder.aac" },
{ "OMX.google.amrnb.decoder", "amrdec", "audio_decoder.amrnb" },
{ "OMX.google.amrnb.encoder", "amrnbenc", "audio_encoder.amrnb" },
{ "OMX.google.amrwb.decoder", "amrdec", "audio_decoder.amrwb" },
{ "OMX.google.amrwb.encoder", "amrwbenc", "audio_encoder.amrwb" },
{ "OMX.google.h264.decoder", "h264dec", "video_decoder.avc" },
{ "OMX.google.h264.encoder", "h264enc", "video_encoder.avc" },
{ "OMX.google.g711.alaw.decoder", "g711dec", "audio_decoder.g711alaw" },
{ "OMX.google.g711.mlaw.decoder", "g711dec", "audio_decoder.g711mlaw" },
{ "OMX.google.h263.decoder", "mpeg4dec", "video_decoder.h263" },
{ "OMX.google.h263.encoder", "mpeg4enc", "video_encoder.h263" },
{ "OMX.google.mpeg4.decoder", "mpeg4dec", "video_decoder.mpeg4" },
{ "OMX.google.mpeg4.encoder", "mpeg4enc", "video_encoder.mpeg4" },
{ "OMX.google.mp3.decoder", "mp3dec", "audio_decoder.mp3" },
{ "OMX.google.vorbis.decoder", "vorbisdec", "audio_decoder.vorbis" },
{ "OMX.google.vpx.decoder", "vpxdec", "video_decoder.vpx" },
{ "OMX.google.raw.decoder", "rawdec", "audio_decoder.raw" },
{ "OMX.google.flac.encoder", "flacenc", "audio_encoder.flac" },
};

硬件编解码是通过 addVendorPlugin();加载libstagefrighthw.so.各个芯片平台可以遵循openmax
标准,生成libstagefrighthw.so的库来提供android应用。

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void OMXMaster::addVendorPlugin() { addPlugin("libstagefrighthw.so"); }

void OMXMaster::addVendorPlugin() {
addPlugin("libstagefrighthw.so");
}

然后通过dlopen、dlsym来调用库中的函数。

这部分准备工作是在AwesomePlayer的构造函数中
CHECK_EQ(mClient.connect(), (status_t)OK); 已经完成了。

2.创建mVideoSource

有了上面的OMX，接下来会在AwesomePlayer::initVideoDecoder中创建mVideoSource
实例，下面代码只保留的主要部分：

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status_t AwesomePlayer::initVideoDecoder(uint32_t flags) {
ATRACE_CALL();
mVideoSource = OMXCodec::Create(
mClient.interface(), mVideoTrack->getFormat(),
false, // createEncoder

mVideoTrack,
NULL, flags, USE_SURFACE_ALLOC ? mNativeWindow : NULL);
status_t err = mVideoSource->start();
return mVideoSource != NULL ? OK : UNKNOWN_ERROR;
}

status_t AwesomePlayer::initVideoDecoder(uint32_t flags) {
ATRACE_CALL();
mVideoSource = OMXCodec::Create(
mClient.interface(), mVideoTrack->getFormat(),
false, // createEncoder
mVideoTrack,
NULL, flags, USE_SURFACE_ALLOC ? mNativeWindow : NULL);
status_t err = mVideoSource->start();
return mVideoSource != NULL ? OK : UNKNOWN_ERROR;
}

保留主要部分，去除编码相关

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sp<MediaSource> OMXCodec::Create(
const sp<IOMX> &omx,
const sp<MetaData> &meta, bool createEncoder,
const sp<MediaSource> &source,
const char *matchComponentName,
uint32_t flags,
const sp<ANativeWindow> &nativeWindow) {
int32_t requiresSecureBuffers;

const char *mime;
bool success = meta->findCString(kKeyMIMEType, &mime);
CHECK(success);

Vector<String8> matchingCodecs;
Vector<uint32_t> matchingCodecQuirks;
findMatchingCodecs(
mime, createEncoder, matchComponentName, flags,
&matchingCodecs, &matchingCodecQuirks);

sp<OMXCodecObserver> observer = new OMXCodecObserver; IOMX::node_id node = 0;

for (size_t i = 0; i < matchingCodecs.size(); ++i) {
const char *componentNameBase = matchingCodecs[i].string();
uint32_t quirks = matchingCodecQuirks[i];
const char *componentName = componentNameBase;

AString tmp;

status_t err = omx->allocateNode(componentName, observer, &node);
if (err == OK) {
ALOGV("Successfully allocated OMX node '%s'", componentName);

sp<OMXCodec> codec = new OMXCodec(
omx, node, quirks, flags,
createEncoder, mime, componentName,
source, nativeWindow);

observer->setCodec(codec);

err = codec->configureCodec(meta);

if (err == OK) {
if (!strcmp("OMX.Nvidia.mpeg2v.decode", componentName)) {
codec->mFlags |= kOnlySubmitOneInputBufferAtOneTime;
}

return codec;
}

ALOGV("Failed to configure codec '%s'", componentName);
}
}

return NULL;
}

sp<MediaSource> OMXCodec::Create(
const sp<IOMX> &omx,
const sp<MetaData> &meta, bool createEncoder,
const sp<MediaSource> &source,
const char *matchComponentName,
uint32_t flags,
const sp<ANativeWindow> &nativeWindow) {
int32_t requiresSecureBuffers;

const char *mime;
bool success = meta->findCString(kKeyMIMEType, &mime);
CHECK(success);

Vector<String8> matchingCodecs;
Vector<uint32_t> matchingCodecQuirks;
findMatchingCodecs(
mime, createEncoder, matchComponentName, flags,
&matchingCodecs, &matchingCodecQuirks);

sp<OMXCodecObserver> observer = new OMXCodecObserver;
IOMX::node_id node = 0;

for (size_t i = 0; i < matchingCodecs.size(); ++i) {
const char *componentNameBase = matchingCodecs[i].string();
uint32_t quirks = matchingCodecQuirks[i];
const char *componentName = componentNameBase;

AString tmp;

status_t err = omx->allocateNode(componentName, observer, &node);
if (err == OK) {
ALOGV("Successfully allocated OMX node '%s'", componentName);

sp<OMXCodec> codec = new OMXCodec(
omx, node, quirks, flags,
createEncoder, mime, componentName,
source, nativeWindow);

observer->setCodec(codec);

err = codec->configureCodec(meta);

if (err == OK) {
if (!strcmp("OMX.Nvidia.mpeg2v.decode", componentName)) {
codec->mFlags |= kOnlySubmitOneInputBufferAtOneTime;
}

return codec;
}

ALOGV("Failed to configure codec '%s'", componentName);
}
}

return NULL;
}

1.根据mVideoTrack传进来的视频信息，查找相匹配的解码器。

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bool success = meta->findCString(kKeyMIMEType, &mime);
findMatchingCodecs(
mime, createEncoder, matchComponentName, flags,
&matchingCodecs, &matchingCodecQuirks);

bool success = meta->findCString(kKeyMIMEType, &mime);
findMatchingCodecs(
mime, createEncoder, matchComponentName, flags,
&matchingCodecs, &matchingCodecQuirks);

2. 创建OMXCodecObserver 实例，OMXCodecObserver功能后续会详细介绍。创建一个node 并初始化为0.

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sp<OMXCodecObserver> observer = new OMXCodecObserver; IOMX::node_id node = 0;

sp<OMXCodecObserver> observer = new OMXCodecObserver;
IOMX::node_id node = 0;

3. 通过omx入口 依靠binder 机制调用OMX服务中的allocateNode()，这一步把匹配得到的解码器组件名、OMXCodecObserver实例和初始化为0的node一并传入。

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status_t err = omx->allocateNode(componentName, observer, &node);

status_t err = omx->allocateNode(componentName, observer, &node);

这个allocateNode 就是文章最开始讲的，在OMX那头创建一个和mVideoSource相匹配的解码实例。用node值作为唯一标识。

让我们来看看真正的omx中allocateNode做了啥？

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status_t OMX::allocateNode( const char *name, const sp<IOMXObserver> &observer, node_id *node) { Mutex::Autolock autoLock(mLock); *node = 0; OMXNodeInstance *instance = new OMXNodeInstance(this, observer); OMX_COMPONENTTYPE *handle; OMX_ERRORTYPE err = mMaster->makeComponentInstance( name, &OMXNodeInstance::kCallbacks, instance, &handle); if (err != OMX_ErrorNone) { ALOGV("FAILED to allocate omx component '%s'", name); instance->onGetHandleFailed(); return UNKNOWN_ERROR; } *node = makeNodeID(instance); mDispatchers.add(*node, new CallbackDispatcher(instance)); instance->setHandle(*node, handle); mLiveNodes.add(observer->asBinder(), instance); observer->asBinder()->linkToDeath(this); return OK; }

status_t OMX::allocateNode(
const char *name, const sp<IOMXObserver> &observer, node_id *node) {
Mutex::Autolock autoLock(mLock);

*node = 0;

OMXNodeInstance *instance = new OMXNodeInstance(this, observer);

OMX_COMPONENTTYPE *handle;
OMX_ERRORTYPE err = mMaster->makeComponentInstance(
name, &OMXNodeInstance::kCallbacks,
instance, &handle);

if (err != OMX_ErrorNone) {
ALOGV("FAILED to allocate omx component '%s'", name);

instance->onGetHandleFailed();

return UNKNOWN_ERROR;
}

*node = makeNodeID(instance);
mDispatchers.add(*node, new CallbackDispatcher(instance));

instance->setHandle(*node, handle);

mLiveNodes.add(observer->asBinder(), instance);
observer->asBinder()->linkToDeath(this);

return OK;
}

创建一个OMXNodeInstance实例。
通过mMaster->makeComponentInstance创建真正解码器的组件，并通过handle与OMXNodeInstance关联。
所以说mMaster->makeComponentInstance这里是建立解码器组件的核心。会把mVideoSource需要的解码器name一直传递下去。

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OMX_ERRORTYPE OMXMaster::makeComponentInstance( const char *name, const OMX_CALLBACKTYPE *callbacks, OMX_PTR appData, OMX_COMPONENTTYPE **component) { Mutex::Autolock autoLock(mLock); *component = NULL; ssize_t index = mPluginByComponentName.indexOfKey(String8(name)); if (index < 0) { return OMX_ErrorInvalidComponentName; } OMXPluginBase *plugin = mPluginByComponentName.valueAt(index); OMX_ERRORTYPE err = plugin->makeComponentInstance(name, callbacks, appData, component); if (err != OMX_ErrorNone) { return err; } mPluginByInstance.add(*component, plugin); return err; }

OMX_ERRORTYPE OMXMaster::makeComponentInstance(
const char *name,
const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData,
OMX_COMPONENTTYPE **component) {
Mutex::Autolock autoLock(mLock);

*component = NULL;

ssize_t index = mPluginByComponentName.indexOfKey(String8(name));

if (index < 0) {
return OMX_ErrorInvalidComponentName;
}

OMXPluginBase *plugin = mPluginByComponentName.valueAt(index);
OMX_ERRORTYPE err =
plugin->makeComponentInstance(name, callbacks, appData, component);

if (err != OMX_ErrorNone) {
return err;
}

mPluginByInstance.add(*component, plugin);

return err;
}

最开始OMXMaster通过 addPlugin(new SoftOMXPlugin);把支持的软解码放在mPluginByComponentName中，在makeComponentInstance中通过上面传下来的解码器的name值从mPluginByComponentName找到相对应的plugin，然后调用 plugin->makeComponentInstance(name,
callbacks, appData, component);
这里的plugin 值得就是软解SoftOMXPlugin 也就是调用了

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OMX_ERRORTYPE SoftOMXPlugin::makeComponentInstance(
const char *name,
const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData,
OMX_COMPONENTTYPE **component) {
ALOGV("makeComponentInstance '%s'", name);

for (size_t i = 0; i < kNumComponents; ++i) {
if (strcmp(name, kComponents[i].mName)) {
continue;
}

AString libName = "libstagefright_soft_";
libName.append(kComponents[i].mLibNameSuffix);
libName.append(".so");

void *libHandle = dlopen(libName.c_str(), RTLD_NOW);

if (libHandle == NULL) {
ALOGE("unable to dlopen %s", libName.c_str());

return OMX_ErrorComponentNotFound;
}

typedef SoftOMXComponent *(*CreateSoftOMXComponentFunc)(
const char *, const OMX_CALLBACKTYPE *,
OMX_PTR, OMX_COMPONENTTYPE **);

CreateSoftOMXComponentFunc createSoftOMXComponent =
(CreateSoftOMXComponentFunc)dlsym(
libHandle,
"_Z22createSoftOMXComponentPKcPK16OMX_CALLBACKTYPE"
"PvPP17OMX_COMPONENTTYPE");

if (createSoftOMXComponent == NULL) {
dlclose(libHandle);
libHandle = NULL;

return OMX_ErrorComponentNotFound;
}

sp<SoftOMXComponent> codec =
(*createSoftOMXComponent)(name, callbacks, appData, component);

if (codec == NULL) {
dlclose(libHandle);
libHandle = NULL;

return OMX_ErrorInsufficientResources;
}

OMX_ERRORTYPE err = codec->initCheck();
if (err != OMX_ErrorNone) {
dlclose(libHandle);
libHandle = NULL;

return err;
}

codec->incStrong(this);
codec->setLibHandle(libHandle);

return OMX_ErrorNone;
}

return OMX_ErrorInvalidComponentName;
}

OMX_ERRORTYPE SoftOMXPlugin::makeComponentInstance(
const char *name,
const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData,
OMX_COMPONENTTYPE **component) {
ALOGV("makeComponentInstance '%s'", name);

for (size_t i = 0; i < kNumComponents; ++i) {
if (strcmp(name, kComponents[i].mName)) {
continue;
}

AString libName = "libstagefright_soft_";
libName.append(kComponents[i].mLibNameSuffix);
libName.append(".so");

void *libHandle = dlopen(libName.c_str(), RTLD_NOW);

if (libHandle == NULL) {
ALOGE("unable to dlopen %s", libName.c_str());

return OMX_ErrorComponentNotFound;
}

typedef SoftOMXComponent *(*CreateSoftOMXComponentFunc)(
const char *, const OMX_CALLBACKTYPE *,
OMX_PTR, OMX_COMPONENTTYPE **);

CreateSoftOMXComponentFunc createSoftOMXComponent =
(CreateSoftOMXComponentFunc)dlsym(
libHandle,
"_Z22createSoftOMXComponentPKcPK16OMX_CALLBACKTYPE"
"PvPP17OMX_COMPONENTTYPE");

if (createSoftOMXComponent == NULL) {
dlclose(libHandle);
libHandle = NULL;

return OMX_ErrorComponentNotFound;
}

sp<SoftOMXComponent> codec =
(*createSoftOMXComponent)(name, callbacks, appData, component);

if (codec == NULL) {
dlclose(libHandle);
libHandle = NULL;

return OMX_ErrorInsufficientResources;
}

OMX_ERRORTYPE err = codec->initCheck();
if (err != OMX_ErrorNone) {
dlclose(libHandle);
libHandle = NULL;

return err;
}

codec->incStrong(this);
codec->setLibHandle(libHandle);

return OMX_ErrorNone;
}

return OMX_ErrorInvalidComponentName;
}

通过上面传下来的解码器的name，找到对应库的名字。假如是264的话，要加载的库就是 libstagefright_soft_h264dec.so,也就是对应上层264解码的话，omx解码组件会加载对应的libstagefright_soft_h264dec.so库。相对应的软解代码在
Android4.1.1\frameworks\av\media\libstagefright\codecs\on2\h264dec 中。
加载完264解码库后
通过dlopen、dlsym来调用库中函数。
通过调用 SoftAVC 中的 createSoftOMXComponent 来创建真正264解码器实例SoftOMXComponent。以后真正视频解码的工作都是通过avc 这个SoftAVC实例完成的

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android::SoftOMXComponent *createSoftOMXComponent( const char *name, const OMX_CALLBACKTYPE *callbacks, OMX_PTR appData, OMX_COMPONENTTYPE **component) { return new android::SoftAVC(name, callbacks, appData, component); }

android::SoftOMXComponent *createSoftOMXComponent(
const char *name, const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData, OMX_COMPONENTTYPE **component) {
return new android::SoftAVC(name, callbacks, appData, component);
}

经过这一路下来，终于完成了解码器的创建工作。简单总结一下。
1.AwesomePlayer中通过initVideoDecoder 来创建video解码器mVideoSource
2.mVideoSource 中通过上部分demux后的视频流 mVideoTrack来获得解码器的类型，通过类型调用omx->allocateNode 创建omx node实例与自己对应。以后都是通过node实例来操作解码器。
3.在 omx->allocateNode中 通过mMaster->makeComponentInstance 来创建真正对应的解码器组件。这个解码器组件是完成之后解码实际工作的。
4.在创建mMaster->makeComponentInstance过程中，也是通过上面mVideoTrack
过来的解码器类型名，找到相对应的解码器的库，然后实例化。