"本文转载自：[VNanyesheshou]的Android MultiMedia框架——OMXPlugin"
1.概述
上一篇《Android多媒体框架--19：OMX服务启动》中提到OMXMaster添加两个Plugin，一个是SoftOMXPlugin，另一个是供应商实现的硬件Plugin，其父类是OMXPluginBase。
2.OMXPluginBase
OMXPluginBase主要包含四个虚拟方法。硬件厂商接入自己的编解码器，需要继承OMXPluginBase 类，并实现抽象方法。
- frameworks/native/headers/media_plugin/media/hardware/OMXPluginBase.h
struct OMXPluginBase {
OMXPluginBase() {}
virtual ~OMXPluginBase() {}
virtual OMX_ERRORTYPE makeComponentInstance(
const char *name,
const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData,
OMX_COMPONENTTYPE **component) = 0;
virtual OMX_ERRORTYPE destroyComponentInstance(
OMX_COMPONENTTYPE *component) = 0;
virtual OMX_ERRORTYPE enumerateComponents(
OMX_STRING name,
size_t size,
OMX_U32 index) = 0;
virtual OMX_ERRORTYPE getRolesOfComponent(
const char *name,
Vector<String8> *roles) = 0;
private:
OMXPluginBase(const OMXPluginBase &);
OMXPluginBase &operator=(const OMXPluginBase &);
};
下面先看下硬件厂商实现的Plugin。
3.QComOMXPlugin
&emspQComOMXPlugin继承自OMXPluginBase，是高通实现的硬编解码。
- hardware/qcom/media/**/libstagefrighthw/QComOMXPlugin.cpp
OMXPluginBase *createOMXPlugin() {
//（1）创建QComOMXPlugin
return new QComOMXPlugin;
}
---------------
QComOMXPlugin::QComOMXPlugin()
//（2）加载libOmxCore.so
: mLibHandle(dlopen("libOmxCore.so", RTLD_NOW)),
mInit(NULL),
mDeinit(NULL),
mComponentNameEnum(NULL),
mGetHandle(NULL),
mFreeHandle(NULL),
mGetRolesOfComponentHandle(NULL) {
if (mLibHandle != NULL) {
//（3）通过dlsym查找OMX_Init、OMX_Deinit等符号值
mInit = (InitFunc)dlsym(mLibHandle, "OMX_Init");
mDeinit = (DeinitFunc)dlsym(mLibHandle, "OMX_Deinit");
mComponentNameEnum =
(ComponentNameEnumFunc)dlsym(mLibHandle, "OMX_ComponentNameEnum");
mGetHandle = (GetHandleFunc)dlsym(mLibHandle, "OMX_GetHandle");
mFreeHandle = (FreeHandleFunc)dlsym(mLibHandle, "OMX_FreeHandle");
mGetRolesOfComponentHandle =
(GetRolesOfComponentFunc)dlsym(
mLibHandle, "OMX_GetRolesOfComponent");
if (!mInit || !mDeinit || !mComponentNameEnum || !mGetHandle ||
!mFreeHandle || !mGetRolesOfComponentHandle) {
dlclose(mLibHandle);
mLibHandle = NULL;
} else
(*mInit)();
}
}
（1）OMXMaster通过createOMXPlugin创建QComOMXPlugin对象；
（2）QComOMXPlugin构造函数中加载libOmxCore.so；
（3）通过dlsym查找OMX_Init、OMX_Deinit等符号值；
（4）这些方法对应OMXPluginBase中的抽象方法。
libOmxCore.so中为厂商高通实现的具体硬件编解码功能，这里先不分析了。
4.SoftOMXPlugin
SoftOMXPlugin是google提供的原生编解码插件，它并不负责实现编解码，而是负责管理加载编解码器。如下是其支持的音视频编解码格式，这些都是软件编解码。
- frameworks/av/media/libstagefright/omx/SoftOMXPlugin.cpp
static const struct {
const char *mName;
const char *mLibNameSuffix;
const char *mRole;
} kComponents[] = {
// two choices for aac decoding.
// configurable in media/libstagefright/data/media_codecs_google_audio.xml
// default implementation
{ "OMX.google.aac.decoder", "aacdec", "audio_decoder.aac" },
// alternate implementation
{ "OMX.google.xaac.decoder", "xaacdec", "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", "avcdec", "video_decoder.avc" },
{ "OMX.google.h264.encoder", "avcenc", "video_encoder.avc" },
{ "OMX.google.hevc.decoder", "hevcdec", "video_decoder.hevc" },
{ "OMX.google.g711.alaw.decoder", "g711dec", "audio_decoder.g711alaw" },
{ "OMX.google.g711.mlaw.decoder", "g711dec", "audio_decoder.g711mlaw" },
{ "OMX.google.mpeg2.decoder", "mpeg2dec", "video_decoder.mpeg2" },
{ "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.opus.decoder", "opusdec", "audio_decoder.opus" },
{ "OMX.google.vp8.decoder", "vpxdec", "video_decoder.vp8" },
{ "OMX.google.vp9.decoder", "vpxdec", "video_decoder.vp9" },
{ "OMX.google.vp8.encoder", "vpxenc", "video_encoder.vp8" },
{ "OMX.google.vp9.encoder", "vpxenc", "video_encoder.vp9" },
{ "OMX.google.raw.decoder", "rawdec", "audio_decoder.raw" },
{ "OMX.google.flac.decoder", "flacdec", "audio_decoder.flac" },
{ "OMX.google.flac.encoder", "flacenc", "audio_encoder.flac" },
{ "OMX.google.gsm.decoder", "gsmdec", "audio_decoder.gsm" },
};
makeComponentInstance函数：
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");
// RTLD_NODELETE means we keep the shared library around forever.
// this eliminates thrashing during sequences like loading soundpools.
// It also leaves the rest of the logic around the dlopen()/dlclose()
// calls in this file unchanged.
//
// Implications of the change:
// -- the codec process (where this happens) will have a slightly larger
// long-term memory footprint as it accumulates the loaded shared libraries.
// This is expected to be a small amount of memory.
// -- plugin codecs can no longer (and never should have) depend on a
// free reset of any static data as the library would have crossed
// a dlclose/dlopen cycle.
//
//（1）动态加载对应的编解码器
void *libHandle = dlopen(libName.c_str(), RTLD_NOW|RTLD_NODELETE);
if (libHandle == NULL) {
ALOGE("unable to dlopen %s: %s", libName.c_str(), dlerror());
return OMX_ErrorComponentNotFound;
}
typedef SoftOMXComponent *(*CreateSoftOMXComponentFunc)(
const char *, const OMX_CALLBACKTYPE *,
OMX_PTR, OMX_COMPONENTTYPE **);
//（2）dlsym查找createSoftOMXComponent符号值
CreateSoftOMXComponentFunc createSoftOMXComponent =
(CreateSoftOMXComponentFunc)dlsym(
libHandle,
"_Z22createSoftOMXComponentPKcPK16OMX_CALLBACKTYPE"
"PvPP17OMX_COMPONENTTYPE");
if (createSoftOMXComponent == NULL) {
dlclose(libHandle);
libHandle = NULL;
return OMX_ErrorComponentNotFound;
}
// （3）创建编解码器
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;
}
（1）该函数通过ACodec传递的name参数，动态加载对应的编解码器，如 libstagefright_soft_aacdec.so。其包含的so如下：
01.png
（2）dlsym 查找createSoftOMXComponent符号值；
（3）创建编解码器；
android::SoftOMXComponent *createSoftOMXComponent(
const char *name, const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData, OMX_COMPONENTTYPE **component) {
return new android::SoftAACEncoder2(name, callbacks, appData, component);
}
SoftAACEncoder2为aac编码器对应的实现类。其继承关系如下：
02.png
5.总结
OMXPlugin分为软硬编解码库，分别加载不同的so，实现其对应的编解码。
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