前言

RenderEngine是一個靜態(tài)庫，主要作用于SurfaceFlinger進行client layer合成。
簡單理解就是將client buffer layer合成到FramebufferSurface

frameworks\native\libs\renderengine\

一、RenderEngineType

RenderEngineType是指RenderEngine的合成方式實現(xiàn)類型，目前有四種類型實現(xiàn)如下。

enum class RenderEngineType {
    GLES = 1,//opengl
    THREADED = 2,//opengl異步線程
    SKIA_GL = 3,//skia
    SKIA_GL_THREADED = 4,//skia異步線程
};

異步的意思會另起一個名叫RenderEngine的線程中處理合成任務(wù)。
同步的意思是會在SurfaceFlinger的主線程中處理合成任務(wù)
如圖：異步處理合成任務(wù)的Trace。

二、RenderEngine初始化流程

基于最新的代碼安卓代碼

1.1 SurfaceFlinger.cpp

SurfaceFlinger init的時候會構(gòu)建RenderEngine，并且確認RenderEngineType

frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

void SurfaceFlinger::init() {
...
    // Get a RenderEngine for the given display / config (can't fail)
    // TODO(b/77156734): We need to stop casting and use HAL types when possible.
    // Sending maxFrameBufferAcquiredBuffers as the cache size is tightly tuned to single-display.
    mCompositionEngine->setRenderEngine(renderengine::RenderEngine::create(
            renderengine::RenderEngineCreationArgs::Builder()
                    .setPixelFormat(static_cast<int32_t>(defaultCompositionPixelFormat))
                    .setImageCacheSize(maxFrameBufferAcquiredBuffers)
                    .setUseColorManagerment(useColorManagement)
                    .setEnableProtectedContext(enable_protected_contents(false))
                    .setPrecacheToneMapperShaderOnly(false)
                    .setSupportsBackgroundBlur(mSupportsBlur)
                    .setContextPriority(
                            useContextPriority
                                    ? renderengine::RenderEngine::ContextPriority::REALTIME
                                    : renderengine::RenderEngine::ContextPriority::MEDIUM)
                    .build()));//跳轉(zhuǎn)到1.2
    mMaxRenderTargetSize =
            std::min(getRenderEngine().getMaxTextureSize(), getRenderEngine().getMaxViewportDims());

    ....
}

1.2 RenderEngine

提供了setRenderEngineType的接口，默認缺省值是SKIA_GL_THREADED

frameworks/native/libs/renderengine/include/renderengine/RenderEngine.h

struct RenderEngineCreationArgs::Builder {
...
    Builder& setRenderEngineType(RenderEngine::RenderEngineType renderEngineType) {
        this->renderEngineType = renderEngineType;
        return *this;
    }
...

private:
    // 1 means RGBA_8888
    int pixelFormat = 1;
    uint32_t imageCacheSize = 0;
    bool useColorManagement = true;
    bool enableProtectedContext = false;
    bool precacheToneMapperShaderOnly = false;
    bool supportsBackgroundBlur = false;
    RenderEngine::ContextPriority contextPriority = RenderEngine::ContextPriority::MEDIUM;
    RenderEngine::RenderEngineType renderEngineType =
            RenderEngine::RenderEngineType::SKIA_GL_THREADED;//缺省值
};

} // namespace renderengine

可以通過設(shè)置"debug.renderengine.backend"，改變RenderEngineType。
配置項：gles,threaded, skiagl,skiaglthread 分別對應(yīng)四種類型

frameworks/native/libs/renderengine/RenderEngine.cpp

#define PROPERTY_DEBUG_RENDERENGINE_BACKEND "debug.renderengine.backend"

std::unique_ptr<RenderEngine> RenderEngine::create(RenderEngineCreationArgs args) {
    // Keep the ability to override by PROPERTIES:
    char prop[PROPERTY_VALUE_MAX];
    property_get(PROPERTY_DEBUG_RENDERENGINE_BACKEND, prop, "");
    if (strcmp(prop, "gles") == 0) {
        args.renderEngineType = RenderEngineType::GLES;
    }
    if (strcmp(prop, "threaded") == 0) {
        args.renderEngineType = RenderEngineType::THREADED;
    }
    if (strcmp(prop, "skiagl") == 0) {
        args.renderEngineType = RenderEngineType::SKIA_GL;
    }
    if (strcmp(prop, "skiaglthreaded") == 0) {
        args.renderEngineType = RenderEngineType::SKIA_GL_THREADED;
    }

    switch (args.renderEngineType) {
        case RenderEngineType::THREADED:
            ALOGD("Threaded RenderEngine with GLES Backend");
            return renderengine::threaded::RenderEngineThreaded::create(
                    [args]() { return android::renderengine::gl::GLESRenderEngine::create(args); },
                    args.renderEngineType);
        case RenderEngineType::SKIA_GL:
            ALOGD("RenderEngine with SkiaGL Backend");
            return renderengine::skia::SkiaGLRenderEngine::create(args);
        case RenderEngineType::SKIA_GL_THREADED: {
            ALOGD("Threaded RenderEngine with SkiaGL Backend");
            return renderengine::threaded::RenderEngineThreaded::create(
                    [args]() {
                        return android::renderengine::skia::SkiaGLRenderEngine::create(args);
                    },
                    args.renderEngineType);
        }
        case RenderEngineType::GLES:
        default:
            ALOGD("RenderEngine with GLES Backend");
            return renderengine::gl::GLESRenderEngine::create(args);
    }
}

三、小結(jié)

android S以后原生配置的是SKIA_GL_THREADED，目前窗口的blurs效果，只在skia實現(xiàn)上
opengl模式不支持blurs。

Some new features (e.g. cross window blurs) are only implemented in Skia
RenderEngine. Turn it on for all Pixel devices. Other devices can still
turn it off with PROPERTY_DEBUG_RENDERENGINE_BACKEND.

細心你可能會發(fā)現(xiàn)前面SKIA_GL_THREADED的drawlayers中并沒有發(fā)揮異步的作用。本質(zhì)上還是會Block SF的主線程。

目前我只發(fā)現(xiàn)REThreaded::unmapExternalTextureBuffer這個方法是異步處理了，相比SKIA_GL，有一定程度上的性能提升。

但是據(jù)說可能一些極限情況的下并發(fā)讀寫的問題，我相信后續(xù)android會不斷的完善SKIA_GL_THREADED，后續(xù)可能就漸漸放棄GLES了。