Android-View繪制原理(14)-RenderPipeline

在上一篇關(guān)于幀繪制的原理中，做好了EGLSuface切換，同步好了UI的更新，為需要進(jìn)行GPU繪制的RenderNode創(chuàng)好了SKSurface, 最后通過(guò)ANativeWindow為下一幀調(diào)用了dequeueBuffer。所有的資源和數(shù)據(jù)都準(zhǔn)備好了，從而可以進(jìn)行繪制，這個(gè)任務(wù)將由RenderPipeline來(lái)完成。我們先不考慮Fence的邏輯，直接接著上一篇文章，從CanvasContext.draw方法出發(fā)。

1 CanvasContext.draw

frameworks/base/libs/hwui/renderthread/CanvasContext.cpp

nsecs_t CanvasContext::draw() {
    ...
    Frame frame = mRenderPipeline->getFrame();
    SkRect windowDirty = computeDirtyRect(frame, &dirty);

    bool drew = mRenderPipeline->draw(frame, windowDirty, dirty, mLightGeometry, &mLayerUpdateQueue,
                                      mContentDrawBounds, mOpaque, mLightInfo, mRenderNodes,
                                      &(profiler()));
    ...
   bool didSwap =  mRenderPipeline->swapBuffers(frame, drew, windowDirty, mCurrentFrameInfo, &requireSwap);

    ...
     for (auto& func : mFrameCompleteCallbacks) {
            std::invoke(func, frameCompleteNr);
        }
        mFrameCompleteCallbacks.clear();
    }
   ...
    cleanupResources();
    mRenderThread.cacheManager().onFrameCompleted();
    return mCurrentFrameInfo->get(FrameInfoIndex::DequeueBufferDuration);
}

這個(gè)draw方法比較復(fù)雜，這里摘取主要邏輯來(lái)分析一下。可以看到具體的繪制是繼續(xù)委托到mRenderPipeline去完成的，僅僅分析一下使用OpenGL繪制的情況，因此對(duì)應(yīng)的SkiaOpenGLPipeline。這里主要由這個(gè)幾個(gè)步驟

  • 創(chuàng)建Frame對(duì)象
  • 調(diào)用mRenderPipeline->draw進(jìn)行繪制
  • mRenderPipeline->swapBuffers 切換GragphicBuffer
  • 回調(diào)FrameCompleteCalback。

下面就按這個(gè)流程來(lái)分析

2 mRenderPipeline->getFrame

Frame是一個(gè)幀的模型

class Frame {
public:
    Frame(int32_t width, int32_t height, int32_t bufferAge)
            : mWidth(width), mHeight(height), mBufferAge(bufferAge) {}

    int32_t width() const { return mWidth; }
    int32_t height() const { return mHeight; }
    int32_t bufferAge() const { return mBufferAge; }

private:
    Frame() {}
    friend class EglManager;

    int32_t mWidth;
    int32_t mHeight;
    int32_t mBufferAge;

    EGLSurface mSurface;

    // Maps from 0,0 in top-left to 0,0 in bottom-left
    // If out is not an int32_t[4] you're going to have a bad time
    void map(const SkRect& in, int32_t* out) const;
};

它封裝的是一個(gè)EGLSurface, 前面分析過(guò)，EGLSurface關(guān)聯(lián)著一個(gè)ANativeWindow, 也就是一個(gè)Surface對(duì)象，所以Frame可以代表一個(gè)Surface對(duì)象。

frameworks/base/libs/hwui/pipeline/skia/SkiaOpenGLPipeline.cpp

Frame SkiaOpenGLPipeline::getFrame() {
    LOG_ALWAYS_FATAL_IF(mEglSurface == EGL_NO_SURFACE,
                        "drawRenderNode called on a context with no surface!");
    return mEglManager.beginFrame(mEglSurface);
}

進(jìn)入到mEglManager的beginFrame方法

Frame EglManager::beginFrame(EGLSurface surface) {
    LOG_ALWAYS_FATAL_IF(surface == EGL_NO_SURFACE, "Tried to beginFrame on EGL_NO_SURFACE!");
    makeCurrent(surface);
    Frame frame;
    frame.mSurface = surface;
    eglQuerySurface(mEglDisplay, surface, EGL_WIDTH, &frame.mWidth);
    eglQuerySurface(mEglDisplay, surface, EGL_HEIGHT, &frame.mHeight);
    frame.mBufferAge = queryBufferAge(surface);
    eglBeginFrame(mEglDisplay, surface);
    return frame;
}

將這個(gè)surface切換到當(dāng)前后，新創(chuàng)建一個(gè)Frame對(duì)象，并將surface賦給這個(gè)frame對(duì)象，后設(shè)在對(duì)象的長(zhǎng)寬屬性等，然后返回這個(gè)新的Frame對(duì)象。

3 mRenderPipeline->draw

使用OpenGL來(lái)繪制的時(shí)候，mRenderPipeline是一個(gè)SkiaOpenGLPipeline對(duì)象，它是SkiaPipeline的子類，我們來(lái)分析一下它的draw方法

frameworks/base/libs/hwui/pipeline/skia/SkiaOpenGLPipeline.cpp

bool SkiaOpenGLPipeline::draw(const Frame& frame, const SkRect& screenDirty, const SkRect& dirty,
                              const LightGeometry& lightGeometry,
                              LayerUpdateQueue* layerUpdateQueue, const Rect& contentDrawBounds,
                              bool opaque, const LightInfo& lightInfo,
                              const std::vector<sp<RenderNode>>& renderNodes,
                              FrameInfoVisualizer* profiler) {
    ...
    GrGLFramebufferInfo fboInfo;
    fboInfo.fFBOID = 0;
    ....
    GrBackendRenderTarget backendRT(frame.width(), frame.height(), 0, STENCIL_BUFFER_SIZE, fboInfo);
    ...
    SkSurfaceProps props(0, kUnknown_SkPixelGeometry);
     sk_sp<SkSurface> surface(SkSurface::MakeFromBackendRenderTarget(
            mRenderThread.getGrContext(), backendRT, this->getSurfaceOrigin(), colorType,
            mSurfaceColorSpace, &props));
     ...
    renderFrame(*layerUpdateQueue, dirty, renderNodes, opaque, contentDrawBounds, surface,
                SkMatrix::I());

    ...
    {
        ATRACE_NAME("flush commands");
        surface->flushAndSubmit();
    }
    layerUpdateQueue->clear();
    
    return true;
}

這里首先生成了一個(gè)GrGLFramebufferInfo和GrBackendRenderTarget，它是屬于skia庫(kù)里的api，但是只包含一些j簡(jiǎn)單的屬性信息。

external/skia/include/gpu/gl/GrGLTypes.h

struct GrGLFramebufferInfo {
    GrGLuint fFBOID;
    GrGLenum fFormat = 0;

    bool operator==(const GrGLFramebufferInfo& that) const {
        return fFBOID == that.fFBOID && fFormat == that.fFormat;
    }
};

external/skia/src/gpu/GrBackendSurface.cpp

GrBackendRenderTarget::GrBackendRenderTarget(int width,
                                             int height,
                                             int sampleCnt,
                                             int stencilBits,
                                             const GrGLFramebufferInfo& glInfo)
        : fWidth(width)
        , fHeight(height)
        , fSampleCnt(std::max(1, sampleCnt))
        , fStencilBits(stencilBits)
        , fBackend(GrBackendApi::kOpenGL)
        , fGLInfo(glInfo) {
    fIsValid = SkToBool(glInfo.fFormat); // the glInfo must have a valid format
}

隨后調(diào)用SkSurface::MakeFromBackendRenderTarget生成一個(gè)SkSurface，這是Skia在GPU上申請(qǐng)用于繪制的surface。我們來(lái)看看這個(gè)sksurface 的生成流程。

sk_sp<SkSurface> SkSurface::MakeFromBackendRenderTarget(GrRecordingContext* context,
                                                        const GrBackendRenderTarget& rt,
                                                        GrSurfaceOrigin origin,
                                                        SkColorType colorType,
                                                        sk_sp<SkColorSpace> colorSpace,
                                                        const SkSurfaceProps* props,
                                                        SkSurface::RenderTargetReleaseProc relProc,
                                                        SkSurface::ReleaseContext releaseContext) {
    ...
    auto sdc = GrSurfaceDrawContext::MakeFromBackendRenderTarget(context,
                                                                 grColorType,
                                                                 std::move(colorSpace),
                                                                 rt,
                                                                 origin,
                                                                 SkSurfacePropsCopyOrDefault(props),
                                                                 std::move(releaseHelper));
    if (!sdc) {
        return nullptr;
    }

    auto device = SkGpuDevice::Make(std::move(sdc), SkGpuDevice::kUninit_InitContents);
    if (!device) {
        return nullptr;
    }

    return sk_make_sp<SkSurface_Gpu>(std::move(device));
}

首先生成一個(gè)GrSurfaceDrawContext的對(duì)象sdc，由它來(lái)持有上面生成的GrBackendRenderTarget和GrRecordingContext，

sk_sp<SkGpuDevice> SkGpuDevice::Make(std::unique_ptr<GrSurfaceDrawContext> surfaceDrawContext,
                                     InitContents init) {
    if (!surfaceDrawContext) {
        return nullptr;
    }

    GrRecordingContext* rContext = surfaceDrawContext->recordingContext();
    if (rContext->abandoned()) {
        return nullptr;
    }

    SkColorType ct = GrColorTypeToSkColorType(surfaceDrawContext->colorInfo().colorType());

    unsigned flags;
    if (!rContext->colorTypeSupportedAsSurface(ct) ||
        !CheckAlphaTypeAndGetFlags(nullptr, init, &flags)) {
        return nullptr;
    }
    return sk_sp<SkGpuDevice>(new SkGpuDevice(std::move(surfaceDrawContext), flags));
}


SkGpuDevice::SkGpuDevice(std::unique_ptr<GrSurfaceDrawContext> surfaceDrawContext, unsigned flags)
        : INHERITED(make_info(surfaceDrawContext.get(), SkToBool(flags & kIsOpaque_Flag)),
                    surfaceDrawContext->surfaceProps())
        , fContext(sk_ref_sp(surfaceDrawContext->recordingContext()))
        , fSurfaceDrawContext(std::move(surfaceDrawContext))
#if !defined(SK_DISABLE_NEW_GR_CLIP_STACK)
        , fClip(SkIRect::MakeSize(fSurfaceDrawContext->dimensions()),
                &this->asMatrixProvider(),
                force_aa_clip(fSurfaceDrawContext.get())) {
#else
        , fClip(fSurfaceDrawContext->dimensions(), &this->cs(), &this->asMatrixProvider()) {
#endif
    if (flags & kNeedClear_Flag) {
        this->clearAll();
    }
}

這里創(chuàng)將了一個(gè)SkGpuDevice對(duì)象，它的成員變量 fContext 是通過(guò)外部傳入的 surfaceDrawContext 調(diào)用 recordingContext 方法的得來(lái)的，而這個(gè)surfaceDrawContext就是上面的 sdc 局部變量，它的 recordingContext 實(shí)質(zhì)上來(lái)自 mRenderThread.getGrContext() 方法。因此 SkGpuDevice的fContext指向的是mRenderThread.getGrContext()返回的對(duì)象 。

之后此構(gòu)建了一個(gè)SkSurface_Gpu對(duì)象，它是SkSurface的子類，因?yàn)閭魅氲氖荢kGpuDevice，所以繪制命令將通過(guò)它提交到GPU進(jìn)行像素渲染。準(zhǔn)備好了SkSurface之后，調(diào)用renderFrame在該SkSurface上繪制。最后調(diào)用 surface->flushAndSubmit();提交到GPU。這里的內(nèi)容比較多，在后面的文章中再展開。

4 總結(jié)

本文分析了幀繪制的流程，這個(gè)抽象成了一個(gè)RenderPipeline，根據(jù)使用不同渲染引擎，提供了SkiaOpenGLPipeline和SkiaVulkanPipeline兩個(gè)實(shí)現(xiàn)，本文僅僅分析SkiaOpenGLPipeline，它的繪制總共被分成了3個(gè)步驟：

  • 創(chuàng)建SkSurface
  • renderFrame 將記錄的描述數(shù)據(jù)記錄的SkSurface
  • flushAndSubmit 提交繪制命令到GPU進(jìn)行像素渲染

在更大的視角上看，繪制的步驟包含：

  • 繪制前準(zhǔn)備Frame模型
  • SkiaOpenGLPipeline 調(diào)用GPU繪制
  • mRenderPipeline->swapBuffers 通知HWComposer進(jìn)行屏幕合成
  • 回調(diào)FrameCompleteCalback結(jié)尾
最后編輯于 ：
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