Android SurfaceFlinger服务（七） ----- 图像的合成

android应用中申请的Surface在SurfaceFlinger服务中都有对应有图层Layer与之对应。将这些图层合并且输出到显示外设是SurfaceFlinger的工作重点。本文来分析下合成的过程。合成工作在接收到VSync消息（MessageQueue::REFRESH）后开始。

void SurfaceFlinger::onMessageReceived(int32_t what) {
    ATRACE_CALL();
    switch (what) {
        
        ......
        
        case MessageQueue::REFRESH: {  
            handleMessageRefresh();        
            break;
        }
    }  
}

• 调用消息处理函数handleMessageRefresh来进行显示刷新工作,图像合成与输出的核心就在这个处理函数里

void SurfaceFlinger::handleMessageRefresh() {
    ATRACE_CALL();

    ......
    
        preComposition();
        rebuildLayerStacks();
        setUpHWComposer();
        doDebugFlashRegions();
        doComposition();
        postComposition();
    
    ......


    previousExpectedPresent = mPrimaryDispSync.computeNextRefresh(0);
}

• preComposition、rebuildLayerStacks、setUpHWComposer图层合成准备工作
• setUpHWComposer会去设置图层属性compositionType（HWC_OVERLAY、HWC_FRAMEBUFFER等），这个属性将决定该图层在合成采用什么方式（硬件合成或软件合成）
• doComposition图层合成的主要工作，合成后的图像也在这里进行输出显示，重点关注
• postComposition主要做一些收尾工作，不是关注的重点

void SurfaceFlinger::doComposition() {
    ATRACE_CALL();
    const bool repaintEverything = android_atomic_and(0, &mRepaintEverything);
    for (size_t dpy=0 ; dpy<mDisplays.size() ; dpy++) {
        const sp<DisplayDevice>& hw(mDisplays[dpy]);
        if (hw->isDisplayOn()) {           
            // transform the dirty region into this screen's coordinate space
            const Region dirtyRegion(hw->getDirtyRegion(repaintEverything));

            // repaint the framebuffer (if needed)
            doDisplayComposition(hw, dirtyRegion);

            hw->dirtyRegion.clear();       
            hw->flip(hw->swapRegion);      
            hw->swapRegion.clear();    
        }
        // inform the h/w that we're done compositing
        hw->compositionComplete(); 
    }  
    postFramebuffer();
}

• doDisplayComposition函数对第个DisplayDevice的图层进行合成，在没有硬件合成模块的情况下，还进行输出显示
• postFramebuffer在有硬件合成模块时，提交图层到HWComposer

void SurfaceFlinger::doDisplayComposition(const sp<const DisplayDevice>& hw,
        const Region& inDirtyRegion)
{
    ......
    
    if (CC_LIKELY(!mDaltonize && !mHasColorMatrix)) {
        if (!doComposeSurfaces(hw, dirtyRegion)) return;
    } else {
        RenderEngine& engine(getRenderEngine());
        mat4 colorMatrix = mColorMatrix;
        if (mDaltonize) {
            colorMatrix = colorMatrix * mDaltonizer();
        }
        mat4 oldMatrix = engine.setupColorTransform(colorMatrix);
        doComposeSurfaces(hw, dirtyRegion);
        engine.setupColorTransform(oldMatrix);
    }

    // update the swap region and clear the dirty region
    hw->swapRegion.orSelf(dirtyRegion);

    // swap buffers (presentation)
    hw->swapBuffers(getHwComposer());
}

• 调用doComposeSurfaces函数进行图层的合成，图像合成的真正执行者
• 调用hw->swapBuffers将软件合成的结果提交到图像缓冲区，待消费者去处理（后文分析）

bool SurfaceFlinger::doComposeSurfaces(const sp<const DisplayDevice>& hw, const Region& dirty)
{
    

    ......
    

    /*
     * and then, render the layers targeted at the framebuffer
     */

    const Vector< sp<Layer> >& layers(hw->getVisibleLayersSortedByZ());
    const size_t count = layers.size();
    const Transform& tr = hw->getTransform();
    if (cur != end) {
        // we're using h/w composer
        for (size_t i=0 ; i<count && cur!=end ; ++i, ++cur) {
            const sp<Layer>& layer(layers[i]);
            const Region clip(dirty.intersect(tr.transform(layer->visibleRegion)));
            if (!clip.isEmpty()) {
                switch (cur->getCompositionType()) {
                    case HWC_CURSOR_OVERLAY:
                    case HWC_OVERLAY: {
                        const Layer::State& state(layer->getDrawingState());
                        if ((cur->getHints() & HWC_HINT_CLEAR_FB)
                                && i
                                && layer->isOpaque(state) && (state.alpha == 0xFF)
                                && hasGlesComposition) {
                            // never clear the very first layer since we're
                            // guaranteed the FB is already cleared
                            layer->clearWithOpenGL(hw, clip);
                        }
                        break;
                    }
                    case HWC_FRAMEBUFFER: {
                        layer->draw(hw, clip);
                        break;
                    }
                    case HWC_FRAMEBUFFER_TARGET: {
                        // this should not happen as the iterator shouldn't
                        // let us get there.
                        ALOGW("HWC_FRAMEBUFFER_TARGET found in hwc list (index=%zu)", i);
                        break;
                    }
                }
            }
            layer->setAcquireFence(hw, *cur);
        }
    } else {
        // we're not using h/w composer
        for (size_t i=0 ; i<count ; ++i) {
            const sp<Layer>& layer(layers[i]);
            const Region clip(dirty.intersect(
                    tr.transform(layer->visibleRegion)));
            if (!clip.isEmpty()) {
                layer->draw(hw, clip);
            }
        }
    }

    // disable scissor at the end of the frame
    engine.disableScissor();
    return true;
}

• 当图层的compositionType值为HWC_CURSOR_OVERLAY或HWC_OVERLAY，采用硬件合成。既将图层提交给HWComposer去处理
• 当图层的compositionType值为HWC_FRAMEBUFFER，采用软件gl合成。既调用Layer的draw方法绘制

至此，图层合成流程大体走完，图像输出的过程将在下文分析。