[RK3399][Android7.1] 使用opengl进行界面合成流程

平台	内核版本	安卓版本
RK3399	Linux4.4	Android7.1

如何合成

方案一

在Framebuffer上依次画出（使用opengl）每个app，然后把Framebuffer显示出来

方案二（HWComposer）

各个app写入不同的window，硬件合成后在LCD上直接显示

使用OpenGL

void SurfaceFlinger::handleMessageRefresh() {
    ATRACE_CALL();
    preComposition();
    rebuildLayerStacks(); //计算dirty区域
    setUpHWComposer();    //如果有硬件合成单元的话
    doDebugFlashRegions();
    doComposition();      //对于opengl
    postComposition();
}

• 绘画出每一个Layer

对于每个Layer，执行layer->draw()===>drawWithOpenGL()

• swapBuffer

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);     //重绘buffer

            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

void SurfaceFlinger::doDisplayComposition(const sp<const DisplayDevice>& hw,
        const Region& inDirtyRegion)
{
    // We only need to actually compose the display if:
    // 1) It is being handled by hardware composer, which may need this to
    //    keep its virtual display state machine in sync, or
    // 2) There is work to be done (the dirty region isn't empty)
    bool isHwcDisplay = hw->getHwcDisplayId() >= 0;
    if (!isHwcDisplay && inDirtyRegion.isEmpty()) {
        return;
    }

    Region dirtyRegion(inDirtyRegion);

    // compute the invalid region
    hw->swapRegion.orSelf(dirtyRegion);

    uint32_t flags = hw->getFlags();
    if (flags & DisplayDevice::SWAP_RECTANGLE) {
        // we can redraw only what's dirty, but since SWAP_RECTANGLE only
        // takes a rectangle, we must make sure to update that whole
        // rectangle in that case
        dirtyRegion.set(hw->swapRegion.bounds());
    } else {
        if (flags & DisplayDevice::PARTIAL_UPDATES) {
            // We need to redraw the rectangle that will be updated
            // (pushed to the framebuffer).
            // This is needed because PARTIAL_UPDATES only takes one
            // rectangle instead of a region (see DisplayDevice::flip())
            dirtyRegion.set(hw->swapRegion.bounds());
        } else {
            // we need to redraw everything (the whole screen)
            dirtyRegion.set(hw->bounds());
            hw->swapRegion = dirtyRegion;
        }
    }

    if (CC_LIKELY(!mDaltonize && !mHasColorMatrix)) {
        if (!doComposeSurfaces(hw, dirtyRegion)) return;
    } else {
        RenderEngine& engine(getRenderEngine());
        mat4 colorMatrix = mColorMatrix;
        if (mDaltonize) {
            colorMatrix = colorMatrix * mDaltonizer();
        }
        engine.beginGroup(colorMatrix);
★        doComposeSurfaces(hw, dirtyRegion);
        engine.endGroup();
    }

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

    // swap buffers (presentation)
★    hw->swapBuffers(getHwComposer());//重绘完图层后，应该把绘制完的图层替换过来以显示
}

• 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

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