缘起

最近App开发同事提出了一个需求，需要系统团队提供一个截屏的接口，可以过滤一些特定Layer中的内容，我这顺便记录一下调试过程。

流程

截屏流程一般通过 SurfaceControl.screenshot(frame, dw, dh, false, ROTATION_0)函数返回一个bitmap的图片。
函数参数包含裁剪区域sourceCrop 和旋转角度。

frameworks/base/core/java/android/view/SurfaceControl.java

public static Bitmap screenshot(Rect sourceCrop 和, int width, int height,
        boolean useIdentityTransform, int rotation) {
    // TODO: should take the display as a parameter
    final IBinder displayToken = SurfaceControl.getInternalDisplayToken();
    if (displayToken == null) {
        Log.w(TAG, "Failed to take screenshot because internal display is disconnected");
        return null;
    }

    if (rotation == ROTATION_90 || rotation == ROTATION_270) {
        rotation = (rotation == ROTATION_90) ? ROTATION_270 : ROTATION_90;
    }

    SurfaceControl.rotateCropForSF(sourceCrop, rotation);
    final ScreenshotGraphicBuffer buffer = screenshotToBuffer(displayToken, sourceCrop, width,
            height, useIdentityTransform, rotation);

    if (buffer == null) {
        Log.w(TAG, "Failed to take screenshot");
        return null;
    }
    return Bitmap.wrapHardwareBuffer(buffer.getGraphicBuffer(), buffer.getColorSpace());
}

值得留意的2点
1.displayToken 固定截取为第一个物理屏幕设备，如果我们需要截取第2个物理屏怎么办？
2.通过screenshotToBuffer 会返回一个buf，然后使用Bitmap的warp功能转换成图片上传给app。

这样我们跟踪起来，流程就显得很简单了，我们只需要跟踪buffer（实际是就是一块GraphicBuffer的封装）的流转就好了。

public static ScreenshotGraphicBuffer screenshotToBuffer(IBinder display, Rect sourceCrop,
        int width, int height, boolean useIdentityTransform, int rotation) {
    if (display == null) {
        throw new IllegalArgumentException("displayToken must not be null");
    }

    return nativeScreenshot(display, sourceCrop, width, height, useIdentityTransform, rotation,
            false /* captureSecureLayers */);
}

nativeScreenshot 是一个native函数，通过jni进入native世界
frameworks/base/core/jni/android_view_SurfaceControl.cpp

static jobject nativeScreenshot(JNIEnv* env, jclass clazz,
        jobject displayTokenObj, jobject sourceCropObj, jint width, jint height,
        bool useIdentityTransform, int rotation, bool captureSecureLayers) {
    sp<IBinder> displayToken = ibinderForJavaObject(env, displayTokenObj);
    if (displayToken == NULL) {
        return NULL;
    }
    const ui::ColorMode colorMode = SurfaceComposerClient::getActiveColorMode(displayToken);
    const ui::Dataspace dataspace = pickDataspaceFromColorMode(colorMode);

    Rect sourceCrop = rectFromObj(env, sourceCropObj);
    sp<GraphicBuffer> buffer;
    bool capturedSecureLayers = false;
    status_t res = ScreenshotClient::capture(displayToken, dataspace,
            ui::PixelFormat::RGBA_8888,
            sourceCrop, width, height,
            useIdentityTransform, ui::toRotation(rotation),
            captureSecureLayers, &buffer, capturedSecureLayers);
    if (res != NO_ERROR) {
        return NULL;
    }

    const jint namedColorSpace = fromDataspaceToNamedColorSpaceValue(dataspace);
    return env->CallStaticObjectMethod(gScreenshotGraphicBufferClassInfo.clazz,
            gScreenshotGraphicBufferClassInfo.builder,
            buffer->getWidth(),
            buffer->getHeight(),
            buffer->getPixelFormat(),
            (jint)buffer->getUsage(),
            (jlong)buffer.get(),
            namedColorSpace,
            capturedSecureLayers);
}

ScreenshotClient::capture 为static函数，最终通过getComposerService函数调用SurfaceFlinger的captureScreen函数。
env->CallStaticObjectMethod 是native层调用静态java的方法，把native的GraphicBuffer buffer对象转化成java层的 ScreenshotGraphicBuffer对象，上传到app。

status_t ScreenshotClient::capture(const sp<IBinder>& display, ui::Dataspace reqDataSpace,
                                   ui::PixelFormat reqPixelFormat, const Rect& sourceCrop,
                                   uint32_t reqWidth, uint32_t reqHeight, bool useIdentityTransform,
                                   ui::Rotation rotation, bool captureSecureLayers,
                                   sp<GraphicBuffer>* outBuffer, bool& outCapturedSecureLayers) {
    sp<ISurfaceComposer> s(ComposerService::getComposerService());
    if (s == nullptr) return NO_INIT;
    status_t ret = s->captureScreen(display, outBuffer, outCapturedSecureLayers, reqDataSpace,
                                    reqPixelFormat, sourceCrop, reqWidth, reqHeight,
                                    useIdentityTransform, rotation, captureSecureLayers);
    if (ret != NO_ERROR) {
        return ret;
    }
    return ret;
}

其中ComposerService s 服务实际上就是Surfaceflinger，如下

frameworks/native/libs/gui/SurfaceComposerClient.cpp

ComposerService::ComposerService()
: Singleton<ComposerService>() {
    Mutex::Autolock _l(mLock);
    connectLocked();
}

void ComposerService::connectLocked() {
    const String16 name("SurfaceFlinger");
    while (getService(name, &mComposerService) != NO_ERROR) {
        usleep(250000);
  }

所以这里通过binder 调用SurfaceFlinger 进程的captureScreen函数。

status_t SurfaceFlinger::captureScreen(const sp<IBinder>& displayToken,
                                       sp<GraphicBuffer>* outBuffer, bool& outCapturedSecureLayers,
                                       Dataspace reqDataspace, ui::PixelFormat reqPixelFormat,
                                       const Rect& sourceCrop, uint32_t reqWidth,
                                       uint32_t reqHeight, bool useIdentityTransform,
                                       ui::Rotation rotation, bool captureSecureLayers) {
    ATRACE_CALL();

    if (!displayToken) return BAD_VALUE;

    auto renderAreaRotation = ui::Transform::toRotationFlags(rotation);
    if (renderAreaRotation == ui::Transform::ROT_INVALID) {
        ALOGE("%s: Invalid rotation: %s", __FUNCTION__, toCString(rotation));
        renderAreaRotation = ui::Transform::ROT_0;
    }

    sp<DisplayDevice> display;
    {
        Mutex::Autolock lock(mStateLock);

        display = getDisplayDeviceLocked(displayToken);
        if (!display) return NAME_NOT_FOUND;

        // set the requested width/height to the logical display viewport size
        // by default
        if (reqWidth == 0 || reqHeight == 0) {
            reqWidth = uint32_t(display->getViewport().width());
            reqHeight = uint32_t(display->getViewport().height());
        }
    }

    DisplayRenderArea renderArea(display, sourceCrop, reqWidth, reqHeight, reqDataspace,
                                 renderAreaRotation, captureSecureLayers);
    auto traverseLayers = std::bind(&SurfaceFlinger::traverseLayersInDisplay, this, display,
                                    std::placeholders::_1);
    return captureScreenCommon(renderArea, traverseLayers, outBuffer, reqPixelFormat,
                               useIdentityTransform, outCapturedSecureLayers);
}

主要工作都是在captureScreenCommon函数中完成，其他都是一些Display和RenderArea区域的赋值。
在看captureScreenCommon之前，我们看一下参数 traverseLayersInDisplay.

void SurfaceFlinger::traverseLayersInDisplay(const sp<const DisplayDevice>& display,
                                             const LayerVector::Visitor& visitor) {
    // We loop through the first level of layers without traversing,
    // as we need to determine which layers belong to the requested display.
    for (const auto& layer : mDrawingState.layersSortedByZ) {
        if (!layer->belongsToDisplay(display->getLayerStack(), false)) {
            continue;
        }
        // relative layers are traversed in Layer::traverseInZOrder
        layer->traverseInZOrder(LayerVector::StateSet::Drawing, [&](Layer* layer) {
            if (!layer->belongsToDisplay(display->getLayerStack(), false)) {
                return;
            }
            if (!layer->isVisible()) {
                return;
            }
            visitor(layer);
        });
    }
}

故名思议，traverseLayersInDisplay的作用是遍历mDrawingState 中可用的layer .后面通过opengl把每层layer的内容绘制到buff中，当然这是后话。

我们接着看captureScreenCommon，最终调用renderScreenImplLocked函数

void SurfaceFlinger::renderScreenImplLocked(const RenderArea& renderArea,
                                            TraverseLayersFunction traverseLayers,
                                            ANativeWindowBuffer* buffer, bool useIdentityTransform,
                                            bool regionSampling, int* outSyncFd) {
    ATRACE_CALL();

    const auto reqWidth = renderArea.getReqWidth();
    const auto reqHeight = renderArea.getReqHeight();
    const auto sourceCrop = renderArea.getSourceCrop();
    const auto transform = renderArea.getTransform();
    const auto rotation = renderArea.getRotationFlags();
    const auto& displayViewport = renderArea.getDisplayViewport();

    renderengine::DisplaySettings clientCompositionDisplay;
    std::vector<compositionengine::LayerFE::LayerSettings> clientCompositionLayers;

    // assume that bounds are never offset, and that they are the same as the
    // buffer bounds.
    clientCompositionDisplay.physicalDisplay = Rect(reqWidth, reqHeight);
    clientCompositionDisplay.clip = sourceCrop;
    clientCompositionDisplay.orientation = rotation;

    clientCompositionDisplay.outputDataspace = renderArea.getReqDataSpace();
    clientCompositionDisplay.maxLuminance = DisplayDevice::sDefaultMaxLumiance;

    const float alpha = RenderArea::getCaptureFillValue(renderArea.getCaptureFill());

    compositionengine::LayerFE::LayerSettings fillLayer;
    fillLayer.source.buffer.buffer = nullptr;
    fillLayer.source.solidColor = half3(0.0, 0.0, 0.0);
    fillLayer.geometry.boundaries =
            FloatRect(sourceCrop.left, sourceCrop.top, sourceCrop.right, sourceCrop.bottom);
    fillLayer.alpha = half(alpha);
    clientCompositionLayers.push_back(fillLayer);

    const auto display = renderArea.getDisplayDevice();
    std::vector<Layer*> renderedLayers;
    Region clearRegion = Region::INVALID_REGION;
    traverseLayers([&](Layer* layer) {
        const bool supportProtectedContent = false;
        Region clip(renderArea.getBounds());
        compositionengine::LayerFE::ClientCompositionTargetSettings targetSettings{
                clip,
                useIdentityTransform,
                layer->needsFilteringForScreenshots(display.get(), transform) ||
                        renderArea.needsFiltering(),
                renderArea.isSecure(),
                supportProtectedContent,
                clearRegion,
                displayViewport,
                clientCompositionDisplay.outputDataspace,
                true,  /* realContentIsVisible */
                false, /* clearContent */
        };
        std::vector<compositionengine::LayerFE::LayerSettings> results =
                layer->prepareClientCompositionList(targetSettings);
        if (results.size() > 0) {
            for (auto& settings : results) {
                settings.geometry.positionTransform =
                        transform.asMatrix4() * settings.geometry.positionTransform;
                // There's no need to process blurs when we're executing region sampling,
                // we're just trying to understand what we're drawing, and doing so without
                // blurs is already a pretty good approximation.
                if (regionSampling) {
                    settings.backgroundBlurRadius = 0;
                }
            }
            clientCompositionLayers.insert(clientCompositionLayers.end(),
                                           std::make_move_iterator(results.begin()),
                                           std::make_move_iterator(results.end()));
            renderedLayers.push_back(layer);
        }
    });

    std::vector<const renderengine::LayerSettings*> clientCompositionLayerPointers(
            clientCompositionLayers.size());
    std::transform(clientCompositionLayers.begin(), clientCompositionLayers.end(),
                   clientCompositionLayerPointers.begin(),
                   std::pointer_traits<renderengine::LayerSettings*>::pointer_to);

    clientCompositionDisplay.clearRegion = clearRegion;
    // Use an empty fence for the buffer fence, since we just created the buffer so
    // there is no need for synchronization with the GPU.
    base::unique_fd bufferFence;
    base::unique_fd drawFence;
    //huzy3 add for
    getRenderEngine().useProtectedContext(false);
    getRenderEngine().drawLayers(clientCompositionDisplay, clientCompositionLayerPointers, buffer,
                                 /*useFramebufferCache=*/false, std::move(bufferFence), &drawFence);

    *outSyncFd = drawFence.release();

    if (*outSyncFd >= 0) {
        sp<Fence> releaseFence = new Fence(dup(*outSyncFd));
        for (auto* layer : renderedLayers) {
            layer->onLayerDisplayed(releaseFence);
        }
    }
}

主要步骤

• 1.通过traverseLayers 把需要合成的layer 添加到 clientCompositionLayers

• 2.调用getRenderEngine().drawLayers 把所有layer通过OpengL 离屏幕渲染绘制到buffer 中。

我们先看看RenderEngine 是什么

renderengine::RenderEngine& SurfaceFlinger::getRenderEngine() const {
    return mCompositionEngine->getRenderEngine();
}

mCompositionEngine对象是在SurfaceFlinger对象初始化的时候创建

SurfaceFlinger::SurfaceFlinger(Factory& factory, SkipInitializationTag)
      : mFactory(factory),
        mInterceptor(mFactory.createSurfaceInterceptor(this)),
        mTimeStats(std::make_shared<impl::TimeStats>()),
        mFrameTracer(std::make_unique<FrameTracer>()),
        mEventQueue(mFactory.createMessageQueue()),
        mCompositionEngine(mFactory.createCompositionEngine()),

std::unique_ptr<compositionengine::CompositionEngine> createCompositionEngine() {
    return std::make_unique<CompositionEngine>();
}

renderengine::RenderEngine& CompositionEngine::getRenderEngine() const {
    return *mRenderEngine.get();
}

mRenderEngine 是在Surfaceflinger的init函数中初始化的。

void SurfaceFlinger::init() {
    ALOGI(  "SurfaceFlinger's main thread ready to run. "
            "Initializing graphics H/W...");
    Mutex::Autolock _l(mStateLock);

    // 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::HIGH
                        : renderengine::RenderEngine::ContextPriority::MEDIUM)
                .build()));

继续跟踪 RenderEngine 的 create函数
frameworks/native/libs/renderengine/RenderEngine.cpp

std::unique_ptr<impl::RenderEngine> RenderEngine::create(const RenderEngineCreationArgs& args) {
    char prop[PROPERTY_VALUE_MAX];
    property_get(PROPERTY_DEBUG_RENDERENGINE_BACKEND, prop, "gles");
    if (strcmp(prop, "gles") == 0) {
        ALOGD("RenderEngine GLES Backend");
        return renderengine::gl::GLESRenderEngine::create(args);
    }
    ALOGE("UNKNOWN BackendType: %s, create GLES RenderEngine.", prop);
    return renderengine::gl::GLESRenderEngine::create(args);
}

frameworks/native/libs/renderengine/gl/GLESRenderEngine.cpp

std::unique_ptr<GLESRenderEngine> GLESRenderEngine::create(const RenderEngineCreationArgs& args) {
    // initialize EGL for the default display
    EGLDisplay display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
    if (!eglInitialize(display, nullptr, nullptr)) {
        LOG_ALWAYS_FATAL("failed to initialize EGL");
    }

    const auto eglVersion = eglQueryStringImplementationANDROID(display, EGL_VERSION);
    if (!eglVersion) {
        checkGlError(__FUNCTION__, __LINE__);
        LOG_ALWAYS_FATAL("eglQueryStringImplementationANDROID(EGL_VERSION) failed");
    }

    const auto eglExtensions = eglQueryStringImplementationANDROID(display, EGL_EXTENSIONS);
    if (!eglExtensions) {
        checkGlError(__FUNCTION__, __LINE__);
        LOG_ALWAYS_FATAL("eglQueryStringImplementationANDROID(EGL_EXTENSIONS) failed");
    }

    GLExtensions& extensions = GLExtensions::getInstance();
    extensions.initWithEGLStrings(eglVersion, eglExtensions);

    // The code assumes that ES2 or later is available if this extension is
    // supported.
    EGLConfig config = EGL_NO_CONFIG;
    if (!extensions.hasNoConfigContext()) {
        config = chooseEglConfig(display, args.pixelFormat, /*logConfig*/ true);
    }

    bool useContextPriority =
            extensions.hasContextPriority() && args.contextPriority == ContextPriority::HIGH;
    EGLContext protectedContext = EGL_NO_CONTEXT;
    if (args.enableProtectedContext && extensions.hasProtectedContent()) {
        protectedContext = createEglContext(display, config, nullptr, useContextPriority,
                                            Protection::PROTECTED);
        ALOGE_IF(protectedContext == EGL_NO_CONTEXT, "Can't create protected context");
    }

    EGLContext ctxt = createEglContext(display, config, protectedContext, useContextPriority,
                                       Protection::UNPROTECTED);

    // if can't create a GL context, we can only abort.
    LOG_ALWAYS_FATAL_IF(ctxt == EGL_NO_CONTEXT, "EGLContext creation failed");

    EGLSurface dummy = EGL_NO_SURFACE;
    if (!extensions.hasSurfacelessContext()) {
        dummy = createDummyEglPbufferSurface(display, config, args.pixelFormat,
                                             Protection::UNPROTECTED);
        LOG_ALWAYS_FATAL_IF(dummy == EGL_NO_SURFACE, "can't create dummy pbuffer");
    }
    EGLBoolean success = eglMakeCurrent(display, dummy, dummy, ctxt);
    LOG_ALWAYS_FATAL_IF(!success, "can't make dummy pbuffer current");
    extensions.initWithGLStrings(glGetString(GL_VENDOR), glGetString(GL_RENDERER),
                                 glGetString(GL_VERSION), glGetString(GL_EXTENSIONS));

    EGLSurface protectedDummy = EGL_NO_SURFACE;
    if (protectedContext != EGL_NO_CONTEXT && !extensions.hasSurfacelessContext()) {
        protectedDummy = createDummyEglPbufferSurface(display, config, args.pixelFormat,
                                                      Protection::PROTECTED);
        ALOGE_IF(protectedDummy == EGL_NO_SURFACE, "can't create protected dummy pbuffer");
    }

    // now figure out what version of GL did we actually get
    GlesVersion version = parseGlesVersion(extensions.getVersion());

    LOG_ALWAYS_FATAL_IF(args.supportsBackgroundBlur && version < GLES_VERSION_3_0,
        "Blurs require OpenGL ES 3.0. Please unset ro.surface_flinger.supports_background_blur");

    // initialize the renderer while GL is current
    std::unique_ptr<GLESRenderEngine> engine;
    switch (version) {
        case GLES_VERSION_1_0:
        case GLES_VERSION_1_1:
            LOG_ALWAYS_FATAL("SurfaceFlinger requires OpenGL ES 2.0 minimum to run.");
            break;
        case GLES_VERSION_2_0:
        case GLES_VERSION_3_0:
            engine = std::make_unique<GLESRenderEngine>(args, display, config, ctxt, dummy,
                                                        protectedContext, protectedDummy);
            break;
    }

    ALOGI("OpenGL ES informations:");
    ALOGI("vendor    : %s", extensions.getVendor());
    ALOGI("renderer  : %s", extensions.getRenderer());
    ALOGI("version   : %s", extensions.getVersion());
    ALOGI("extensions: %s", extensions.getExtensions());
    ALOGI("GL_MAX_TEXTURE_SIZE = %zu", engine->getMaxTextureSize());
    ALOGI("GL_MAX_VIEWPORT_DIMS = %zu", engine->getMaxViewportDims());

    return engine;
}

可以看到renderengine 为OPENGL 渲染引擎GLESRenderEngine对象。其中几个重要EGL变量，这个简单介绍一下

• EGLDisplay ——系统显示 ID 或句柄，可以理解为一个前端的显示窗口
• EGLConfig ——Surface的EGL配置，可以理解为绘制目标framebuffer的配置属性
• EGLSurface ——系统窗口或 frame buffer 句柄 ，可以理解为一个后端的渲染目标窗口。
• EGLContext ——OpenGL ES 图形上下文，它代表了OpenGL状态机；如果没有它，OpenGL指令就没有执行的环境。

关于Android OPENGL和EGL后面会另起文章介绍。上面找一大圈， 主要得到getRenderEngine().drawLayers函数，最终会实现是GLESRenderEngine中的drawLayers函数。

status_t GLESRenderEngine::drawLayers(const DisplaySettings& display,
                                      const std::vector<const LayerSettings*>& layers,
                                      ANativeWindowBuffer* const buffer,
                                      const bool useFramebufferCache, base::unique_fd&& bufferFence,
                                      base::unique_fd* drawFence) {
    ATRACE_CALL();
    if (layers.empty()) {
        ALOGV("Drawing empty layer stack");
        return NO_ERROR;
    }

    if (bufferFence.get() >= 0) {
        // Duplicate the fence for passing to waitFence.
        base::unique_fd bufferFenceDup(dup(bufferFence.get()));
        if (bufferFenceDup < 0 || !waitFence(std::move(bufferFenceDup))) {
            ATRACE_NAME("Waiting before draw");
            sync_wait(bufferFence.get(), -1);
        }
    }

    if (buffer == nullptr) {
        ALOGE("No output buffer provided. Aborting GPU composition.");
        return BAD_VALUE;
    }

    std::unique_ptr<BindNativeBufferAsFramebuffer> fbo;
    // Gathering layers that requested blur, we'll need them to decide when to render to an
    // offscreen buffer, and when to render to the native buffer.
    std::deque<const LayerSettings*> blurLayers;
    if (CC_LIKELY(mBlurFilter != nullptr)) {
        for (auto layer : layers) {
            if (layer->backgroundBlurRadius > 0) {
                blurLayers.push_back(layer);
            }
        }
    }
    const auto blurLayersSize = blurLayers.size();

    if (blurLayersSize == 0) {
        fbo = std::make_unique<BindNativeBufferAsFramebuffer>(*this, buffer, useFramebufferCache);
        if (fbo->getStatus() != NO_ERROR) {
            ALOGE("Failed to bind framebuffer! Aborting GPU composition for buffer (%p).",
                  buffer->handle);
            checkErrors();
            return fbo->getStatus();
        }
        setViewportAndProjection(display.physicalDisplay, display.clip);
    } else {
        setViewportAndProjection(display.physicalDisplay, display.clip);
        auto status =
                mBlurFilter->setAsDrawTarget(display, blurLayers.front()->backgroundBlurRadius);
        if (status != NO_ERROR) {
            ALOGE("Failed to prepare blur filter! Aborting GPU composition for buffer (%p).",
                  buffer->handle);
            checkErrors();
            return status;
        }
    }

    // clear the entire buffer, sometimes when we reuse buffers we'd persist
    // ghost images otherwise.
    // we also require a full transparent framebuffer for overlays. This is
    // probably not quite efficient on all GPUs, since we could filter out
    // opaque layers.
    clearWithColor(0.0, 0.0, 0.0, 0.0);

    setOutputDataSpace(display.outputDataspace);
    setDisplayMaxLuminance(display.maxLuminance);

    const mat4 projectionMatrix =
            ui::Transform(display.orientation).asMatrix4() * mState.projectionMatrix;
    if (!display.clearRegion.isEmpty()) {
        glDisable(GL_BLEND);
        fillRegionWithColor(display.clearRegion, 0.0, 0.0, 0.0, 1.0);
    }

    Mesh mesh = Mesh::Builder()
                        .setPrimitive(Mesh::TRIANGLE_FAN)
                        .setVertices(4 /* count */, 2 /* size */)
                        .setTexCoords(2 /* size */)
                        .setCropCoords(2 /* size */)
                        .build();
    for (auto const layer : layers) {
        if (blurLayers.size() > 0 && blurLayers.front() == layer) {
            blurLayers.pop_front();

            auto status = mBlurFilter->prepare();
            if (status != NO_ERROR) {
                ALOGE("Failed to render blur effect! Aborting GPU composition for buffer (%p).",
                      buffer->handle);
                checkErrors("Can't render first blur pass");
                return status;
            }

            if (blurLayers.size() == 0) {
                // Done blurring, time to bind the native FBO and render our blur onto it.
                fbo = std::make_unique<BindNativeBufferAsFramebuffer>(*this, buffer,
                                                                      useFramebufferCache);
                status = fbo->getStatus();
                setViewportAndProjection(display.physicalDisplay, display.clip);
            } else {
                // There's still something else to blur, so let's keep rendering to our FBO
                // instead of to the display.
                status = mBlurFilter->setAsDrawTarget(display,
                                                      blurLayers.front()->backgroundBlurRadius);
            }
            if (status != NO_ERROR) {
                ALOGE("Failed to bind framebuffer! Aborting GPU composition for buffer (%p).",
                      buffer->handle);
                checkErrors("Can't bind native framebuffer");
                return status;
            }

            status = mBlurFilter->render(blurLayersSize > 1);
            if (status != NO_ERROR) {
                ALOGE("Failed to render blur effect! Aborting GPU composition for buffer (%p).",
                      buffer->handle);
                checkErrors("Can't render blur filter");
                return status;
            }
        }

        mState.maxMasteringLuminance = layer->source.buffer.maxMasteringLuminance;
        mState.maxContentLuminance = layer->source.buffer.maxContentLuminance;
        mState.projectionMatrix = projectionMatrix * layer->geometry.positionTransform;

        const FloatRect bounds = layer->geometry.boundaries;
        Mesh::VertexArray<vec2> position(mesh.getPositionArray<vec2>());
        position[0] = vec2(bounds.left, bounds.top);
        position[1] = vec2(bounds.left, bounds.bottom);
        position[2] = vec2(bounds.right, bounds.bottom);
        position[3] = vec2(bounds.right, bounds.top);

        setupLayerCropping(*layer, mesh);
        setColorTransform(display.colorTransform * layer->colorTransform);

        bool usePremultipliedAlpha = true;
        bool disableTexture = true;
        bool isOpaque = false;
        if (layer->source.buffer.buffer != nullptr) {
            disableTexture = false;
            isOpaque = layer->source.buffer.isOpaque;

            sp<GraphicBuffer> gBuf = layer->source.buffer.buffer;
            bindExternalTextureBuffer(layer->source.buffer.textureName, gBuf,
                                      layer->source.buffer.fence);

            usePremultipliedAlpha = layer->source.buffer.usePremultipliedAlpha;
            Texture texture(Texture::TEXTURE_EXTERNAL, layer->source.buffer.textureName);
            mat4 texMatrix = layer->source.buffer.textureTransform;

            texture.setMatrix(texMatrix.asArray());
            texture.setFiltering(layer->source.buffer.useTextureFiltering);

            texture.setDimensions(gBuf->getWidth(), gBuf->getHeight());
            setSourceY410BT2020(layer->source.buffer.isY410BT2020);

            renderengine::Mesh::VertexArray<vec2> texCoords(mesh.getTexCoordArray<vec2>());
            texCoords[0] = vec2(0.0, 0.0);
            texCoords[1] = vec2(0.0, 1.0);
            texCoords[2] = vec2(1.0, 1.0);
            texCoords[3] = vec2(1.0, 0.0);
            setupLayerTexturing(texture);
        }

        const half3 solidColor = layer->source.solidColor;
        const half4 color = half4(solidColor.r, solidColor.g, solidColor.b, layer->alpha);
        // Buffer sources will have a black solid color ignored in the shader,
        // so in that scenario the solid color passed here is arbitrary.
        setupLayerBlending(usePremultipliedAlpha, isOpaque, disableTexture, color,
                           layer->geometry.roundedCornersRadius);
        if (layer->disableBlending) {
            glDisable(GL_BLEND);
        }
        setSourceDataSpace(layer->sourceDataspace);

        if (layer->shadow.length > 0.0f) {
            handleShadow(layer->geometry.boundaries, layer->geometry.roundedCornersRadius,
                         layer->shadow);
        }
        // We only want to do a special handling for rounded corners when having rounded corners
        // is the only reason it needs to turn on blending, otherwise, we handle it like the
        // usual way since it needs to turn on blending anyway.
        else if (layer->geometry.roundedCornersRadius > 0.0 && color.a >= 1.0f && isOpaque) {
            handleRoundedCorners(display, *layer, mesh);
        } else {
            drawMesh(mesh);
        }

        // Cleanup if there's a buffer source
        if (layer->source.buffer.buffer != nullptr) {
            disableBlending();
            setSourceY410BT2020(false);
            disableTexturing();
        }
    }

    if (drawFence != nullptr) {
        *drawFence = flush();
    }
    // If flush failed or we don't support native fences, we need to force the
    // gl command stream to be executed.
    if (drawFence == nullptr || drawFence->get() < 0) {
        bool success = finish();
        if (!success) {
            ALOGE("Failed to flush RenderEngine commands");
            checkErrors();
            // Chances are, something illegal happened (either the caller passed
            // us bad parameters, or we messed up our shader generation).
            return INVALID_OPERATION;
        }
        mLastDrawFence = nullptr;
    } else {
        // The caller takes ownership of drawFence, so we need to duplicate the
        // fd here.
        mLastDrawFence = new Fence(dup(drawFence->get()));
    }
    mPriorResourcesCleaned = false;

    checkErrors();
    return NO_ERROR;
}

这里面函数众多，大多是关于opengl的，这里不作介绍，主要是2个步骤

• mesh相关信息初始化，包括顶点，纹理，纹理坐标 ，crop等。
• 调用drawMesh(mesh)回执绘制网格。

其中buffer在这函数通过 BindNativeBufferAsFramebuffer 把buff 转化成Framebuffer。

fbo = std::make_unique<BindNativeBufferAsFramebuffer>(*this, buffer,
                                                         useFramebufferCache);
                                                     

并且通过mEngine.bindFrameBuffer(mFramebuffer) ，绑定FBO,一旦FBO被绑定，之后的所有的OpenGL操作都会对当前所绑定的FBO造成影响.
即之后绘制的内容都会在FBO上面进行。

class BindNativeBufferAsFramebuffer {
public:
    BindNativeBufferAsFramebuffer(RenderEngine& engine, ANativeWindowBuffer* buffer,
                                  const bool useFramebufferCache)
          : mEngine(engine), mFramebuffer(mEngine.getFramebufferForDrawing()), mStatus(NO_ERROR) {
        mStatus = mFramebuffer->setNativeWindowBuffer(buffer, mEngine.isProtected(),
                                                      useFramebufferCache)
                ? mEngine.bindFrameBuffer(mFramebuffer)
                : NO_MEMORY;
    }
    ~BindNativeBufferAsFramebuffer() {
        mFramebuffer->setNativeWindowBuffer(nullptr, false, /*arbitrary*/ true);
        mEngine.unbindFrameBuffer(mFramebuffer);
    }
    status_t getStatus() const { return mStatus; }

private:
    RenderEngine& mEngine;
    Framebuffer* mFramebuffer;
    status_t mStatus;
};

在opengl里面网格(Mesh)代表的是单个的可绘制实体.

void GLESRenderEngine::drawMesh(const Mesh& mesh) {
    ATRACE_CALL();
    if (mesh.getTexCoordsSize()) {
        glEnableVertexAttribArray(Program::texCoords);
        glVertexAttribPointer(Program::texCoords, mesh.getTexCoordsSize(), GL_FLOAT, GL_FALSE,
                              mesh.getByteStride(), mesh.getTexCoords());
    }

    glVertexAttribPointer(Program::position, mesh.getVertexSize(), GL_FLOAT, GL_FALSE,
                          mesh.getByteStride(), mesh.getPositions());

    if (mState.cornerRadius > 0.0f) {
        glEnableVertexAttribArray(Program::cropCoords);
        glVertexAttribPointer(Program::cropCoords, mesh.getVertexSize(), GL_FLOAT, GL_FALSE,
                              mesh.getByteStride(), mesh.getCropCoords());
    }

    if (mState.drawShadows) {
        glEnableVertexAttribArray(Program::shadowColor);
        glVertexAttribPointer(Program::shadowColor, mesh.getShadowColorSize(), GL_FLOAT, GL_FALSE,
                              mesh.getByteStride(), mesh.getShadowColor());

        glEnableVertexAttribArray(Program::shadowParams);
        glVertexAttribPointer(Program::shadowParams, mesh.getShadowParamsSize(), GL_FLOAT, GL_FALSE,
                              mesh.getByteStride(), mesh.getShadowParams());
    }

    Description managedState = mState;
    // By default, DISPLAY_P3 is the only supported wide color output. However,
    // when HDR content is present, hardware composer may be able to handle
    // BT2020 data space, in that case, the output data space is set to be
    // BT2020_HLG or BT2020_PQ respectively. In GPU fall back we need
    // to respect this and convert non-HDR content to HDR format.
    if (mUseColorManagement) {
        Dataspace inputStandard = static_cast<Dataspace>(mDataSpace & Dataspace::STANDARD_MASK);
        Dataspace inputTransfer = static_cast<Dataspace>(mDataSpace & Dataspace::TRANSFER_MASK);
        Dataspace outputStandard =
                static_cast<Dataspace>(mOutputDataSpace & Dataspace::STANDARD_MASK);
        Dataspace outputTransfer =
                static_cast<Dataspace>(mOutputDataSpace & Dataspace::TRANSFER_MASK);
        bool needsXYZConversion = needsXYZTransformMatrix();

        // NOTE: if the input standard of the input dataspace is not STANDARD_DCI_P3 or
        // STANDARD_BT2020, it will be  treated as STANDARD_BT709
        if (inputStandard != Dataspace::STANDARD_DCI_P3 &&
            inputStandard != Dataspace::STANDARD_BT2020) {
            inputStandard = Dataspace::STANDARD_BT709;
        }

        if (needsXYZConversion) {
            // The supported input color spaces are standard RGB, Display P3 and BT2020.
            switch (inputStandard) {
                case Dataspace::STANDARD_DCI_P3:
                    managedState.inputTransformMatrix = mDisplayP3ToXyz;
                    break;
                case Dataspace::STANDARD_BT2020:
                    managedState.inputTransformMatrix = mBt2020ToXyz;
                    break;
                default:
                    managedState.inputTransformMatrix = mSrgbToXyz;
                    break;
            }

            // The supported output color spaces are BT2020, Display P3 and standard RGB.
            switch (outputStandard) {
                case Dataspace::STANDARD_BT2020:
                    managedState.outputTransformMatrix = mXyzToBt2020;
                    break;
                case Dataspace::STANDARD_DCI_P3:
                    managedState.outputTransformMatrix = mXyzToDisplayP3;
                    break;
                default:
                    managedState.outputTransformMatrix = mXyzToSrgb;
                    break;
            }
        } else if (inputStandard != outputStandard) {
            // At this point, the input data space and output data space could be both
            // HDR data spaces, but they match each other, we do nothing in this case.
            // In addition to the case above, the input data space could be
            // - scRGB linear
            // - scRGB non-linear
            // - sRGB
            // - Display P3
            // - BT2020
            // The output data spaces could be
            // - sRGB
            // - Display P3
            // - BT2020
            switch (outputStandard) {
                case Dataspace::STANDARD_BT2020:
                    if (inputStandard == Dataspace::STANDARD_BT709) {
                        managedState.outputTransformMatrix = mSrgbToBt2020;
                    } else if (inputStandard == Dataspace::STANDARD_DCI_P3) {
                        managedState.outputTransformMatrix = mDisplayP3ToBt2020;
                    }
                    break;
                case Dataspace::STANDARD_DCI_P3:
                    if (inputStandard == Dataspace::STANDARD_BT709) {
                        managedState.outputTransformMatrix = mSrgbToDisplayP3;
                    } else if (inputStandard == Dataspace::STANDARD_BT2020) {
                        managedState.outputTransformMatrix = mBt2020ToDisplayP3;
                    }
                    break;
                default:
                    if (inputStandard == Dataspace::STANDARD_DCI_P3) {
                        managedState.outputTransformMatrix = mDisplayP3ToSrgb;
                    } else if (inputStandard == Dataspace::STANDARD_BT2020) {
                        managedState.outputTransformMatrix = mBt2020ToSrgb;
                    }
                    break;
            }
        }

        // we need to convert the RGB value to linear space and convert it back when:
        // - there is a color matrix that is not an identity matrix, or
        // - there is an output transform matrix that is not an identity matrix, or
        // - the input transfer function doesn't match the output transfer function.
        if (managedState.hasColorMatrix() || managedState.hasOutputTransformMatrix() ||
            inputTransfer != outputTransfer) {
            managedState.inputTransferFunction =
                    Description::dataSpaceToTransferFunction(inputTransfer);
            managedState.outputTransferFunction =
                    Description::dataSpaceToTransferFunction(outputTransfer);
        }
    }

    ProgramCache::getInstance().useProgram(mInProtectedContext ? mProtectedEGLContext : mEGLContext,
                                           managedState);

    if (mState.drawShadows) {
        glDrawElements(mesh.getPrimitive(), mesh.getIndexCount(), GL_UNSIGNED_SHORT,
                       mesh.getIndices());
    } else {
        glDrawArrays(mesh.getPrimitive(), 0, mesh.getVertexCount());
    }

    if (mUseColorManagement && outputDebugPPMs) {
        static uint64_t managedColorFrameCount = 0;
        std::ostringstream out;
        out << "/data/texture_out" << managedColorFrameCount++;
        writePPM(out.str().c_str(), mVpWidth, mVpHeight);
    }

    if (mesh.getTexCoordsSize()) {
        glDisableVertexAttribArray(Program::texCoords);
    }

    if (mState.cornerRadius > 0.0f) {
        glDisableVertexAttribArray(Program::cropCoords);
    }

    if (mState.drawShadows) {
        glDisableVertexAttribArray(Program::shadowColor);
        glDisableVertexAttribArray(Program::shadowParams);
    }
}

最终执行绘制 的是 glDrawElements 或者glDrawArrays 函数。到此绘制流程已经完成。

总结