// An image can be directly compositing if it's the sole content of the layer, and has no box decorations
// that require painting. Direct compositing saves backing store.
bool CompositedLayerMapping::isDirectlyCompositedImage() const
{
    RenderObject* renderObject = renderer();

    if (!renderObject->isImage() || m_owningLayer->hasBoxDecorationsOrBackground() || renderObject->hasClip())
        return false;

    RenderImage* imageRenderer = toRenderImage(renderObject);
    if (ImageResource* cachedImage = imageRenderer->cachedImage()) {
        if (!cachedImage->hasImage())
            return false;

        Image* image = cachedImage->imageForRenderer(imageRenderer);
        return image->isBitmapImage();
    }

    return false;
}

void CompositedLayerMapping::contentChanged(ContentChangeType changeType)
{
    if ((changeType == ImageChanged) && isDirectlyCompositedImage()) {
        updateImageContents();
        return;
    }

    if ((changeType == MaskImageChanged) && m_maskLayer) {
        // The composited layer bounds relies on box->maskClipRect(), which changes
        // when the mask image becomes available.
        updateAfterLayout(CompositingChildrenOnly | IsUpdateRoot);
    }

    if ((changeType == CanvasChanged || changeType == CanvasPixelsChanged) && isAcceleratedCanvas(renderer())) {
        m_graphicsLayer->setContentsNeedsDisplay();
        return;
    }
}

void CompositedLayerMapping::updateImageContents()
{
    ASSERT(renderer()->isImage());
    RenderImage* imageRenderer = toRenderImage(renderer());

    ImageResource* cachedImage = imageRenderer->cachedImage();
    if (!cachedImage)
        return;

    Image* image = cachedImage->imageForRenderer(imageRenderer);
    if (!image)
        return;

    // We have to wait until the image is fully loaded before setting it on the layer.
    if (!cachedImage->isLoaded())
        return;

    // This is a no-op if the layer doesn't have an inner layer for the image.
    m_graphicsLayer->setContentsToImage(image);
    bool isSimpleContainer = false;
    updateDrawsContent(isSimpleContainer);

void GraphicsLayer::setContentsToImage(Image* image)
{
    RefPtr<NativeImageSkia> nativeImage = image ? image->nativeImageForCurrentFrame() : 0;
    if (nativeImage) {
        if (!m_imageLayer) {
            m_imageLayer = adoptPtr(Platform::current()->compositorSupport()->createImageLayer());
            registerContentsLayer(m_imageLayer->layer());
        }
        m_imageLayer->setBitmap(nativeImage->bitmap());
        m_imageLayer->layer()->setOpaque(image->currentFrameKnownToBeOpaque());
        updateContentsRect();
    } else {
        if (m_imageLayer) {
            unregisterContentsLayer(m_imageLayer->layer());
            m_imageLayer.clear();
        }
    }

    setContentsTo(m_imageLayer ? m_imageLayer->layer() : 0);
}

Chromium M34上图像（IMG元素）的直接合成，不走NativeImageSkia::draw路径的相关代码