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Commit 1c740bce authored by Romain Guy's avatar Romain Guy
Browse files

Document the implementation of saveLayer(). 

The implementation is simple but tricky. Leave explanations to my
future self so that I don't invent a time machine to come back to
now and slap my self.

This change also simplifies the way the GL blending function is
chosen when compositing a layer. It reuses existing OpenGLRenderer
APIs and is easier to understand.

Change-Id: I1b9cf8c5d51e09836d85b8cf157a1c284aa65c59
parent 81ab046a

libs/hwui/OpenGLRenderer.cpp

+50 −15
Original line number Diff line number Diff line
@@ -280,7 +280,49 @@ int OpenGLRenderer::saveLayerAlpha(float left, float top, float right, float bot 
    }

}





/**

 * Layers are viewed by Skia are slightly different than layers in image editing

 * programs (for instance.) When a layer is created, previously created layers

 * and the frame buffer still receive every drawing command. For instance, if a

 * layer is created and a shape intersecting the bounds of the layers and the

 * framebuffer is draw, the shape will be drawn on both (unless the layer was

 * created with the SkCanvas::kClipToLayer_SaveFlag flag.)

 *

 * A way to implement layers is to create an FBO for each layer, backed by an RGBA

 * texture. Unfortunately, this is inefficient as it requires every primitive to

 * be drawn n + 1 times, where n is the number of active layers. In practice this

 * means, for every primitive:

 *   - Switch active frame buffer

 *   - Change viewport, clip and projection matrix

 *   - Issue the drawing

 *

 * Switching rendering target n + 1 times per drawn primitive is extremely costly.

 * To avoid this, layers are implemented in a different way here.

 *

 * This implementation relies on the frame buffer being at least RGBA 8888. When

 * a layer is created, only a texture is created, not an FBO. The content of the

 * frame buffer contained within the layer's bounds is copied into this texture

 * using glCopyTexImage2D(). The layer's region is then cleared in the frame

 * buffer and drawing continues as normal. This technique therefore treats the

 * frame buffer as a scratch buffer for the layers.

 *

 * To compose the layers back onto the frame buffer, each layer texture

 * (containing the original frame buffer data) is drawn as a simple quad over

 * the frame buffer. The trick is that the quad is set as the composition

 * destination in the blending equation, and the frame buffer becomes the source

 * of the composition.

 *

 * Drawing layers with an alpha value requires an extra step before composition.

 * An empty quad is drawn over the layer's region in the frame buffer. This quad

 * is drawn with the rgba color (0,0,0,alpha). The alpha value offered by the

 * quad is used to multiply the colors in the frame buffer. This is achieved by

 * changing the GL blend functions for the GL_FUNC_ADD blend equation to

 * GL_ZERO, GL_SRC_ALPHA.

 *

 * Because glCopyTexImage2D() can be slow, an alternative implementation might

 * be use to draw a single clipped layer. The implementation described above

 * is correct in every case.

 */

bool OpenGLRenderer::createLayer(sp<Snapshot> snapshot, float left, float top,

        float right, float bottom, int alpha, SkXfermode::Mode mode,int flags) {

    LAYER_LOGD("Requesting layer %fx%f", right - left, bottom - top);
@@ -323,6 +365,9 @@ bool OpenGLRenderer::createLayer(sp<Snapshot> snapshot, float left, float top, 
    return true;

}



/**

 * Read the documentation of createLayer() before doing anything in this method.

 */

void OpenGLRenderer::composeLayer(sp<Snapshot> current, sp<Snapshot> previous) {

    if (!current->layer) {

        LOGE("Attempting to compose a layer that does not exist");
@@ -337,16 +382,8 @@ void OpenGLRenderer::composeLayer(sp<Snapshot> current, sp<Snapshot> previous) { 
    const Rect& rect = layer->layer;



    if (layer->alpha < 255) {

        glEnable(GL_BLEND);

        glBlendFuncSeparate(GL_ZERO, GL_SRC_ALPHA, GL_DST_ALPHA, GL_ZERO);



        drawColorRect(rect.left, rect.top, rect.right, rect.bottom,

                layer->alpha << 24, SkXfermode::kSrcOver_Mode, true, true);



        glBlendFunc(mCaches.lastSrcMode, mCaches.lastDstMode);

        if (!mCaches.blend) {

            glDisable(GL_BLEND);

        }

                layer->alpha << 24, SkXfermode::kDstIn_Mode, true);

    }



    // Layers are already drawn with a top-left origin, don't flip the texture
@@ -846,7 +883,7 @@ void OpenGLRenderer::drawTextDecorations(const char* text, int bytesCount, float 
}



void OpenGLRenderer::drawColorRect(float left, float top, float right, float bottom,

        int color, SkXfermode::Mode mode, bool ignoreTransform, bool ignoreBlending) {

        int color, SkXfermode::Mode mode, bool ignoreTransform) {

    clearLayerRegions();



    // If a shader is set, preserve only the alpha
@@ -872,10 +909,8 @@ void OpenGLRenderer::drawColorRect(float left, float top, float right, float bot 
        mColorFilter->describe(description, mExtensions);

    }



    if (!ignoreBlending) {

    // Setup the blending mode

    chooseBlending(alpha < 255 || (mShader && mShader->blend()), mode, description);

    }



    // Build and use the appropriate shader

    useProgram(mCaches.programCache.get(description));

libs/hwui/OpenGLRenderer.h

+1 −2
Original line number Diff line number Diff line
@@ -176,8 +176,7 @@ private: 
     * @paran ignoreBlending True if the blending is set by the caller

     */

    void drawColorRect(float left, float top, float right, float bottom,

    		int color, SkXfermode::Mode mode, bool ignoreTransform = false,

    		bool ignoreBlending = false);

            int color, SkXfermode::Mode mode, bool ignoreTransform = false);



    /**

     * Draws a textured rectangle with the specified texture. The specified coordinates