diff options
| -rw-r--r-- | libs/hwui/OpenGLRenderer.cpp | 65 | ||||
| -rw-r--r-- | libs/hwui/OpenGLRenderer.h | 3 |
2 files changed, 51 insertions, 17 deletions
diff --git a/libs/hwui/OpenGLRenderer.cpp b/libs/hwui/OpenGLRenderer.cpp index ca4babe..a65edcd 100644 --- a/libs/hwui/OpenGLRenderer.cpp +++ b/libs/hwui/OpenGLRenderer.cpp @@ -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); - } + // Setup the blending mode + chooseBlending(alpha < 255 || (mShader && mShader->blend()), mode, description); // Build and use the appropriate shader useProgram(mCaches.programCache.get(description)); diff --git a/libs/hwui/OpenGLRenderer.h b/libs/hwui/OpenGLRenderer.h index 12ec276..3126754 100644 --- a/libs/hwui/OpenGLRenderer.h +++ b/libs/hwui/OpenGLRenderer.h @@ -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 |