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
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 @@
}
}
-
+/**
+ * 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 @@
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 @@
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::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 @@
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));