Docs updates.

Change-Id: Iff395e1c6d81a484a2c4ad6e88357c0134971b78

2 files changed, 14 insertions, 1 deletions

diff --git a/libs/hwui/OpenGLRenderer.cpp b/libs/hwui/OpenGLRenderer.cpp
index a65edcd..b1f5f6b 100644
--- a/libs/hwui/OpenGLRenderer.cpp
+++ b/libs/hwui/OpenGLRenderer.cpp
@@ -80,6 +80,9 @@ static const Blender gBlends[] = {
         { SkXfermode::kXor_Mode,     GL_ONE_MINUS_DST_ALPHA,  GL_ONE_MINUS_SRC_ALPHA }
 };
 
+// This array contains the swapped version of each SkXfermode. For instance
+// this array's SrcOver blending mode is actually DstOver. You can refer to
+// createLayer() for more information on the purpose of this array.
 static const Blender gBlendsSwap[] = {
         { SkXfermode::kClear_Mode,   GL_ZERO,                 GL_ZERO },
         { SkXfermode::kSrc_Mode,     GL_ZERO,                 GL_ONE },
@@ -302,7 +305,7 @@ int OpenGLRenderer::saveLayerAlpha(float left, float top, float right, float bot
  * 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
+ * using glCopyTexImage2D(). The layer's region is then cleared(1) in the frame
  * buffer and drawing continues as normal. This technique therefore treats the
  * frame buffer as a scratch buffer for the layers.
  *
@@ -322,6 +325,11 @@ int OpenGLRenderer::saveLayerAlpha(float left, float top, float right, float bot
  * 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.
+ *
+ * (1) The frame buffer is actually not cleared right away. To allow the GPU
+ *     to potentially optimize series of calls to glCopyTexImage2D, the frame
+ *     buffer is left untouched until the first drawing operation. Only when
+ *     something actually gets drawn are the layers regions cleared.
  */
 bool OpenGLRenderer::createLayer(sp<Snapshot> snapshot, float left, float top,
         float right, float bottom, int alpha, SkXfermode::Mode mode,int flags) {
diff --git a/libs/hwui/Snapshot.h b/libs/hwui/Snapshot.h
index ebedf53..062c986 100644
--- a/libs/hwui/Snapshot.h
+++ b/libs/hwui/Snapshot.h
@@ -116,6 +116,11 @@ public:
     bool clipTransformed(const Rect& r, SkRegion::Op op = SkRegion::kIntersect_Op) {
         bool clipped = false;
 
+        // NOTE: The unimplemented operations require support for regions
+        // Supporting regions would require using a stencil buffer instead
+        // of the scissor. The stencil buffer itself is not too expensive
+        // (memory cost excluded) but on fillrate limited devices, managing
+        // the stencil might have a negative impact on the framerate.
         switch (op) {
             case SkRegion::kDifference_Op:
                 break;