Fix T-junctions in layers' generated meshes

bug:4128442

Currently we fix T-junctions for hardware layers, and for
Canvas.saveLayer if the transform isn't rectAsRect. Ideally, hardware
layers should lazily re-generate T-junction free geometry if the
transform becomes non-rectAsRect

Depends on frameworks/native change:
https://googleplex-android-review.googlesource.com/#/c/277668/
which adds Region::createTJunctionFreeRegion()

Change-Id: Ice75b4a1a4459ff835059ea81d70f7d1563fd96d

1 files changed, 10 insertions, 8 deletions

diff --git a/libs/hwui/LayerRenderer.cpp b/libs/hwui/LayerRenderer.cpp
index bc660cd8c2a9..14c7c39b39e3 100644
--- a/libs/hwui/LayerRenderer.cpp
+++ b/libs/hwui/LayerRenderer.cpp
@@ -123,13 +123,9 @@ Region* LayerRenderer::getRegion() {
     return &mLayer->region;
 }
 
-// TODO: This implementation is flawed and can generate T-junctions
-//       in the mesh, which will in turn produce cracks when the
-//       mesh is rotated/skewed. The easiest way to fix this would
-//       be, for each row, to add new vertices shared with the previous
-//       row when the two rows share an edge.
-//       In practice, T-junctions do not appear often so this has yet
-//       to be fixed.
+// TODO: This implementation uses a very simple approach to fixing T-junctions which keeps the
+//       results as rectangles, and is thus not necessarily efficient in the geometry
+//       produced. Eventually, it may be better to develop triangle-based mechanism.
 void LayerRenderer::generateMesh() {
     if (mLayer->region.isRect() || mLayer->region.isEmpty()) {
         if (mLayer->mesh) {
@@ -145,8 +141,14 @@ void LayerRenderer::generateMesh() {
         return;
     }
 
+    // avoid T-junctions as they cause artifacts in between the resultant
+    // geometry when complex transforms occur.
+    // TODO: generate the safeRegion only if necessary based on drawing transform (see
+    // OpenGLRenderer::composeLayerRegion())
+    Region safeRegion = Region::createTJunctionFreeRegion(mLayer->region);
+
     size_t count;
-    const android::Rect* rects = mLayer->region.getArray(&count);
+    const android::Rect* rects = safeRegion.getArray(&count);
 
     GLsizei elementCount = count * 6;