Fix SkLocalMatrixShader nesting semantics

Inherited/outer local matrices are supposed to compose to the right of
(preconcat) other/nested local matrices.

BUG=skia:7781

Change-Id: Icd3c24f226845427be849a8be3d78293aef176b3
Reviewed-on: https://skia-review.googlesource.com/118344
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>

1 files changed, 11 insertions, 9 deletions

diff --git a/src/shaders/SkLocalMatrixShader.cpp b/src/shaders/SkLocalMatrixShader.cpp
index a08fea14a5..e352d31299 100644
--- a/src/shaders/SkLocalMatrixShader.cpp
+++ b/src/shaders/SkLocalMatrixShader.cpp
@@ -6,6 +6,7 @@
  */
 
 #include "SkLocalMatrixShader.h"
+#include "SkTLazy.h"
 
 #if SK_SUPPORT_GPU
 #include "GrFragmentProcessor.h"
@@ -42,14 +43,14 @@ void SkLocalMatrixShader::flatten(SkWriteBuffer& buffer) const {
 SkShaderBase::Context* SkLocalMatrixShader::onMakeContext(
     const ContextRec& rec, SkArenaAlloc* alloc) const
 {
-    ContextRec newRec(rec);
-    SkMatrix tmp;
+    SkTCopyOnFirstWrite<SkMatrix> lm(this->getLocalMatrix());
     if (rec.fLocalMatrix) {
-        tmp.setConcat(*rec.fLocalMatrix, this->getLocalMatrix());
-        newRec.fLocalMatrix = &tmp;
-    } else {
-        newRec.fLocalMatrix = &this->getLocalMatrix();
+        lm.writable()->preConcat(*rec.fLocalMatrix);
     }
+
+    ContextRec newRec(rec);
+    newRec.fLocalMatrix = lm;
+
     return as_SB(fProxyShader)->makeContext(newRec, alloc);
 }
 
@@ -65,12 +66,13 @@ SkImage* SkLocalMatrixShader::onIsAImage(SkMatrix* outMatrix, enum TileMode* mod
 }
 
 bool SkLocalMatrixShader::onAppendStages(const StageRec& rec) const {
-    SkMatrix tmp;
+    SkTCopyOnFirstWrite<SkMatrix> lm(this->getLocalMatrix());
     if (rec.fLocalM) {
-        tmp.setConcat(*rec.fLocalM, this->getLocalMatrix());
+        lm.writable()->preConcat(*rec.fLocalM);
     }
+
     StageRec newRec = rec;
-    newRec.fLocalM = rec.fLocalM ? &tmp : &this->getLocalMatrix();
+    newRec.fLocalM = lm;
     return as_SB(fProxyShader)->appendStages(newRec);
 }