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>

3 files changed, 113 insertions, 9 deletions

diff --git a/gm/localmatrixshader.cpp b/gm/localmatrixshader.cpp
new file mode 100644
index 0000000000..bbd2775b7c
--- /dev/null
+++ b/gm/localmatrixshader.cpp
@@ -0,0 +1,101 @@
+/*
+ * Copyright 2018 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkCanvas.h"
+#include "SkShader.h"
+#include "SkSurface.h"
+#include "sk_tool_utils.h"
+
+static sk_sp<SkImage> make_image(SkCanvas* rootCanvas) {
+    static constexpr SkScalar kSize = 50;
+    SkImageInfo info = SkImageInfo::MakeN32Premul(kSize, kSize);
+    auto surface = sk_tool_utils::makeSurface(rootCanvas, info);
+
+    SkPaint p;
+    p.setAntiAlias(true);
+    p.setColor(SK_ColorGREEN);
+
+    surface->getCanvas()->drawCircle(kSize / 2, kSize / 2, kSize / 2, p);
+
+    p.setStyle(SkPaint::kStroke_Style);
+    p.setColor(SK_ColorRED);
+    surface->getCanvas()->drawLine(kSize * .25f, kSize * .50f, kSize * .75f, kSize * .50f, p);
+    surface->getCanvas()->drawLine(kSize * .50f, kSize * .25f, kSize * .50f, kSize * .75f, p);
+
+    return surface->makeImageSnapshot();
+}
+
+DEF_SIMPLE_GM(localmatrixshader_nested, canvas, 450, 1200) {
+    auto image = make_image(canvas);
+
+    using FactoryT = sk_sp<SkShader> (*)(const sk_sp<SkImage>&,
+                                         const SkMatrix& inner,
+                                         const SkMatrix& outer);
+    static const FactoryT gFactories[] = {
+        // SkLocalMatrixShader(SkImageShader(inner), outer)
+        [](const sk_sp<SkImage>& img, const SkMatrix& inner, const SkMatrix& outer) {
+            return img->makeShader(&inner)->makeWithLocalMatrix(outer);
+        },
+
+        // SkLocalMatrixShader(SkLocalMatrixShader(SkImageShader(I), inner), outer)
+        [](const sk_sp<SkImage>& img, const SkMatrix& inner, const SkMatrix& outer) {
+            return img->makeShader()->makeWithLocalMatrix(inner)->makeWithLocalMatrix(outer);
+        },
+
+        // SkLocalMatrixShader(SkComposeShader(SkImageShader(inner)), outer)
+        [](const sk_sp<SkImage>& img, const SkMatrix& inner, const SkMatrix& outer) {
+            return SkShader::MakeCompose(SkShader::MakeColorShader(SK_ColorTRANSPARENT),
+                                         img->makeShader(&inner),
+                                         SkBlendMode::kSrcOver)
+                   ->makeWithLocalMatrix(outer);
+        },
+
+        // SkLocalMatrixShader(SkComposeShader(SkLocalMatrixShader(SkImageShader(I), inner)), outer)
+        [](const sk_sp<SkImage>& img, const SkMatrix& inner, const SkMatrix& outer) {
+            return SkShader::MakeCompose(SkShader::MakeColorShader(SK_ColorTRANSPARENT),
+                                         img->makeShader()->makeWithLocalMatrix(inner),
+                                         SkBlendMode::kSrcOver)
+                   ->makeWithLocalMatrix(outer);
+        },
+    };
+
+    static const auto inner = SkMatrix::MakeScale(2, 2),
+                      outer = SkMatrix::MakeTrans(20, 20);
+
+    SkPaint border;
+    border.setAntiAlias(true);
+    border.setStyle(SkPaint::kStroke_Style);
+
+    auto rect = SkRect::Make(image->bounds());
+    SkAssertResult(SkMatrix::Concat(inner, outer).mapRect(&rect));
+
+    const auto drawColumn = [&]() {
+        SkAutoCanvasRestore acr(canvas, true);
+        for (const auto& f : gFactories) {
+            SkPaint p;
+            p.setShader(f(image, inner, outer));
+
+            canvas->drawRect(rect, p);
+            canvas->drawRect(rect, border);
+
+            canvas->translate(0, rect.height() * 1.5f);
+        }
+    };
+
+    drawColumn();
+
+    {
+        SkAutoCanvasRestore acr(canvas, true);
+        canvas->translate(0, rect.height() * SK_ARRAY_COUNT(gFactories) * 1.5f);
+        drawColumn();
+    }
+
+    canvas->translate(rect.width() * 1.5f, 0);
+    canvas->scale(2, 2);
+    drawColumn();
+}
diff --git a/gn/gm.gni b/gn/gm.gni
index 1558881e06..b1f5135068 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -201,6 +201,7 @@ gm_sources = [
   "$_gm/linepaths.cpp",
   "$_gm/localmatriximagefilter.cpp",
   "$_gm/localmatriximageshader.cpp",
+  "$_gm/localmatrixshader.cpp",
   "$_gm/lumafilter.cpp",
   "$_gm/makecolorspace.cpp",
   "$_gm/makeRasterImage.cpp",
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);
 }