SkMatrix::preScale() is too conservative

SkMatrix::preScale() always sets the kScale bit, which means something
like

  m = SkMatrix::MakeScale(2, 2);
  m.preScale(0.5, 0.5);

leaves m.getType() == kScale_Mask, and can throw off the bitmap proc
heuristics.

We could detect the inverse case and clear the scale bit instead.

R=reed@google.com,caryclark@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1531323002

Review URL: https://codereview.chromium.org/1531323002

1 files changed, 13 insertions, 0 deletions

diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index 6101eb9f9a..67ae052e06 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -380,7 +380,20 @@ void SkMatrix::preScale(SkScalar sx, SkScalar sy) {
     fMat[kMScaleY] *= sy;
     fMat[kMPersp1] *= sy;
 
+#ifndef SK_SUPPORT_LEGACY_PRESCALE_SEMANTICS
+    // Attempt to simplify our type when applying an inverse scale.
+    // TODO: The persp/affine preconditions are in place to keep the mask consistent with
+    //       what computeTypeMask() would produce (persp/skew always implies kScale).
+    //       We should investigate whether these flag dependencies are truly needed.
+    if (fMat[kMScaleX] == 1 && fMat[kMScaleY] == 1
+        && !(fTypeMask & (kPerspective_Mask | kAffine_Mask))) {
+        this->clearTypeMask(kScale_Mask);
+    } else {
+        this->orTypeMask(kScale_Mask);
+    }
+#else
     this->orTypeMask(kScale_Mask);
+#endif
 }
 
 void SkMatrix::postScale(SkScalar sx, SkScalar sy, SkScalar px, SkScalar py) {