use forward matrix to determine if we can ignore scale part of a matrix

src/core/SkBitmapProcState.cpp

Index: src/core/SkBitmapProcState.cpp
===================================================================
--- src/core/SkBitmapProcState.cpp	(revision 8685)
+++ src/core/SkBitmapProcState.cpp	(working copy)
@@ -41,6 +41,7 @@
         return false;
     }
     if (mask & SkMatrix::kScale_Mask) {
+#if 0
         SkScalar sx = matrix[SkMatrix::kMScaleX];
         SkScalar sy = matrix[SkMatrix::kMScaleY];
         int w = bitmap.width();
@@ -48,6 +49,14 @@
         int sw = SkScalarRound(SkScalarMul(sx, SkIntToScalar(w)));
         int sh = SkScalarRound(SkScalarMul(sy, SkIntToScalar(h)));
         return sw == w && sh == h;
+#else
+        SkRect src, dst;
+        bitmap.getBounds(&src);
+        matrix.mapRect(&dst, src);
+        SkIRect idst;
+        dst.round(&idst);
+        return idst.width() == bitmap.width() && idst.height() == bitmap.height();
+#endif
     }
     // if we got here, we're either kTranslate_Mask or identity
     return true;
@@ -81,6 +90,11 @@
     return (dimension & ~0x3FFF) == 0;
 }
 
+static bool matrix_only_scale_translate(const SkMatrix& m) {

[robertphillips, 2013/04/15 19:43:02]

This is done as "if (mask <= (SkMatrix::kTranslate_Mask |
SkMatrix::kScale_Mask))" elsewhere.

[reed1, 2013/04/15 20:00:26]

Done.

+    unsigned mask = SkMatrix::kScale_Mask | SkMatrix::kTranslate_Mask;
+    return 0 == (m.getType() & ~mask);
+}
+
 bool SkBitmapProcState::chooseProcs(const SkMatrix& inv, const SkPaint& paint) {
     if (fOrigBitmap.width() == 0 || fOrigBitmap.height() == 0) {
         return false;
@@ -120,7 +134,7 @@
     }
 
     // wack our matrix to exactly no-scale, if we're really close to begin with
-    {
+    if (false) {
         bool fixupMatrix = clamp_clamp ?
         just_trans_clamp(*m, *fBitmap) : just_trans_general(*m);
         if (fixupMatrix) {
@@ -139,6 +153,20 @@
             }
         }
     }
+    if (matrix_only_scale_translate(*m)) {
+        SkMatrix forward;
+        if (m->invert(&forward)) {
+            if (clamp_clamp ? just_trans_clamp(forward, *fBitmap)
+                            : just_trans_general(forward)) {
+                SkScalar tx = -SkScalarRoundToScalar(forward.getTranslateX());
+                SkScalar ty = -SkScalarRoundToScalar(forward.getTranslateY());
+                fUnitInvMatrix.setTranslate(tx, ty);
+                m = &fUnitInvMatrix;
+                // now the following code will sniff m, and decide to take the
+                // fast case (since m is purely translate).
+            }
+        }
+    }
 
     // Below this point, we should never refer to the inv parameter, since we
     // may be using a munged version for "our" inverse.