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 8695)
+++ src/core/SkBitmapProcState.cpp	(working copy)
@@ -30,36 +30,39 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
+// true iff the matrix contains, at most, scale and translate elements
+static bool matrix_only_scale_translate(const SkMatrix& m) {
+    return m.getType() <= SkMatrix::kScale_Mask | SkMatrix::kTranslate_Mask;
+}
+
 /**
  *  For the purposes of drawing bitmaps, if a matrix is "almost" translate
  *  go ahead and treat it as if it were, so that subsequent code can go fast.
  */
 static bool just_trans_clamp(const SkMatrix& matrix, const SkBitmap& bitmap) {
-    SkMatrix::TypeMask mask = matrix.getType();
+    SkASSERT(matrix_only_scale_translate(matrix));
 
-    if (mask & (SkMatrix::kAffine_Mask | SkMatrix::kPerspective_Mask)) {
-        return false;
+    if (matrix.getType() & SkMatrix::kScale_Mask) {
+        SkRect src, dst;
+        bitmap.getBounds(&src);
+        matrix.mapRect(&dst, src);
+
+        // Now round all 4 edges to device space, and then compare the device
+        // width/height to the original. Note: we must map all 4 and subtract
+        // rather than map the "width" and compare, since we care about the
+        // phase (in pixel space) that any translate in the matrix might impart.
+        SkIRect idst;
+        dst.round(&idst);
+        return idst.width() == bitmap.width() && idst.height() == bitmap.height();
     }
-    if (mask & SkMatrix::kScale_Mask) {
-        SkScalar sx = matrix[SkMatrix::kMScaleX];
-        SkScalar sy = matrix[SkMatrix::kMScaleY];
-        int w = bitmap.width();
-        int h = bitmap.height();
-        int sw = SkScalarRound(SkScalarMul(sx, SkIntToScalar(w)));
-        int sh = SkScalarRound(SkScalarMul(sy, SkIntToScalar(h)));
-        return sw == w && sh == h;
-    }
     // if we got here, we're either kTranslate_Mask or identity
     return true;
 }
 
 static bool just_trans_general(const SkMatrix& matrix) {
-    SkMatrix::TypeMask mask = matrix.getType();
+    SkASSERT(matrix_only_scale_translate(matrix));
 
-    if (mask & (SkMatrix::kAffine_Mask | SkMatrix::kPerspective_Mask)) {
-        return false;
-    }
-    if (mask & SkMatrix::kScale_Mask) {
+    if (matrix.getType() & SkMatrix::kScale_Mask) {
         const SkScalar tol = SK_Scalar1 / 32768;
 
         if (!SkScalarNearlyZero(matrix[SkMatrix::kMScaleX] - SK_Scalar1, tol)) {
@@ -120,16 +123,11 @@
     }
 
     // wack our matrix to exactly no-scale, if we're really close to begin with
-    {
-        bool fixupMatrix = clamp_clamp ?
-        just_trans_clamp(*m, *fBitmap) : just_trans_general(*m);
-        if (fixupMatrix) {
-            // If we can be treated just like translate, construct that inverse
-            // such that we landed in the proper place. Given that m may have
-            // some slight scale, we have to invert it to compute this new
-            // matrix.
-            SkMatrix forward;
-            if (m->invert(&forward)) {
+    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);