add SkMatrix::decomposeScale

src/core/SkMatrix.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMatrix.h"
 #include "SkFloatBits.h"
 #include "SkString.h"
@@ skipping 1585 matching lines @@
     SK_COMPILE_ASSERT(offsetof(SkMatrix, fTypeMask) == offsetof(PODMatrix, typemask), BadfTypeMask);
 
     static const PODMatrix invalid =
         { {SK_ScalarMax, SK_ScalarMax, SK_ScalarMax,
            SK_ScalarMax, SK_ScalarMax, SK_ScalarMax,
            SK_ScalarMax, SK_ScalarMax, SK_ScalarMax },
          kTranslate_Mask | kScale_Mask | kAffine_Mask | kPerspective_Mask };
     return invalid.asSkMatrix();
 }
 
+bool SkMatrix::decomposeScale(SkSize* scale, SkMatrix* remaining) const {
+    if (this->hasPerspective()) {
+        return false;
+    }
+
+    const SkScalar sx = SkVector::Length(this->getScaleX(), this->getSkewY());
+    const SkScalar sy = SkVector::Length(this->getSkewX(), this->getScaleY());
+    if (0 == sx || 0 == sy) {
+        return false;
+    }
+
+    if (scale) {
+        scale->set(sx, sy);
+    }
+    if (remaining) {
+        *remaining = *this;
+        remaining->postScale(SkScalarInvert(sx), SkScalarInvert(sy));

[Stephen White, 2015/03/19 18:27:50]

Hmm.. for my uses, I was also extracting the translate component from the
remainder matrix, so that the CTM looks at least positionally similar to what it
was before. Now the translation will be applied by the SkMatrixImageFilter, and
the CTM will always have 0, 0 translation. I guess that's ok, it'll just look a
little weird during debugging.

+    }
+    return true;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 size_t SkMatrix::writeToMemory(void* buffer) const {
     // TODO write less for simple matrices
     static const size_t sizeInMemory = 9 * sizeof(SkScalar);
     if (buffer) {
         memcpy(buffer, fMat, sizeInMemory);
     }
     return sizeInMemory;
 }
@@ skipping 168 matching lines @@
         rotation1->fX = cos1;
         rotation1->fY = sin1;
     }
     if (rotation2) {
         rotation2->fX = cos2;
         rotation2->fY = sin2;
     }
 
     return true;
 }