add helper to create RSXform w/ anchorPt

samplecode/SampleAtlas.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SampleCode.h"
 #include "SkAnimTimer.h"
 #include "SkView.h"
@@ skipping 78 matching lines @@
             if (fAlpha > 1) {
                 fAlpha = 1;
                 fDAlpha = -fDAlpha;
             } else if (fAlpha < 0) {
                 fAlpha = 0;
                 fDAlpha = -fDAlpha;
             }
         }
         
         SkRSXform asRSXform() const {
-            SkMatrix m;
-            m.setTranslate(-8, -8);
-            m.postScale(fScale, fScale);
-            m.postRotate(SkRadiansToDegrees(fRadian));
-            m.postTranslate(fCenter.fX, fCenter.fY);
-
-            SkRSXform x;
-            x.fSCos = m.getScaleX();
-            x.fSSin = m.getSkewY();
-            x.fTx = m.getTranslateX();
-            x.fTy = m.getTranslateY();
-            return x;
+            return SkRSXform::MakeFromRadians(fScale, fRadian, fCenter.x(), fCenter.y(),
+                                              SkScalarHalf(kCellSize), SkScalarHalf(kCellSize));
         }
     };
 
     enum {
         N = 256,
     };
 
     SkAutoTUnref<SkImage> fAtlas;
     Rec         fRec[N];
     SkRect      fTex[N];
@@ skipping 10 matching lines @@
         for (int y = 0; y < kAtlasSize; y += kCellSize) {
             for (int x = 0; x < kAtlasSize; x += kCellSize) {
                 const SkScalar sx = SkIntToScalar(x);
                 const SkScalar sy = SkIntToScalar(y);
                 fTex[i].setXYWH(sx, sy, cell, cell);
                 
                 fRec[i].fCenter.set(sx + cell/2, sy + 3*cell/4);
                 fRec[i].fVelocity.fX = rand.nextSScalar1() * kMaxSpeed;
                 fRec[i].fVelocity.fY = rand.nextSScalar1() * kMaxSpeed;
                 fRec[i].fScale = 1;
-                fRec[i].fDScale = rand.nextSScalar1() / 4;
+                fRec[i].fDScale = rand.nextSScalar1() / 16;
                 fRec[i].fRadian = 0;
                 fRec[i].fDRadian = rand.nextSScalar1() / 8;
                 fRec[i].fAlpha = rand.nextUScalar1();
                 fRec[i].fDAlpha = rand.nextSScalar1() / 10;
                 i += 1;
             }
         }
     }
 
     void toggleUseColors() {
@@ skipping 75 matching lines @@
 #endif
 
 private:
     typedef SampleView INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 static SkView* MyFactory() { return new DrawAtlasView; }
 static SkViewRegister reg(MyFactory);