Provide a GPU implementation of SkArithmeticMode

gm/arithmode.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkCanvas.h"
 #include "SkColorPriv.h"
@@ skipping 36 matching lines @@
     SkColor colors[] = {
         SK_ColorBLUE, SK_ColorYELLOW, SK_ColorBLACK, SK_ColorGREEN, SK_ColorGRAY
     };
     SkShader* s = SkGradientShader::CreateLinear(pts, colors, NULL, SK_ARRAY_COUNT(colors),
                                                  SkShader::kClamp_TileMode);
     paint.setShader(s)->unref();
     canvas.drawPaint(paint);
     return bm;
 }
 
-static SkBitmap make_arith(const SkBitmap& src, const SkBitmap& dst,
-                           const SkScalar k[]) {
-    SkBitmap bm = make_bm();
-    SkCanvas canvas(bm);
-    SkPaint paint;
-    canvas.drawBitmap(dst, 0, 0, NULL);
-    SkXfermode* xfer = SkArithmeticMode::Create(k[0], k[1], k[2], k[3]);
-    paint.setXfermode(xfer)->unref();
-    canvas.drawBitmap(src, 0, 0, &paint);
-    return bm;
-}
-
 static void show_k_text(SkCanvas* canvas, SkScalar x, SkScalar y, const SkScalar k[]) {
     SkPaint paint;
     paint.setTextSize(SkIntToScalar(24));
     paint.setAntiAlias(true);
     for (int i = 0; i < 4; ++i) {
         SkString str;
         str.appendScalar(k[i]);
         SkScalar width = paint.measureText(str.c_str(), str.size());
         canvas->drawText(str.c_str(), str.size(), x, y + paint.getTextSize(), paint);
         x += width + SkIntToScalar(10);
@@ skipping 30 matching lines @@
             one/4, one/2, one/2, 0,
             -one/4, one/2, one/2, 0,
         };
 
         const SkScalar* k = K;
         const SkScalar* stop = k + SK_ARRAY_COUNT(K);
         SkScalar y = 0;
         SkScalar gap = SkIntToScalar(src.width() + 20);
         while (k < stop) {
             SkScalar x = 0;
-            SkBitmap res = make_arith(src, dst, k);
             canvas->drawBitmap(src, x, y, NULL);
             x += gap;
             canvas->drawBitmap(dst, x, y, NULL);
             x += gap;
-            canvas->drawBitmap(res, x, y, NULL);
+            SkRect rect = SkRect::MakeXYWH(x, y, SkIntToScalar(WW), SkIntToScalar(HH));
+            canvas->saveLayer(&rect, NULL);
+            canvas->drawBitmap(dst, x, y, NULL);
+            SkXfermode* xfer = SkArithmeticMode::Create(k[0], k[1], k[2], k[3]);
+            SkPaint paint;
+            paint.setXfermode(xfer)->unref();
+            canvas->drawBitmap(src, x, y, &paint);
+            canvas->restore();
             x += gap;
             show_k_text(canvas, x, y, k);
             k += 4;
             y += SkIntToScalar(src.height() + 12);
         }
     }
 
 private:
     typedef GM INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* MyFactory(void*) { return new ArithmodeGM; }
 static skiagm::GMRegistry reg(MyFactory);