sdf: use linear edge distance approximation for all gradient directions

src/core/SkDistanceFieldGen.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDistanceFieldGen.h"
 #include "SkPoint.h"
 
@@ skipping 89 matching lines @@
         }
         data += 2*pad;
         edges += 2*pad;
     }
 }
 
 // from Gustavson (2011)
 // computes the distance to an edge given an edge normal vector and a pixel's alpha value
 // assumes that direction has been pre-normalized
 static float edge_distance(const SkPoint& direction, float alpha) {
+#if 1    // formula (1)
+    return 0.5f - alpha;
+#else    // formula (4)
     float dx = direction.fX;
     float dy = direction.fY;
     float distance;
     if (SkScalarNearlyZero(dx) || SkScalarNearlyZero(dy)) {
         distance = 0.5f - alpha;
     } else {
         // this is easier if we treat the direction as being in the first octant
         // (other octants are symmetrical)
         dx = SkScalarAbs(dx);
         dy = SkScalarAbs(dy);
@@ skipping 16 matching lines @@
         } else if (alpha*dx < (dx - a1num)) {
             distance = (0.5f - alpha)*dx;
         // if 1 - a1 < alpha <= 1
         } else {
             // TODO: find a way to do this without square roots?
             distance = -0.5f*(dx + dy) + SkScalarSqrt(2.0f*dx*dy*(1.0f - alpha));
         }
     }
 
     return distance;
+#endif
 }
 
 static void init_distances(DFData* data, unsigned char* edges, int width, int height) {
     // skip one pixel border
     DFData* currData = data;
     DFData* prevData = data - width;
     DFData* nextData = data + width;
 
     for (int j = 0; j < height; ++j) {
         for (int i = 0; i < width; ++i) {
@@ skipping 356 matching lines @@
                 *currDestPtr++ = (mask & (1 << i)) ? 0xff : 0;
             }
         }
         currSrcScanLine += rowBytes;
         *currDestPtr++ = 0;
     }
     sk_bzero(currDestPtr, (width+2)*sizeof(char));
 
     return generate_distance_field_from_image(distanceField, copyPtr, width, height);
 }