Update Alligned Rect Shader to match geometry version

src/gpu/GrAARectRenderer.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrAARectRenderer.h"
 #include "GrRefCnt.h"
 #include "GrGpu.h"
@@ skipping 41 matching lines @@
                               const TextureSamplerArray& samplers) SK_OVERRIDE {
             // setup the varying for the Axis aligned rect effect
             //      xy -> interpolated offset
             //      zw -> w/2+0.5, h/2+0.5
             const char *vsRectName, *fsRectName;
             builder->addVarying(kVec4f_GrSLType, "Rect", &vsRectName, &fsRectName);
             const SkString* attr0Name =
                 builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[0]);
             builder->vsCodeAppendf("\t%s = %s;\n", vsRectName, attr0Name->c_str());
 
-            // TODO: compute these scale factors in the VS
-            // These scale factors adjust the coverage for < 1 pixel wide/high rects
-            builder->fsCodeAppendf("\tfloat wScale = max(1.0, 2.0/(0.5+%s.z));\n",
-                                   fsRectName);
-            builder->fsCodeAppendf("\tfloat hScale = max(1.0, 2.0/(0.5+%s.w));\n",
-                                   fsRectName);
+            // TODO: compute all these offsets, spans, and scales in the VS
+            builder->fsCodeAppendf("\tfloat insetW = min(1.0, %s.z) - 0.5;\n", fsRectName);
+            builder->fsCodeAppendf("\tfloat insetH = min(1.0, %s.w) - 0.5;\n", fsRectName);
+            builder->fsCodeAppend("\tfloat outset = 0.5;\n");
+            // For rects > 1 pixel wide and tall the span's are noops (i.e., 1.0). For rects
+            // < 1 pixel wide or tall they serve to normalize the < 1 ramp to a 0 .. 1 range.
+            builder->fsCodeAppend("\tfloat spanW = insetW + outset;\n");
+            builder->fsCodeAppend("\tfloat spanH = insetH + outset;\n");
+            // For rects < 1 pixel wide or tall, these scale factors are used to cap the maximum
+            // value of coverage that is used. In other words it is the coverage that is
+            // used in the interior of the rect after the ramp.
+            builder->fsCodeAppend("\tfloat scaleW = min(1.0, 2.0*insetW/spanW);\n");
+            builder->fsCodeAppend("\tfloat scaleH = min(1.0, 2.0*insetH/spanH);\n");
 
             // Compute the coverage for the rect's width
-            builder->fsCodeAppendf("\tfloat coverage = clamp(wScale*(%s.z-abs(%s.x)), 0.0, 1.0);\n",
-                                   fsRectName,
-                                   fsRectName);
-
+            builder->fsCodeAppendf(
+                "\tfloat coverage = scaleW*clamp((%s.z-abs(%s.x))/spanW, 0.0, 1.0);\n", fsRectName,
+                fsRectName);
             // Compute the coverage for the rect's height and merge with the width
             builder->fsCodeAppendf(
-                    "\tcoverage = min(coverage, clamp(hScale*(%s.w-abs(%s.y)), 0.0, 1.0));\n",
-                    fsRectName,
-                    fsRectName);
+                "\tcoverage = coverage*scaleH*clamp((%s.w-abs(%s.y))/spanH, 0.0, 1.0);\n",
+                fsRectName, fsRectName);
 
             SkString modulate;
             GrGLSLModulatef<4>(&modulate, inputColor, "coverage");
             builder->fsCodeAppendf("\t%s = %s;\n", outputColor, modulate.c_str());
         }
 
         static inline EffectKey GenKey(const GrDrawEffect& drawEffect, const GrGLCaps&) {
             return 0;
         }
 
@@ skipping 689 matching lines @@
     // can't call mapRect for devInside since it calls sort
     combinedMatrix.mapPoints((SkPoint*)&devInside, (const SkPoint*)&rects[1], 2);
 
     if (devInside.isEmpty()) {
         this->fillAARect(gpu, target, devOutside, SkMatrix::I(), devOutside, useVertexCoverage);
         return;
     }
 
     this->geometryStrokeAARect(gpu, target, devOutside, devInside, useVertexCoverage);
 }