Add device space "nudge" to gpu draws

src/gpu/gl/builders/GrGLVertexShaderBuilder.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 "GrGLVertexShaderBuilder.h"
 #include "GrGLProgramBuilder.h"
 #include "GrGLShaderStringBuilder.h"
@@ skipping 29 matching lines @@
     // setup RT Uniform
     fProgramBuilder->fUniformHandles.fRTAdjustmentUni =
             fProgramBuilder->addUniform(GrGLProgramBuilder::kVertex_Visibility,
                                         kVec4f_GrSLType, kDefault_GrSLPrecision,
                                         fProgramBuilder->rtAdjustment(),
                                         &fRtAdjustName);
 
     // Transform from Skia's device coords to GL's normalized device coords. Note that
     // because we want to "nudge" the device space positions we are converting to 
     // non-homogeneous NDC.
+    // The "0.003" nudge serves to match the raster path's rounding for bw draws.
+    // For aa draws we just assume the impact will be minimal - so we always perform the nudge.
     if (kVec3f_GrSLType == posVar.getType()) {
-        this->codeAppendf("gl_Position = vec4(dot(%s.xz, %s.xy)/%s.z, dot(%s.yz, %s.zw)/%s.z, 0, 1);",
-                          posVar.c_str(), fRtAdjustName, posVar.c_str(),
-                          posVar.c_str(), fRtAdjustName, posVar.c_str());
+        this->codeAppendf("gl_Position = vec4((dot(%s.xz, %s.xy)/%s.z) + 0.003 * %s.x, (dot(%s.yz, %s.zw)/%s.z) + 0.003 * %s.z, 0, 1);",
+                          posVar.c_str(), fRtAdjustName, posVar.c_str(), fRtAdjustName,
+                          posVar.c_str(), fRtAdjustName, posVar.c_str(), fRtAdjustName);
     } else {
         SkASSERT(kVec2f_GrSLType == posVar.getType());
-        this->codeAppendf("gl_Position = vec4(%s.x * %s.x + %s.y, %s.y * %s.z + %s.w, 0, 1);",
+        this->codeAppendf("gl_Position = vec4((%s.x + 0.003) * %s.x + %s.y, (%s.y + 0.003) * %s.z + %s.w, 0, 1);",
                           posVar.c_str(), fRtAdjustName, fRtAdjustName,
                           posVar.c_str(), fRtAdjustName, fRtAdjustName);
     }
 
     // We could have the GrGeometryProcessor do this, but its just easier to have it performed here.
     // If we ever need to set variable pointsize, then we can reinvestigate
     this->codeAppend("gl_PointSize = 1.0;");
 }
 
 void GrGLVertexBuilder::bindVertexAttributes(GrGLuint programID) {
@@ skipping 33 matching lines @@
     for (int i = 0; i < fInputs.count(); ++i) {
         const GrGLShaderVar& attr = fInputs[i];
         // if attribute already added, don't add it again
         if (attr.getName().equals(var.getName())) {
             return false;
         }
     }
     fInputs.push_back(var);
     return true;
 }