Add device space "nudge" to gpu draws

This CL nudges all the GPU draws and clips slightly to match raster's round behavior for BW draws. We assume the effect will be negligible and do it for AA draws too.

BUG=423834

Review URL: https://codereview.chromium.org/877473005

1 files changed, 6 insertions, 4 deletions

diff --git a/src/gpu/gl/builders/GrGLVertexShaderBuilder.cpp b/src/gpu/gl/builders/GrGLVertexShaderBuilder.cpp
index f17e741169..f0b645ee42 100644
--- a/src/gpu/gl/builders/GrGLVertexShaderBuilder.cpp
+++ b/src/gpu/gl/builders/GrGLVertexShaderBuilder.cpp
@@ -47,13 +47,15 @@ void GrGLVertexBuilder::transformToNormalizedDeviceSpace(const GrShaderVar& posV
     // 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 "0.05" 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) + 0.05 * %s.x, (dot(%s.yz, %s.zw)/%s.z) + 0.05 * %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 + 0.05) * %s.x + %s.y, (%s.y + 0.05) * %s.z + %s.w, 0, 1);",
                           posVar.c_str(), fRtAdjustName, fRtAdjustName,
                           posVar.c_str(), fRtAdjustName, fRtAdjustName);
     }