Use dFdx in Vulkan to address distance field issues.

On Nexus Player and occasionally Nexus 5x we get transparent boxes around
paths. This appears to be because the dFdy call is not as accurate as
dFdx, which is the opposite of Mali 400. As Mali 400 is not supported with
Vulkan, we can go back to using dFdx in this case.

BUG=skia:5523
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2163213004

Review-Url: https://codereview.chromium.org/2163213004

1 files changed, 32 insertions, 5 deletions

diff --git a/src/gpu/effects/GrDistanceFieldGeoProc.cpp b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
index f43a1e8994..cb1e496f97 100644
--- a/src/gpu/effects/GrDistanceFieldGeoProc.cpp
+++ b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
@@ -119,11 +119,16 @@ public:
             // For uniform scale, we adjust for the effect of the transformation on the distance
             // by using the length of the gradient of the t coordinate in the y direction.
             // We use st coordinates to ensure we're mapping 1:1 from texel space to pixel space.
-            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
 
             // this gives us a smooth step across approximately one fragment
+#ifdef SK_VULKAN
+            fragBuilder->codeAppendf("afwidth = abs(" SK_DistanceFieldAAFactor "*dFdx(%s.x));",
+                                     st.fsIn());
+#else
+            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
             fragBuilder->codeAppendf("afwidth = abs(" SK_DistanceFieldAAFactor "*dFdy(%s.y));",
                                      st.fsIn());
+#endif
         } else if (isSimilarity) {
             // For similarity transform, we adjust the effect of the transformation on the distance
             // by using the length of the gradient of the texture coordinates. We use st coordinates
@@ -131,7 +136,12 @@ public:
             // We use the y gradient because there is a bug in the Mali 400 in the x direction.
 
             // this gives us a smooth step across approximately one fragment
+#ifdef SK_VULKAN
+            fragBuilder->codeAppendf("float st_grad_len = length(dFdx(%s));", st.fsIn());
+#else
+            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
             fragBuilder->codeAppendf("float st_grad_len = length(dFdy(%s));", st.fsIn());
+#endif
             fragBuilder->codeAppend("afwidth = abs(" SK_DistanceFieldAAFactor "*st_grad_len);");
         } else {
             // For general transforms, to determine the amount of correction we multiply a unit
@@ -368,19 +378,26 @@ public:
             // For uniform scale, we adjust for the effect of the transformation on the distance
             // by using the length of the gradient of the t coordinate in the y direction.
             // We use st coordinates to ensure we're mapping 1:1 from texel space to pixel space.
-            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
 
             // this gives us a smooth step across approximately one fragment
+#ifdef SK_VULKAN
+            fragBuilder->codeAppend("afwidth = abs(" SK_DistanceFieldAAFactor "*dFdx(st.x));");
+#else
+            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
             fragBuilder->codeAppend("afwidth = abs(" SK_DistanceFieldAAFactor "*dFdy(st.y));");
-
+#endif
         } else if (isSimilarity) {
             // For similarity transform, we adjust the effect of the transformation on the distance
             // by using the length of the gradient of the texture coordinates. We use st coordinates
             // to ensure we're mapping 1:1 from texel space to pixel space.
-            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
 
             // this gives us a smooth step across approximately one fragment
+#ifdef SK_VULKAN
+            fragBuilder->codeAppend("float st_grad_len = length(dFdx(st));");
+#else
+            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
             fragBuilder->codeAppend("float st_grad_len = length(dFdy(st));");
+#endif
             fragBuilder->codeAppend("afwidth = abs(" SK_DistanceFieldAAFactor "*st_grad_len);");
         } else {
             // For general transforms, to determine the amount of correction we multiply a unit
@@ -610,16 +627,26 @@ public:
             fragBuilder->codeAppendf("float delta = %.*f;\n", SK_FLT_DECIMAL_DIG, lcdDelta);
         }
         if (isUniformScale) {
+#ifdef SK_VULKAN
+            fragBuilder->codeAppendf("float st_grad_len = abs(dFdx(%s.x));", st.fsIn());
+#else
+            // We use the y gradient because there is a bug in the Mali 400 in the x direction.
             fragBuilder->codeAppendf("float st_grad_len = abs(dFdy(%s.y));", st.fsIn());
+#endif
             fragBuilder->codeAppend("vec2 offset = vec2(st_grad_len*delta, 0.0);");
         } else if (isSimilarity) {
             // For a similarity matrix with rotation, the gradient will not be aligned
             // with the texel coordinate axes, so we need to calculate it.
+#ifdef SK_VULKAN
+            fragBuilder->codeAppendf("vec2 st_grad = dFdx(%s);", st.fsIn());
+            fragBuilder->codeAppend("vec2 offset = delta*st_grad;");
+#else
             // We use dFdy because of a Mali 400 bug, and rotate -90 degrees to
             // get the gradient in the x direction.
             fragBuilder->codeAppendf("vec2 st_grad = dFdy(%s);", st.fsIn());
-            fragBuilder->codeAppend("float st_grad_len = length(st_grad);");
             fragBuilder->codeAppend("vec2 offset = delta*vec2(st_grad.y, -st_grad.x);");
+#endif
+            fragBuilder->codeAppend("float st_grad_len = length(st_grad);");
         } else {
             fragBuilder->codeAppendf("vec2 st = %s;\n", st.fsIn());