Add proper aliased support for SDF text

Previously, when asked to render aliased text with distance fields,
we would get the aliased glyph from the cache and then try to
anti-alias the edge. This change instead grabs the anti-aliased glyph,
then deliberately aliases the edge.

Bug: chromium:707979
Change-Id: I05766af17d7ae58bca27aaffd9e08e5c586e789c
Reviewed-on: https://skia-review.googlesource.com/21728
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>

2 files changed, 11 insertions, 5 deletions

diff --git a/src/gpu/effects/GrDistanceFieldGeoProc.cpp b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
index f0f4ec4fa3..c8dd4ebd77 100644
--- a/src/gpu/effects/GrDistanceFieldGeoProc.cpp
+++ b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
@@ -77,6 +77,8 @@ public:
         bool isSimilarity = SkToBool(dfTexEffect.getFlags() & kSimilarity_DistanceFieldEffectFlag);
         bool isGammaCorrect =
             SkToBool(dfTexEffect.getFlags() & kGammaCorrect_DistanceFieldEffectFlag);
+        bool isAliased =
+            SkToBool(dfTexEffect.getFlags() & kAliased_DistanceFieldEffectFlag);
         varyingHandler->addVarying("TextureCoords", &uv, kHigh_GrSLPrecision);
         vertBuilder->codeAppendf("%s = %s;", uv.vsOut(), dfTexEffect.inTextureCoords()->fName);
 
@@ -158,10 +160,12 @@ public:
             fragBuilder->codeAppend("afwidth = " SK_DistanceFieldAAFactor "*length(grad);");
         }
 
-        // The smoothstep falloff compensates for the non-linear sRGB response curve. If we are
-        // doing gamma-correct rendering (to an sRGB or F16 buffer), then we actually want distance
-        // mapped linearly to coverage, so use a linear step:
-        if (isGammaCorrect) {
+        if (isAliased) {
+            fragBuilder->codeAppend("float val = distance > 0 ? 1.0 : 0.0;");
+        } else if (isGammaCorrect) {
+            // The smoothstep falloff compensates for the non-linear sRGB response curve. If we are
+            // doing gamma-correct rendering (to an sRGB or F16 buffer), then we actually want
+            // distance mapped linearly to coverage, so use a linear step:
             fragBuilder->codeAppend(
                 "float val = clamp((distance + afwidth) / (2.0 * afwidth), 0.0, 1.0);");
         } else {
diff --git a/src/gpu/effects/GrDistanceFieldGeoProc.h b/src/gpu/effects/GrDistanceFieldGeoProc.h
index 3f616e9480..344ac4a092 100644
--- a/src/gpu/effects/GrDistanceFieldGeoProc.h
+++ b/src/gpu/effects/GrDistanceFieldGeoProc.h
@@ -23,6 +23,7 @@ enum GrDistanceFieldEffectFlags {
     kBGR_DistanceFieldEffectFlag          = 0x08, // lcd display has bgr order
     kPortrait_DistanceFieldEffectFlag     = 0x10, // lcd display is in portrait mode (not used yet)
     kGammaCorrect_DistanceFieldEffectFlag = 0x20, // assume gamma-correct output (linear blending)
+    kAliased_DistanceFieldEffectFlag      = 0x40, // monochrome output
 
     kInvalid_DistanceFieldEffectFlag    = 0x80,   // invalid state (for initialization)
 
@@ -31,7 +32,8 @@ enum GrDistanceFieldEffectFlags {
     // The subset of the flags relevant to GrDistanceFieldA8TextGeoProc
     kNonLCD_DistanceFieldEffectMask       = kSimilarity_DistanceFieldEffectFlag |
                                             kScaleOnly_DistanceFieldEffectFlag |
-                                            kGammaCorrect_DistanceFieldEffectFlag,
+                                            kGammaCorrect_DistanceFieldEffectFlag |
+                                            kAliased_DistanceFieldEffectFlag,
     // The subset of the flags relevant to GrDistanceFieldLCDTextGeoProc
     kLCD_DistanceFieldEffectMask          = kSimilarity_DistanceFieldEffectFlag |
                                             kScaleOnly_DistanceFieldEffectFlag |