ccpr: Cull extremely thin triangles

When triangles get too thin it's possible for FP round-off error to
actually give us the wrong winding direction, causing rendering
artifacts. This change also allows us to unblacklist ANGLE.

Bug: skia:7805
Bug: skia:7820
Change-Id: Ibaa0f033eba625d720e3a594c4515d8264cc413d
Reviewed-on: https://skia-review.googlesource.com/123262
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>

7 files changed, 86 insertions, 57 deletions

diff --git a/src/gpu/ccpr/GrCCCoverageProcessor.cpp b/src/gpu/ccpr/GrCCCoverageProcessor.cpp
index d38db27a19..75d0667b09 100644
--- a/src/gpu/ccpr/GrCCCoverageProcessor.cpp
+++ b/src/gpu/ccpr/GrCCCoverageProcessor.cpp
@@ -44,6 +44,36 @@ class GrCCCoverageProcessor::TriangleShader : public GrCCCoverageProcessor::Shad
     GrGLSLVarying fCoverages;
 };
 
+void GrCCCoverageProcessor::Shader::CalcWind(const GrCCCoverageProcessor& proc,
+                                             GrGLSLVertexGeoBuilder* s, const char* pts,
+                                             const char* outputWind) {
+    if (3 == proc.numInputPoints()) {
+        s->codeAppendf("float2 a = %s[0] - %s[1], "
+                              "b = %s[0] - %s[2];", pts, pts, pts, pts);
+    } else {
+        // All inputs are convex, so it's sufficient to just average the middle two input points.
+        SkASSERT(4 == proc.numInputPoints());
+        s->codeAppendf("float2 p12 = (%s[1] + %s[2]) * .5;", pts, pts);
+        s->codeAppendf("float2 a = %s[0] - p12, "
+                              "b = %s[0] - %s[3];", pts, pts, pts);
+    }
+
+    s->codeAppend ("float area_x2 = determinant(float2x2(a, b));");
+    if (proc.isTriangles()) {
+        // We cull extremely thin triangles by zeroing wind. When a triangle gets too thin it's
+        // possible for FP round-off error to actually give us the wrong winding direction, causing
+        // rendering artifacts. The criteria we choose is "height <~ 1/1024". So we drop a triangle
+        // if the max effect it can have on any single pixel is <~ 1/1024, or 1/4 of a bit in 8888.
+        s->codeAppend ("float2 bbox_size = max(abs(a), abs(b));");
+        s->codeAppend ("float basewidth = max(bbox_size.x + bbox_size.y, 1);");
+        s->codeAppendf("%s = (abs(area_x2 * 1024) > basewidth) ? sign(area_x2) : 0;", outputWind);
+    } else {
+        // We already converted nearly-flat curves to lines on the CPU, so no need to worry about
+        // thin curve hulls at this point.
+        s->codeAppendf("%s = sign(area_x2);", outputWind);
+    }
+}
+
 void GrCCCoverageProcessor::Shader::EmitEdgeDistanceEquation(GrGLSLVertexGeoBuilder* s,
                                                              const char* leftPt,
                                                              const char* rightPt,
diff --git a/src/gpu/ccpr/GrCCCoverageProcessor.h b/src/gpu/ccpr/GrCCCoverageProcessor.h
index 454e728ae9..e3ea34f8dd 100644
--- a/src/gpu/ccpr/GrCCCoverageProcessor.h
+++ b/src/gpu/ccpr/GrCCCoverageProcessor.h
@@ -133,6 +133,11 @@ public:
         void emitFragmentCode(const GrCCCoverageProcessor&, GrGLSLFPFragmentBuilder*,
                               const char* skOutputColor, const char* skOutputCoverage) const;
 
+        // Calculates the winding direction of the input points (+1, -1, or 0). Wind for extremely
+        // thin triangles gets rounded to zero.
+        static void CalcWind(const GrCCCoverageProcessor&, GrGLSLVertexGeoBuilder*, const char* pts,
+                             const char* outputWind);
+
         // Defines an equation ("dot(float3(pt, 1), distance_equation)") that is -1 on the outside
         // border of a conservative raster edge and 0 on the inside. 'leftPt' and 'rightPt' must be
         // ordered clockwise.
@@ -207,6 +212,11 @@ private:
     // Number of bezier points for curves, or 3 for triangles.
     int numInputPoints() const { return PrimitiveType::kCubics == fPrimitiveType ? 4 : 3; }
 
+    bool isTriangles() const {
+        return PrimitiveType::kTriangles == fPrimitiveType ||
+               PrimitiveType::kWeightedTriangles == fPrimitiveType;
+    }
+
     int hasInputWeight() const {
         return PrimitiveType::kWeightedTriangles == fPrimitiveType ||
                PrimitiveType::kConics == fPrimitiveType;
diff --git a/src/gpu/ccpr/GrCCCoverageProcessor_GSImpl.cpp b/src/gpu/ccpr/GrCCCoverageProcessor_GSImpl.cpp
index b1d886cf8c..61bb4ec7cf 100644
--- a/src/gpu/ccpr/GrCCCoverageProcessor_GSImpl.cpp
+++ b/src/gpu/ccpr/GrCCCoverageProcessor_GSImpl.cpp
@@ -59,18 +59,11 @@ protected:
 
         GrShaderVar wind("wind", kHalf_GrSLType);
         g->declareGlobal(wind);
-        if (PrimitiveType::kWeightedTriangles != proc.fPrimitiveType) {
-            g->codeAppend ("float area_x2 = determinant(float2x2(pts[0] - pts[1], "
-                                                                "pts[0] - pts[2]));");
-            if (4 == numInputPoints) {
-                g->codeAppend ("area_x2 += determinant(float2x2(pts[0] - pts[2], "
-                                                               "pts[0] - pts[3]));");
-            }
-            g->codeAppendf("%s = sign(area_x2);", wind.c_str());
-        } else {
+        Shader::CalcWind(proc, g, "pts", wind.c_str());
+        if (PrimitiveType::kWeightedTriangles == proc.fPrimitiveType) {
             SkASSERT(3 == numInputPoints);
             SkASSERT(kFloat4_GrVertexAttribType == proc.getAttrib(0).fType);
-            g->codeAppendf("%s = sk_in[0].sk_Position.w;", wind.c_str());
+            g->codeAppendf("%s *= sk_in[0].sk_Position.w;", wind.c_str());
         }
 
         SkString emitVertexFn;
@@ -306,10 +299,8 @@ public:
                               const GrShaderVar& wind, const char* emitVertexFn) const override {
         fShader->emitSetupCode(g, "pts", wind.c_str());
 
-        bool isTriangle = PrimitiveType::kTriangles == proc.fPrimitiveType ||
-                          PrimitiveType::kWeightedTriangles == proc.fPrimitiveType;
         g->codeAppendf("int corneridx = sk_InvocationID;");
-        if (!isTriangle) {
+        if (!proc.isTriangles()) {
             g->codeAppendf("corneridx *= %i;", proc.numInputPoints() - 1);
         }
 
@@ -336,7 +327,7 @@ public:
         Shader::CalcCornerAttenuation(g, "leftdir", "rightdir", "attenuation");
         g->codeAppend ("}");
 
-        if (isTriangle) {
+        if (proc.isTriangles()) {
             g->codeAppend ("half2 left_coverages; {");
             Shader::CalcEdgeCoveragesAtBloatVertices(g, "left", "corner", "-outbloat",
                                                      "-crossbloat", "left_coverages");
@@ -384,7 +375,7 @@ public:
             g->codeAppendf("%s(corner + crossbloat * bloat, -1, half2(1));", emitVertexFn);
         }
 
-        g->configure(InputType::kLines, OutputType::kTriangleStrip, 4, isTriangle ? 3 : 2);
+        g->configure(InputType::kLines, OutputType::kTriangleStrip, 4, proc.isTriangles() ? 3 : 2);
     }
 };
 
@@ -416,8 +407,7 @@ void GrCCCoverageProcessor::appendGSMesh(GrBuffer* instanceBuffer, int instanceC
 
 GrGLSLPrimitiveProcessor* GrCCCoverageProcessor::createGSImpl(std::unique_ptr<Shader> shadr) const {
     if (GSSubpass::kHulls == fGSSubpass) {
-        return (PrimitiveType::kTriangles == fPrimitiveType ||
-                PrimitiveType::kWeightedTriangles == fPrimitiveType)
+        return this->isTriangles()
                    ? (GSImpl*) new GSTriangleHullImpl(std::move(shadr))
                    : (GSImpl*) new GSCurveHullImpl(std::move(shadr));
     }
diff --git a/src/gpu/ccpr/GrCCCoverageProcessor_VSImpl.cpp b/src/gpu/ccpr/GrCCCoverageProcessor_VSImpl.cpp
index dd8da96f82..08b8886aed 100644
--- a/src/gpu/ccpr/GrCCCoverageProcessor_VSImpl.cpp
+++ b/src/gpu/ccpr/GrCCCoverageProcessor_VSImpl.cpp
@@ -263,18 +263,12 @@ void GrCCCoverageProcessor::VSImpl::onEmitCode(EmitArgs& args, GrGPArgs* gpArgs)
                    inputWidth, inputWidth, proc.getAttrib(kAttribIdx_X).fName, swizzle,
                    proc.getAttrib(kAttribIdx_Y).fName, swizzle);
 
-    if (PrimitiveType::kWeightedTriangles != proc.fPrimitiveType) {
-        v->codeAppend ("float area_x2 = determinant(float2x2(pts[0] - pts[1], "
-                                                            "pts[0] - pts[2]));");
-        if (4 == numInputPoints) {
-            v->codeAppend ("area_x2 += determinant(float2x2(pts[0] - pts[2], "
-                                                           "pts[0] - pts[3]));");
-        }
-        v->codeAppend ("half wind = sign(area_x2);");
-    } else {
+    v->codeAppend ("half wind;");
+    Shader::CalcWind(proc, v, "pts", "wind");
+    if (PrimitiveType::kWeightedTriangles == proc.fPrimitiveType) {
         SkASSERT(3 == numInputPoints);
         SkASSERT(kFloat4_GrVertexAttribType == proc.getAttrib(kAttribIdx_X).fType);
-        v->codeAppendf("half wind = %s.w;", proc.getAttrib(kAttribIdx_X).fName);
+        v->codeAppendf("wind *= %s.w;", proc.getAttrib(kAttribIdx_X).fName);
     }
 
     float bloat = kAABloatRadius;
diff --git a/src/gpu/ccpr/GrCCGeometry.cpp b/src/gpu/ccpr/GrCCGeometry.cpp
index 9fbf3e61f8..30d93ad416 100644
--- a/src/gpu/ccpr/GrCCGeometry.cpp
+++ b/src/gpu/ccpr/GrCCGeometry.cpp
@@ -38,14 +38,20 @@ void GrCCGeometry::beginContour(const SkPoint& pt) {
     SkDEBUGCODE(fBuildingContour = true);
 }
 
-void GrCCGeometry::lineTo(const SkPoint& pt) {
+void GrCCGeometry::lineTo(const SkPoint P[2]) {
     SkASSERT(fBuildingContour);
-    fPoints.push_back(pt);
-    fVerbs.push_back(Verb::kLineTo);
+    SkASSERT(P[0] == fPoints.back());
+    Sk2f p0 = Sk2f::Load(P);
+    Sk2f p1 = Sk2f::Load(P+1);
+    this->appendLine(p0, p1);
 }
 
-void GrCCGeometry::appendLine(const Sk2f& endpt) {
-    endpt.store(&fPoints.push_back());
+inline void GrCCGeometry::appendLine(const Sk2f& p0, const Sk2f& p1) {
+    SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
+    if ((p0 == p1).allTrue()) {
+        return;
+    }
+    p1.store(&fPoints.push_back());
     fVerbs.push_back(Verb::kLineTo);
 }
 
@@ -142,7 +148,7 @@ void GrCCGeometry::quadraticTo(const SkPoint P[3]) {
     // Don't crunch on the curve if it is nearly flat (or just very small). Flat curves can break
     // The monotonic chopping math.
     if (are_collinear(p0, p1, p2)) {
-        this->appendLine(p2);
+        this->appendLine(p0, p2);
         return;
     }
 
@@ -190,12 +196,12 @@ inline void GrCCGeometry::appendQuadratics(const Sk2f& p0, const Sk2f& p1, const
 inline void GrCCGeometry::appendMonotonicQuadratic(const Sk2f& p0, const Sk2f& p1, const Sk2f& p2) {
     // Don't send curves to the GPU if we know they are nearly flat (or just very small).
     if (are_collinear(p0, p1, p2)) {
-        SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
-        this->appendLine(p2);
+        this->appendLine(p0, p2);
         return;
     }
 
     SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
+    SkASSERT((p0 != p2).anyTrue());
     p1.store(&fPoints.push_back());
     p2.store(&fPoints.push_back());
     fVerbs.push_back(Verb::kMonotonicQuadraticTo);
@@ -466,7 +472,9 @@ void GrCCGeometry::cubicTo(const SkPoint P[4], float inflectPad, float loopInter
     // Don't crunch on the curve or inflate geometry if it is nearly flat (or just very small).
     // Flat curves can break the math below.
     if (are_collinear(P)) {
-        this->lineTo(P[3]);
+        Sk2f p0 = Sk2f::Load(P);
+        Sk2f p3 = Sk2f::Load(P+3);
+        this->appendLine(p0, p3);
         return;
     }
 
@@ -570,10 +578,6 @@ void GrCCGeometry::appendCubics(AppendCubicMode mode, const Sk2f& p0, const Sk2f
 
 void GrCCGeometry::appendCubics(AppendCubicMode mode, const Sk2f& p0, const Sk2f& p1,
                                 const Sk2f& p2, const Sk2f& p3, int maxSubdivisions) {
-    if ((p0 == p3).allTrue()) {
-        return;
-    }
-
     if (SkCubicType::kLoop != fCurrCubicType) {
         // Serpentines and cusps are always monotonic after chopping around inflection points.
         SkASSERT(!SkCubicIsDegenerate(fCurrCubicType));
@@ -583,8 +587,7 @@ void GrCCGeometry::appendCubics(AppendCubicMode mode, const Sk2f& p0, const Sk2f
             // This can cause some curves to feel slightly more flat when inspected rigorously back
             // and forth against another renderer, but for now this seems acceptable given the
             // simplicity.
-            SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
-            this->appendLine(p3);
+            this->appendLine(p0, p3);
             return;
         }
     } else {
@@ -613,12 +616,12 @@ void GrCCGeometry::appendCubics(AppendCubicMode mode, const Sk2f& p0, const Sk2f
     // Don't send curves to the GPU if we know they are nearly flat (or just very small).
     // Since the cubic segment is known to be convex at this point, our flatness check is simple.
     if (are_collinear(p0, (p1 + p2) * .5f, p3)) {
-        SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
-        this->appendLine(p3);
+        this->appendLine(p0, p3);
         return;
     }
 
     SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
+    SkASSERT((p0 != p3).anyTrue());
     p1.store(&fPoints.push_back());
     p2.store(&fPoints.push_back());
     p3.store(&fPoints.push_back());
@@ -686,7 +689,7 @@ inline void GrCCGeometry::chopAndAppendCubicAtMidTangent(AppendCubicMode mode, c
     // near-flat cubics in cubicTo().)
     if (!(midT > 0 && midT < 1)) {
         // The cubic is flat. Otherwise there would be a real midtangent inside T=0..1.
-        this->appendLine(p3);
+        this->appendLine(p0, p3);
         return;
     }
 
@@ -720,7 +723,7 @@ void GrCCGeometry::conicTo(const SkPoint P[3], float w) {
         // midtangents.)
         if (!(midT > 0 && midT < 1)) {
             // The conic is flat. Otherwise there would be a real midtangent inside T=0..1.
-            this->appendLine(p2);
+            this->appendLine(p0, p2);
             return;
         }
 
@@ -747,7 +750,6 @@ void GrCCGeometry::conicTo(const SkPoint P[3], float w) {
 
 void GrCCGeometry::appendMonotonicConic(const Sk2f& p0, const Sk2f& p1, const Sk2f& p2, float w) {
     SkASSERT(w >= 0);
-    SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
 
     Sk2f base = p2 - p0;
     Sk2f baseAbs = base.abs();
@@ -758,24 +760,28 @@ void GrCCGeometry::appendMonotonicConic(const Sk2f& p0, const Sk2f& p1, const Sk
     float h1 = std::abs(d[1] - d[0]); // Height of p1 above the base.
     float ht = h1*w, hs = 1 + w; // Height of the conic = ht/hs.
 
-    if (ht < (baseWidth*hs) * kFlatnessThreshold) { // i.e. ht/hs < baseWidth * kFlatnessThreshold
+    // i.e. (ht/hs <= baseWidth * kFlatnessThreshold). Use "<=" in case base == 0.
+    if (ht <= (baseWidth*hs) * kFlatnessThreshold) {
         // We are flat. (See rationale in are_collinear.)
-        this->appendLine(p2);
+        this->appendLine(p0, p2);
         return;
     }
 
-    if (w > 1 && h1*hs - ht < baseWidth*hs) { // i.e. w > 1 && h1 - ht/hs < baseWidth
+    // i.e. (w > 1 && h1 - ht/hs < baseWidth).
+    if (w > 1 && h1*hs - ht < baseWidth*hs) {
         // If we get within 1px of p1 when w > 1, we will pick up artifacts from the implicit
         // function's reflection. Chop at max height (T=.5) and draw a triangle instead.
         Sk2f p1w = p1*w;
         Sk2f ab = p0 + p1w;
         Sk2f bc = p1w + p2;
         Sk2f highpoint = (ab + bc) / (2*(1 + w));
-        this->appendLine(highpoint);
-        this->appendLine(p2);
+        this->appendLine(p0, highpoint);
+        this->appendLine(highpoint, p2);
         return;
     }
 
+    SkASSERT(fPoints.back() == SkPoint::Make(p0[0], p0[1]));
+    SkASSERT((p0 != p2).anyTrue());
     p1.store(&fPoints.push_back());
     p2.store(&fPoints.push_back());
     fConicWeights.push_back(w);
diff --git a/src/gpu/ccpr/GrCCGeometry.h b/src/gpu/ccpr/GrCCGeometry.h
index 96a38e94e7..571b3c0a52 100644
--- a/src/gpu/ccpr/GrCCGeometry.h
+++ b/src/gpu/ccpr/GrCCGeometry.h
@@ -76,7 +76,7 @@ public:
 
     void beginPath();
     void beginContour(const SkPoint&);
-    void lineTo(const SkPoint&);
+    void lineTo(const SkPoint P[2]);
     void quadraticTo(const SkPoint[3]);
 
     // We pass through inflection points and loop intersections using a line and quadratic(s)
@@ -97,7 +97,7 @@ public:
     PrimitiveTallies endContour(); // Returns the numbers of primitives needed to draw the contour.
 
 private:
-    inline void appendLine(const Sk2f& endpt);
+    inline void appendLine(const Sk2f& p0, const Sk2f& p1);
 
     inline void appendQuadratics(const Sk2f& p0, const Sk2f& p1, const Sk2f& p2);
     inline void appendMonotonicQuadratic(const Sk2f& p0, const Sk2f& p1, const Sk2f& p2);
diff --git a/src/gpu/ccpr/GrCCPathParser.cpp b/src/gpu/ccpr/GrCCPathParser.cpp
index 2740569fe6..1629a191a5 100644
--- a/src/gpu/ccpr/GrCCPathParser.cpp
+++ b/src/gpu/ccpr/GrCCPathParser.cpp
@@ -132,7 +132,7 @@ void GrCCPathParser::parsePath(const SkPath& path, const SkPoint* deviceSpacePts
                 insideContour = false;
                 continue;
             case SkPath::kLine_Verb:
-                fGeometry.lineTo(deviceSpacePts[ptsIdx]);
+                fGeometry.lineTo(&deviceSpacePts[ptsIdx - 1]);
                 ++ptsIdx;
                 continue;
             case SkPath::kQuad_Verb:
@@ -340,8 +340,7 @@ static void emit_tessellated_fan(const GrTessellator::WindingVertex* vertices, i
         } else {
             quadPointInstanceData[indices->fWeightedTriangles++].setW(
                     vertices[i].fPos, vertices[i+1].fPos, vertices[i + 2].fPos, atlasOffset,
-                    // Tessellator has opposite winding sense.
-                    -static_cast<float>(vertices[i].fWinding));
+                    static_cast<float>(abs(vertices[i].fWinding)));
         }
     }
 }