GrTessellator: fix artifact with exactly-1-px-wide edges.

When path features are exactly a pixel wide, the extruded inner edges
can become collinear and then be removed, since their winding is
zero. We need these edges to be preserved through triangulation,
otherwise opaque portions of the geometry can become transparent.

Since the simplify() pass can handle zero-winding edges just fine, the
the fix is to simply not remove them.

In addition, this changes refactors out disconnect() from all the calls
to remove_edge_above()/remove_edge_below(). It also renames the remaining
function erase_edge() (since it's now unconditional).

Add a new test to a new "thinconcavepaths" GM.

BUG=680260
NOTRY=true

Change-Id: I1d3a436c95a01c4d4ef5dc05503de4312677f65d
Reviewed-on: https://skia-review.googlesource.com/6902
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephan White <senorblanco@chromium.org>

gm/thinconcavepaths.cpp

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkCanvas.h"
+#include "SkPath.h"
+
+#define WIDTH 400
+#define HEIGHT 400
+
+namespace {
+// Test thin stroked rect (stroked "by hand", not by stroking).
+void draw_thin_stroked_rect(SkCanvas* canvas, const SkPaint& paint, SkScalar width) {
+    SkPath path;
+    path.moveTo(10 + width, 10 + width);
+    path.lineTo(40,         10 + width);
+    path.lineTo(40,         20);
+    path.lineTo(10 + width, 20);
+    path.moveTo(10,         10);
+    path.lineTo(10,         20 + width);
+    path.lineTo(40 + width, 20 + width);
+    path.lineTo(40 + width, 10);
+    canvas->drawPath(path, paint);
+}
+
+};
+
+class ThinConcavePathsGM : public skiagm::GM {
+public:
+    ThinConcavePathsGM() {}
+
+protected:
+    SkString onShortName() override {
+        return SkString("thinconcavepaths");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(WIDTH, HEIGHT);
+    }
+
+    void onDraw(SkCanvas* canvas) override {
+        SkPaint paint;
+
+        paint.setAntiAlias(true);
+        paint.setStyle(SkPaint::kFill_Style);
+
+        canvas->save();
+        for (SkScalar width = 1.0; width < 2.05; width += 0.25) {
+            draw_thin_stroked_rect(canvas, paint, width);
+            canvas->translate(0, 25);
+        }
+        canvas->restore();
+    }
+
+private:
+    typedef skiagm::GM INHERITED;
+};
+
+DEF_GM( return new ThinConcavePathsGM; )

gn/gm.gni

@@ -289,6 +289,7 @@ gm_sources = [
   "$_gm/textblobuseaftergpufree.cpp",
   "$_gm/texteffects.cpp",
   "$_gm/texturedomaineffect.cpp",
+  "$_gm/thinconcavepaths.cpp",
   "$_gm/thinrects.cpp",
   "$_gm/thinstrokedrects.cpp",
   "$_gm/tiledscaledbitmap.cpp",

src/gpu/GrTessellator.cpp

@@ -896,13 +896,15 @@ void remove_edge_below(Edge* edge) {
         edge, &edge->fTop->fFirstEdgeBelow, &edge->fTop->fLastEdgeBelow);
 }
 
-void erase_edge_if_zero_winding(Edge* edge, EdgeList* edges) {
-    if (edge->fWinding != 0) {
-        return;
-    }
-    LOG("erasing edge (%g -> %g)\n", edge->fTop->fID, edge->fBottom->fID);
+void disconnect(Edge* edge)
+{
     remove_edge_above(edge);
     remove_edge_below(edge);
+}
+
+void erase_edge(Edge* edge, EdgeList* edges) {
+    LOG("erasing edge (%g -> %g)\n", edge->fTop->fID, edge->fBottom->fID);
+    disconnect(edge);
     if (edges && edges->contains(edge)) {
         remove_edge(edge, edges);
     }
@@ -934,16 +936,12 @@ void merge_edges_above(Edge* edge, Edge* other, EdgeList* activeEdges, Comparato
             edge->fTop->fPoint.fX, edge->fTop->fPoint.fY,
             edge->fBottom->fPoint.fX, edge->fBottom->fPoint.fY);
         other->fWinding += edge->fWinding;
-        erase_edge_if_zero_winding(other, activeEdges);
-        edge->fWinding = 0;
-        erase_edge_if_zero_winding(edge, activeEdges);
+        erase_edge(edge, activeEdges);
     } else if (c.sweep_lt(edge->fTop->fPoint, other->fTop->fPoint)) {
         other->fWinding += edge->fWinding;
-        erase_edge_if_zero_winding(other, activeEdges);
         set_bottom(edge, other->fTop, activeEdges, c);
     } else {
         edge->fWinding += other->fWinding;
-        erase_edge_if_zero_winding(edge, activeEdges);
         set_bottom(other, edge->fTop, activeEdges, c);
     }
 }
@@ -954,16 +952,12 @@ void merge_edges_below(Edge* edge, Edge* other, EdgeList* activeEdges, Comparato
             edge->fTop->fPoint.fX, edge->fTop->fPoint.fY,
             edge->fBottom->fPoint.fX, edge->fBottom->fPoint.fY);
         other->fWinding += edge->fWinding;
-        erase_edge_if_zero_winding(other, activeEdges);
-        edge->fWinding = 0;
-        erase_edge_if_zero_winding(edge, activeEdges);
+        erase_edge(edge, activeEdges);
     } else if (c.sweep_lt(edge->fBottom->fPoint, other->fBottom->fPoint)) {
         edge->fWinding += other->fWinding;
-        erase_edge_if_zero_winding(edge, activeEdges);
         set_top(other, edge->fBottom, activeEdges, c);
     } else {
         other->fWinding += edge->fWinding;
-        erase_edge_if_zero_winding(other, activeEdges);
         set_top(edge, other->fBottom, activeEdges, c);
     }
 }
@@ -1455,8 +1449,7 @@ void remove_non_boundary_edges(const VertexList& mesh, SkPath::FillType fillType
         for (Edge* e = v->fFirstEdgeBelow; e != nullptr;) {
             Edge* next = e->fNextEdgeBelow;
             if (!is_boundary_edge(e, fillType)) {
-                remove_edge_above(e);
-                remove_edge_below(e);
+                disconnect(e);
             }
             e = next;
         }
@@ -1612,8 +1605,7 @@ void extract_boundary(EdgeList* boundary, Edge* e, SkPath::FillType fillType, Sk
                 down = true;
             }
         }
-        remove_edge_above(e);
-        remove_edge_below(e);
+        disconnect(e);
         e = next;
     }
 }