fix arcto exception handling

To match SVG behavior,
draw a line when either radius is zero,
or when the end point equals the start point.

Update the GM to include these tests, and
take the scale used to test it on a highres display.

R=fmalita@chromium.org
BUG=skia:4849
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1627423002

Review URL: https://codereview.chromium.org/1627423002

2 files changed, 21 insertions, 1 deletions

diff --git a/gm/arcto.cpp b/gm/arcto.cpp
index 900c6b30ea..aac9bde1b3 100644
--- a/gm/arcto.cpp
+++ b/gm/arcto.cpp
@@ -35,6 +35,12 @@ The arcto test below should draw the same as this SVG:
 
 <path d="M250,400  A120,80 0 0,1 250,500"
     fill="none" stroke="blue" stroke-width="5"/>
+
+<path d="M100,100  A  0, 0 0 0,1 200,200"
+    fill="none" stroke="blue" stroke-width="5" stroke-linecap="round"/>
+
+<path d="M200,100  A 80,80 0 0,1 200,100"
+    fill="none" stroke="blue" stroke-width="5" stroke-linecap="round"/>
 </svg>
  */
 
@@ -44,7 +50,7 @@ DEF_SIMPLE_GM(arcto, canvas, 500, 600) {
     paint.setStyle(SkPaint::kStroke_Style);
     paint.setStrokeWidth(2);
     paint.setColor(0xFF660000);
-    canvas->scale(2, 2);
+//    canvas->scale(2, 2);  // for testing on retina
     SkRect oval = SkRect::MakeXYWH(100, 100, 100, 100);
     SkPath svgArc;
 
@@ -83,4 +89,16 @@ DEF_SIMPLE_GM(arcto, canvas, 500, 600) {
         paint.setColor(colors[cIndex++]);
         canvas->drawPath(svgArc, paint);
     }
+
+    // test that zero length arcs still draw round cap
+    paint.setStrokeCap(SkPaint::kRound_Cap);
+    SkPath path;
+    path.moveTo(100, 100);
+    path.arcTo(0, 0, 0, SkPath::kLarge_ArcSize, SkPath::kCW_Direction, 200, 200);
+    canvas->drawPath(path, paint);
+
+    path.reset();
+    path.moveTo(200, 100);
+    path.arcTo(80, 80, 0, SkPath::kLarge_ArcSize, SkPath::kCW_Direction, 200, 100);
+    canvas->drawPath(path, paint);
 }
diff --git a/src/core/SkPath.cpp b/src/core/SkPath.cpp
index 40ca50b5ca..75a4cdcd7a 100644
--- a/src/core/SkPath.cpp
+++ b/src/core/SkPath.cpp
@@ -1271,12 +1271,14 @@ void SkPath::arcTo(SkScalar rx, SkScalar ry, SkScalar angle, SkPath::ArcSize arc
     // joining the endpoints.
     // http://www.w3.org/TR/SVG/implnote.html#ArcOutOfRangeParameters
     if (!rx || !ry) {
+        this->lineTo(x, y);
         return;
     }
     // If the current point and target point for the arc are identical, it should be treated as a
     // zero length path. This ensures continuity in animations.
     srcPts[1].set(x, y);
     if (srcPts[0] == srcPts[1]) {
+        this->lineTo(x, y);
         return;
     }
     rx = SkScalarAbs(rx);