Fix SkPath::arcTo when sweepAngle is tiny and radius is big

In this function, it first check whether this arc is a lone point
or not. If not, it converts angles to unit vectors. The problem
here is that when the radius is huge and the sweepAngle is small,
the function angles_to_unit_vectors() could return a startV ==stopV.
When that happens, it will draw a dot at the point that corresponding
to the startAngle. This CL adds a special branch for this case, and
draw a connecting line between the points at startAngle and endAngle.

BUG=640031, skia:5807
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2388833002

Review-Url: https://codereview.chromium.org/2388833002

1 files changed, 19 insertions, 0 deletions

diff --git a/src/core/SkPath.cpp b/src/core/SkPath.cpp
index 3cbc76ad8f..4c49e6bc12 100644
--- a/src/core/SkPath.cpp
+++ b/src/core/SkPath.cpp
@@ -1290,6 +1290,25 @@ void SkPath::arcTo(const SkRect& oval, SkScalar startAngle, SkScalar sweepAngle,
     angles_to_unit_vectors(startAngle, sweepAngle, &startV, &stopV, &dir);
 
     SkPoint singlePt;
+
+    // At this point, we know that the arc is not a lone point, but startV == stopV
+    // indicates that the sweepAngle is too small such that angles_to_unit_vectors
+    // cannot handle it.
+    if (startV == stopV) {
+        SkScalar endAngle = SkDegreesToRadians(startAngle + sweepAngle);
+        SkScalar radiusX = oval.width() / 2;
+        SkScalar radiusY = oval.height() / 2;
+        // We cannot use SkScalarSinCos function in the next line because
+        // SkScalarSinCos has a threshold *SkScalarNearlyZero*. When sin(startAngle)
+        // is 0 and sweepAngle is very small and radius is huge, the expected
+        // behavior here is to draw a line. But calling SkScalarSinCos will 
+        // make sin(endAngle) to be 0 which will then draw a dot.
+        singlePt.set(oval.centerX() + radiusX * sk_float_cos(endAngle),
+            oval.centerY() + radiusY * sk_float_sin(endAngle));
+        forceMoveTo ? this->moveTo(singlePt) : this->lineTo(singlePt);
+        return;
+    }
+
     SkConic conics[SkConic::kMaxConicsForArc];
     int count = build_arc_conics(oval, startV, stopV, dir, conics, &singlePt);
     if (count) {