keep SVG arcs axis aligned

Computing the arc width introduces rounding errors that cause the
arc to exceed 1/4 circle and cause integer anchored arcs to start
outside their marks. A round rect may lose convexity as a result.

Check if arcTo() inputs are integers and arc is 90 degrees;
if so, output conics which are axis-aligned on integers as well.

This is triggered when using SVG to represent a round rect.

Possible future enhancements are recorded in bug.skia.org/7383

R=reed@google.com,djsollen@google.com
Change-Id: I6609456fcefabcda6c9560a044533ecb5cda2d31
Reviewed-on: https://skia-review.googlesource.com/79423
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Derek Sollenberger <djsollen@google.com>

1 files changed, 19 insertions, 1 deletions

diff --git a/src/core/SkPath.cpp b/src/core/SkPath.cpp
index c433424635..ed3f466a4a 100644
--- a/src/core/SkPath.cpp
+++ b/src/core/SkPath.cpp
@@ -1417,7 +1417,8 @@ void SkPath::arcTo(SkScalar rx, SkScalar ry, SkScalar angle, SkPath::ArcSize arc
     pointTransform.setRotate(angle);
     pointTransform.preScale(rx, ry);
 
-    int segments = SkScalarCeilToInt(SkScalarAbs(thetaArc / (SK_ScalarPI / 2)));
+    // the arc may be slightly bigger than 1/4 circle, so allow up to 1/3rd
+    int segments = SkScalarCeilToInt(SkScalarAbs(thetaArc / (2 * SK_ScalarPI / 3)));
     SkScalar thetaWidth = thetaArc / segments;
     SkScalar t = SkScalarTan(0.5f * thetaWidth);
     if (!SkScalarIsFinite(t)) {
@@ -1425,6 +1426,12 @@ void SkPath::arcTo(SkScalar rx, SkScalar ry, SkScalar angle, SkPath::ArcSize arc
     }
     SkScalar startTheta = theta1;
     SkScalar w = SkScalarSqrt(SK_ScalarHalf + SkScalarCos(thetaWidth) * SK_ScalarHalf);
+    auto scalar_is_integer = [](SkScalar scalar) -> bool {
+        return scalar == SkScalarFloorToScalar(scalar);
+    };
+    bool expectIntegers = SkScalarNearlyZero(SK_ScalarPI/2 - SkScalarAbs(thetaWidth)) &&
+        scalar_is_integer(rx) && scalar_is_integer(ry) &&
+        scalar_is_integer(x) && scalar_is_integer(y);
     for (int i = 0; i < segments; ++i) {
         SkScalar endTheta = startTheta + thetaWidth;
         SkScalar cosEndTheta, sinEndTheta = SkScalarSinCos(endTheta, &cosEndTheta);
@@ -1435,6 +1442,17 @@ void SkPath::arcTo(SkScalar rx, SkScalar ry, SkScalar angle, SkPath::ArcSize arc
         unitPts[0].offset(t * sinEndTheta, -t * cosEndTheta);
         SkPoint mapped[2];
         pointTransform.mapPoints(mapped, unitPts, (int) SK_ARRAY_COUNT(unitPts));
+        /*
+        Computing the arc width introduces rounding errors that cause arcs to start
+        outside their marks. A round rect may lose convexity as a result. If the input
+        values are on integers, place the conic on integers as well.
+         */
+        if (expectIntegers) {
+            SkScalar* mappedScalars = &mapped[0].fX;
+            for (unsigned index = 0; index < sizeof(mapped) / sizeof(SkScalar); ++index) {
+                mappedScalars[index] = SkScalarRoundToScalar(mappedScalars[index]);
+            }
+        }
         this->conicTo(mapped[0], mapped[1], w);
         startTheta = endTheta;
     }