keep SVG arcs axis aligned with suppression

another attempt at https://skia-review.googlesource.com/c/skia/+/79423

this time, bracket change with SK_SUPPORT_LEGACY_SVG_ARC_TO
to avoid prematurely changing SVG layout test results.

R=djsollen@google.com
Bug: b:69622768
Change-Id: Ia929f89d4f4292dfead191a381cdb431f743726a
Reviewed-on: https://skia-review.googlesource.com/82723
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>

2 files changed, 57 insertions, 0 deletions

diff --git a/src/core/SkPath.cpp b/src/core/SkPath.cpp
index c433424635..422c2b911d 100644
--- a/src/core/SkPath.cpp
+++ b/src/core/SkPath.cpp
@@ -1417,7 +1417,12 @@ void SkPath::arcTo(SkScalar rx, SkScalar ry, SkScalar angle, SkPath::ArcSize arc
     pointTransform.setRotate(angle);
     pointTransform.preScale(rx, ry);
 
+#ifdef SK_SUPPORT_LEGACY_SVG_ARC_TO
     int segments = SkScalarCeilToInt(SkScalarAbs(thetaArc / (SK_ScalarPI / 2)));
+#else
+    // 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)));
+#endif
     SkScalar thetaWidth = thetaArc / segments;
     SkScalar t = SkScalarTan(0.5f * thetaWidth);
     if (!SkScalarIsFinite(t)) {
@@ -1425,6 +1430,14 @@ void SkPath::arcTo(SkScalar rx, SkScalar ry, SkScalar angle, SkPath::ArcSize arc
     }
     SkScalar startTheta = theta1;
     SkScalar w = SkScalarSqrt(SK_ScalarHalf + SkScalarCos(thetaWidth) * SK_ScalarHalf);
+#ifndef SK_SUPPORT_LEGACY_SVG_ARC_TO
+    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);
+#endif
     for (int i = 0; i < segments; ++i) {
         SkScalar endTheta = startTheta + thetaWidth;
         SkScalar cosEndTheta, sinEndTheta = SkScalarSinCos(endTheta, &cosEndTheta);
@@ -1435,6 +1448,19 @@ 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.
+         */
+#ifndef SK_SUPPORT_LEGACY_SVG_ARC_TO
+        if (expectIntegers) {
+            SkScalar* mappedScalars = &mapped[0].fX;
+            for (unsigned index = 0; index < sizeof(mapped) / sizeof(SkScalar); ++index) {
+                mappedScalars[index] = SkScalarRoundToScalar(mappedScalars[index]);
+            }
+        }
+#endif
         this->conicTo(mapped[0], mapped[1], w);
         startTheta = endTheta;
     }
diff --git a/tests/PathTest.cpp b/tests/PathTest.cpp
index b275342e2d..88ca3f8f05 100644
--- a/tests/PathTest.cpp
+++ b/tests/PathTest.cpp
@@ -4915,3 +4915,34 @@ DEF_TEST(NonFinitePathIteration, reporter) {
 
     REPORTER_ASSERT(reporter, verbs == 0);
 }
+
+
+#ifndef SK_SUPPORT_LEGACY_SVG_ARC_TO
+DEF_TEST(AndroidArc, reporter) {
+    const char* tests[] = {
+        "M50,0A50,50,0,0 1 100,50 L100,85 A15,15,0,0 1 85,100 L50,100 A50,50,0,0 1 50,0z",
+        "M50,0L92,0 A8,8,0,0 1 100,8 L100,92 A8,8,0,0 1 92,100 L8,100"
+            " A8,8,0,0 1 0,92 L 0,8 A8,8,0,0 1 8,0z",
+        "M50 0A50 50,0,1,1,50 100A50 50,0,1,1,50 0"
+    };
+    for (auto test : tests) {
+        SkPath aPath;
+        SkAssertResult(SkParsePath::FromSVGString(test, &aPath));
+        SkASSERT(aPath.isConvex());
+        for (SkScalar scale = 1; scale < 1000; scale *= 1.1f) {
+            SkPath scalePath = aPath;
+            SkMatrix matrix;
+            matrix.setScale(scale, scale);
+            scalePath.transform(matrix);
+            SkASSERT(scalePath.isConvex());
+        }
+        for (SkScalar scale = 1; scale < .001; scale /= 1.1f) {
+            SkPath scalePath = aPath;
+            SkMatrix matrix;
+            matrix.setScale(scale, scale);
+            scalePath.transform(matrix);
+            SkASSERT(scalePath.isConvex());
+        }
+    }
+}
+#endif