Fix bug where SkPath will convert an arc to an oval and change the starting point.

src/core/SkPath.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
+#include <cmath>
 #include "SkBuffer.h"
 #include "SkCubicClipper.h"
 #include "SkErrorInternals.h"
 #include "SkGeometry.h"
 #include "SkMath.h"
 #include "SkPathPriv.h"
 #include "SkPathRef.h"
 #include "SkRRect.h"
 
 ////////////////////////////////////////////////////////////////////////////
@@ skipping 1394 matching lines @@
     this->getLastPt(&currentPoint);
     this->arcTo(rx, ry, xAxisRotate, largeArc, sweep, currentPoint.fX + dx, currentPoint.fY + dy);
 }
 
 void SkPath::addArc(const SkRect& oval, SkScalar startAngle, SkScalar sweepAngle) {
     if (oval.isEmpty() || 0 == sweepAngle) {
         return;
     }
 
     const SkScalar kFullCircleAngle = SkIntToScalar(360);
 

[robertphillips, 2016/05/27 12:12:29]

It is unfortunate we have to always do this since the start position will only
matter for some paints.

Is there no way to push the pain of the start point into the dash effect?

[bsalomon, 2016/05/27 14:29:32]

I agree that this is painful. The problem is that the geometry and the paint are
constructed completely independently and by the time we have both the start
angle is lost and so there is no way to communicate it to the dash. I think the
way to preserve the oval status of arbitrary addArc start angles is to record
the actual start angle instead of an index to a set of 4 possible start angles.
That can be done in a future change.

     if (sweepAngle >= kFullCircleAngle || sweepAngle <= -kFullCircleAngle) {
-        this->addOval(oval, sweepAngle > 0 ? kCW_Direction : kCCW_Direction);
-    } else {
-        this->arcTo(oval, startAngle, sweepAngle, true);
+        // We can treat the arc as an oval if it begins at one of our legal starting positions.
+        // See SkPath::addOval() docs.
+        float n = startAngle / 90.f;

[robertphillips, 2016/05/27 12:12:29]

i -> intPortion, r -> remainder ?

[bsalomon, 2016/05/27 14:29:32]

This code wasn't quite correct. The new version is fewer lines and uses more
explicit var names.

+        float i;
+        float r = std::modf(n, &i);
+        if (SkScalarNearlyEqual(r, 0.f)) {
+            if (r < 0) {
+                // If we were just under an integer multiple of 90, bump up.
+                i += 1.f;
+            }
+            i += 1;  // index 1 is at angle 0.

[robertphillips, 2016/05/27 12:12:30]

Maybe introduce a new variable (int startIndex) here, rather than reuse 'r' ?

[bsalomon, 2016/05/27 14:29:33]

Done.

+            r = std::fmod(i, 4.f);
+            r = r < 0 ? r + 4.f : r;
+            this->addOval(oval, sweepAngle > 0 ? kCW_Direction : kCCW_Direction, (unsigned) r);
+            return;
+        }
     }
+    this->arcTo(oval, startAngle, sweepAngle, true);
 }
 
 /*
     Need to handle the case when the angle is sharp, and our computed end-points
     for the arc go behind pt1 and/or p2...
 */
 void SkPath::arcTo(SkScalar x1, SkScalar y1, SkScalar x2, SkScalar y2, SkScalar radius) {
     if (radius == 0) {
         this->lineTo(x1, y1);
         return;
@@ skipping 1788 matching lines @@
         }
     } while (!done);
     return SkToBool(tangents.count()) ^ isInverse;
 }
 
 int SkPath::ConvertConicToQuads(const SkPoint& p0, const SkPoint& p1, const SkPoint& p2,
                                 SkScalar w, SkPoint pts[], int pow2) {
     const SkConic conic(p0, p1, p2, w);
     return conic.chopIntoQuadsPOW2(pts, pow2);
 }