use conics for arcTo

src/core/SkGeometry.cpp

Index: src/core/SkGeometry.cpp
diff --git a/src/core/SkGeometry.cpp b/src/core/SkGeometry.cpp
index ac02afc6334644487cc55cb88aa0c6757203e7ab..7896b0db40d8debb0d682b13767bb8f1d151ad5b 100644
--- a/src/core/SkGeometry.cpp
+++ b/src/core/SkGeometry.cpp
@@ -1555,3 +1555,89 @@ SkScalar SkConic::TransformW(const SkPoint pts[], SkScalar w,
     w = SkScalarSqrt((w1 * w1) / (w0 * w2));
     return w;
 }
+
+int SkConic::BuildUnitArc(const SkVector& uStart, const SkVector& uStop, SkRotationDirection dir,
+                          const SkMatrix* userMatrix, SkConic dst[kMaxConicsForArc]) {
+    // rotate by x,y so that uStart is (1.0)
+    SkScalar x = SkPoint::DotProduct(uStart, uStop);
+    SkScalar y = SkPoint::CrossProduct(uStart, uStop);
+
+    SkScalar absY = SkScalarAbs(y);
+
+    // check for (effectively) coincident vectors
+    // this can happen if our angle is nearly 0 or nearly 180 (y == 0)
+    // ... we use the dot-prod to distinguish between 0 and 180 (x > 0)
+    if (absY <= SK_ScalarNearlyZero && x > 0 && ((y >= 0 && kCW_SkRotationDirection == dir) ||
+                                                 (y <= 0 && kCCW_SkRotationDirection == dir))) {
+        return 0;
+    }
+
+    if (dir == kCCW_SkRotationDirection) {
+        y = -y;
+    }
+
+    // We decide to use 1-conic per quadrant of a circle. What quadrant does [xy] lie in?
+    //      0 == [0  .. 90)
+    //      1 == [90 ..180)
+    //      2 == [180..270)
+    //      3 == [270..360)
+    //
+    int quadrant = 0;
+    if (0 == y) {
+        quadrant = 2;        // 180
+        SkASSERT(SkScalarAbs(x + SK_Scalar1) <= SK_ScalarNearlyZero);
+    } else if (0 == x) {
+        SkASSERT(absY - SK_Scalar1 <= SK_ScalarNearlyZero);
+        quadrant = y > 0 ? 1 : 3; // 90 : 270
+    } else {
+        if (y < 0) {
+            quadrant += 2;
+        }
+        if ((x < 0) != (y < 0)) {
+            quadrant += 1;
+        }
+    }
+
+    const SkPoint quadrantPts[] = {
+        { 1, 0 }, { 1, 1 }, { 0, 1 }, { -1, 1 }, { -1, 0 }, { -1, -1 }, { 0, -1 }, { 1, -1 }
+    };
+    const SkScalar quadrantWeight = SK_ScalarRoot2Over2;
+
+    int conicCount = quadrant;
+    for (int i = 0; i < conicCount; ++i) {
+        dst[i].set(&quadrantPts[i * 2], quadrantWeight);
+    }
+
+    // Now compute any remaing (sub-90-degree) arc for the last conic
+    const SkPoint finalP = { x, y };
+    const SkPoint& lastQ = quadrantPts[quadrant * 2];  // will already be a unit-vector
+    const SkScalar dot = SkVector::DotProduct(lastQ, finalP);
+    SkASSERT(0 <= dot && dot <= SK_Scalar1);
+
+    if (dot < 1 - SK_ScalarNearlyZero) {
+        SkVector offCurve = { lastQ.x() + x, lastQ.y() + y };
+        // compute the bisector vector, and then rescale to be the off-curve point.
+        // we compute its length from cos(theta/2) = length / 1, using half-angle identity we get
+        // length = sqrt(2 / (1 + cos(theta)). We already have cos() when to computed the dot.
+        // This is nice, since our computed weight is cos(theta/2) as well!
+        //
+        const SkScalar cosThetaOver2 = SkScalarSqrt((1 + dot) / 2);
+        offCurve.setLength(SkScalarInvert(cosThetaOver2));
+        dst[conicCount].set(lastQ, offCurve, finalP, cosThetaOver2);
+        conicCount += 1;
+    }
+
+    // now handle counter-clockwise and the initial unitStart rotation
+    SkMatrix    matrix;
+    matrix.setSinCos(uStart.fY, uStart.fX);
+    if (dir == kCCW_SkRotationDirection) {
+        matrix.preScale(SK_Scalar1, -SK_Scalar1);
+    }
+    if (userMatrix) {
+        matrix.postConcat(*userMatrix);
+    }
+    for (int i = 0; i < conicCount; ++i) {
+        matrix.mapPoints(dst[i].fPts, 3);
+    }
+    return conicCount;
+}