reimpl parts of arctTo using doubles to handle shallow angles

Bug: 982968
Change-Id: Ieaf70a406265876655447bd0584717063483808f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/226847
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

gm/patharcto.cpp

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2019 Google LLC
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm/gm.h"
+#include "include/core/SkCanvas.h"
+#include "include/core/SkPath.h"
+#include "include/core/SkPaint.h"
+
+// crbug.com/982968
+// Intended to draw a curvy triangle.
+// With the bug, we extend the left side of the triangle with lines, a bit like a flag pole.
+// The bug/fix is related to precision of the calculation. The original impl used all floats,
+// but the very shallow angle causes our sin/cos and other calcs to lose too many bits.
+// The fix was to use doubles for this part of the calc in SkPath::arcTo().
+//
+DEF_SIMPLE_GM(shallow_angle_path_arcto, canvas, 300, 300) {
+    SkPath path;
+    SkPaint paint;
+    paint.setStyle(SkPaint::kStroke_Style);
+
+    path.moveTo(313.44189096331155f, 106.6009423589212f);
+    path.arcTo(284.3113082008462f, 207.1407719157063f,
+               255.15053777129728f, 307.6718505416374f,
+               697212.0011054524f);
+    path.lineTo(255.15053777129728f, 307.6718505416374f);
+    path.arcTo(340.4737465981018f, 252.6907319346971f,
+               433.54333477716153f, 212.18116363345337f,
+               1251.2484277907251f);
+    path.lineTo(433.54333477716153f, 212.18116363345337f);
+    path.arcTo(350.19513833839466f, 185.89280014838369f,
+               313.44189096331155f, 106.6009423589212f,
+               198.03116885327813f);
+
+    canvas->translate(-200, -50);
+    canvas->drawPath(path, paint);
+};

gn/gm.gni

@@ -255,6 +255,7 @@ gm_sources = [
   "$_gm/p3.cpp",
   "$_gm/patch.cpp",
   "$_gm/path_stroke_with_zero_length.cpp",
+  "$_gm/patharcto.cpp",
   "$_gm/pathcontourstart.cpp",
   "$_gm/patheffects.cpp",
   "$_gm/pathfill.cpp",

src/core/SkPath.cpp

@@ -21,6 +21,8 @@
 #include "src/core/SkPointPriv.h"
 #include "src/core/SkSafeMath.h"
 #include "src/core/SkTLazy.h"
+// need SkDVector
+#include "src/pathops/SkPathOpsPoint.h"
 
 #include <cmath>
 #include <utility>
@@ -1572,32 +1574,30 @@ SkPath& SkPath::arcTo(SkScalar x1, SkScalar y1, SkScalar x2, SkScalar y2, SkScal
         return this->lineTo(x1, y1);
     }
 
-    SkVector before, after;
-
     // need to know our prev pt so we can construct tangent vectors
-    {
-        SkPoint start;
-        this->getLastPt(&start);
-        // Handle degenerate cases by adding a line to the first point and
-        // bailing out.
-        before.setNormalize(x1 - start.fX, y1 - start.fY);
-        after.setNormalize(x2 - x1, y2 - y1);
-    }
+    SkPoint start;
+    this->getLastPt(&start);
 
-    SkScalar cosh = SkPoint::DotProduct(before, after);
-    SkScalar sinh = SkPoint::CrossProduct(before, after);
+    // need double precision for these calcs.
+    SkDVector befored, afterd;
+    befored.set({x1 - start.fX, y1 - start.fY}).normalize();
+    afterd.set({x2 - x1, y2 - y1}).normalize();
+    double cosh = befored.dot(afterd);
+    double sinh = befored.cross(afterd);
 
-    if (SkScalarNearlyZero(sinh)) {   // angle is too tight
+    if (!befored.isFinite() || !afterd.isFinite() || SkScalarNearlyZero(SkDoubleToScalar(sinh))) {
         return this->lineTo(x1, y1);
     }
 
-    SkScalar dist = SkScalarAbs(radius * (1 - cosh) / sinh);
-
+    // safe to convert back to floats now
+    SkVector before = befored.asSkVector();
+    SkVector after = afterd.asSkVector();
+    SkScalar dist = SkScalarAbs(SkDoubleToScalar(radius * (1 - cosh) / sinh));
     SkScalar xx = x1 - dist * before.fX;
     SkScalar yy = y1 - dist * before.fY;
     after.setLength(dist);
     this->lineTo(xx, yy);
-    SkScalar weight = SkScalarSqrt(SK_ScalarHalf + cosh * SK_ScalarHalf);
+    SkScalar weight = SkScalarSqrt(SkDoubleToScalar(SK_ScalarHalf + cosh * 0.5));
     return this->conicTo(x1, y1, x1 + after.fX, y1 + after.fY, weight);
 }
 

src/pathops/SkPathOpsPoint.h

@@ -18,9 +18,10 @@ struct SkDVector {
     double fX;
     double fY;
 
-    void set(const SkVector& pt) {
+    SkDVector& set(const SkVector& pt) {
         fX = pt.fX;
         fY = pt.fY;
+        return *this;
     }
 
     // only used by testing
@@ -84,10 +85,15 @@ struct SkDVector {
         return fX * fX + fY * fY;
     }
 
-    void normalize() {
+    SkDVector& normalize() {
         double inverseLength = 1 / this->length();
         fX *= inverseLength;
         fY *= inverseLength;
+        return *this;
+    }
+
+    bool isFinite() const {
+        return std::isfinite(fX) && std::isfinite(fY);
     }
 };