Adjust the in-angle in BisectingAngle() when the difference is -180

We would previously produce the same angle for 180 -> 0 transitions as
for 0 -> 180 (or similarly 90 -> -90 and -90 -> 90 respectively), which
isn't correct. Tweak the condition in BisectingAngle() to adjust the
in-angle when it is smaller than the out-angle to differentiate the two
cases.

Fixed: 1132525
Change-Id: I191e9e3d4889517f1188ec4a19e71beb8a966ed4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446729Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#815604}

third_party/blink/renderer/core/layout/svg/svg_marker_data.cc

@@ -27,8 +27,10 @@
 namespace blink {
 
 static double BisectingAngle(double in_angle, double out_angle) {
+  double diff = in_angle - out_angle;
   // WK193015: Prevent bugs due to angles being non-continuous.
-  if (fabs(in_angle - out_angle) > 180)
+  // Use an inclusive lower limit to not produce the same angle for both limits.
+  if (diff > 180 || diff <= -180)
     in_angle += 360;
   return (in_angle + out_angle) / 2;
 }

third_party/blink/renderer/platform/geometry/float_point.cc

@@ -31,6 +31,7 @@
 #include <algorithm>
 #include <limits>
 
+#include "build/build_config.h"
 #include "third_party/blink/renderer/platform/geometry/layout_point.h"
 #include "third_party/blink/renderer/platform/geometry/layout_size.h"
 #include "third_party/blink/renderer/platform/wtf/math_extras.h"
@@ -41,7 +42,14 @@
 namespace blink {
 
 float FloatPoint::SlopeAngleRadians() const {
+#if defined(OS_MAC)
+  // atan2f(...) returns less accurate results on Mac.
+  // 3.1415925 vs. 3.14159274 for atan2f(0, -50) as an example.
+  return static_cast<float>(
+      atan2(static_cast<double>(y_), static_cast<double>(x_)));
+#else
   return atan2f(y_, x_);
+#endif
 }
 
 float FloatPoint::length() const {

third_party/blink/web_tests/external/wpt/svg/painting/reftests/markers-orient-002.svg

+<svg xmlns="http://www.w3.org/2000/svg" xmlns:h="http://www.w3.org/1999/xhtml">
+  <title>orient='auto' with 180 degree turns</title>
+  <h:link rel="help" href="https://svgwg.org/svg2-draft/painting.html#RenderingMarkers"/>
+  <h:link rel="match" href="../../pservers/reftests/reference/green-100x100.svg"/>
+  <marker id="m" markerWidth="100" markerHeight="50" markerUnits="userSpaceOnUse"
+          orient="auto" refX="50" refY="50">
+    <path d="M50,-5L105,50h-110z" fill="green"/>
+  </marker>
+  <g marker-start="url(#m)">
+    <path d="M50,0v50z"/>
+    <path d="M100,50h-50z"/>
+    <path d="M50,100v-50z"/>
+    <path d="M0,50h50z"/>
+  </g>
+</svg>