From b96f58cfd771855480f585f58c60162ac727d727 Mon Sep 17 00:00:00 2001
From: oceanful <35951082+oceanful@users.noreply.github.com>
Date: Fri, 17 May 2019 15:25:50 -0700
Subject: [PATCH] Improve intersection precision to reduce false positives.
 (#9)

---
 bugs_test.go | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 clipper.go   |  2 +-
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/bugs_test.go b/bugs_test.go
index c5f14c3..ff23ece 100644
--- a/bugs_test.go
+++ b/bugs_test.go
@@ -287,6 +287,55 @@ func TestResweepingIntersectingEndpoints(t *T) {
 	}.verify(t)
 }
 
+func TestIntersectionFalsePositives(t *T) {
+	testCases{
+		{
+			op: polyclip.INTERSECTION,
+			subject: polyclip.Polygon{{
+				{100.0000001, 100},
+				{98.48077539970159, 117.36481778405785},
+				{98.48077539970159, 82.63518221594214},
+			}},
+			clipping: polyclip.Polygon{{
+				{100.0000001, 100},
+				{100, 99.99999885699484},
+				{99.9999999, 100.00000000000001},
+				{100, 100.00000114300516},
+				{100.0000001, 100},
+			}},
+			result: polyclip.Polygon{{
+				{100.0000001, 100},
+				{100, 99.99999885699484},
+				{99.9999999, 100.00000000000001},
+				{100, 100.00000114300516},
+				{100.0000001, 100},
+			}},
+		},
+		{
+			op: polyclip.INTERSECTION,
+			subject: polyclip.Polygon{{
+				{100.00000001, 100},
+				{98.48077531106888, 117.36481776842952},
+				{98.48077531106888, 82.63518223157048},
+			}},
+			clipping: polyclip.Polygon{{
+				{100.00000001, 100},
+				{100, 99.99999988569955},
+				{99.99999999, 100.00000000000001},
+				{100, 100.00000011430046},
+				{100.00000001, 100},
+			}},
+			result: polyclip.Polygon{{
+				{100.00000001, 100},
+				{100, 99.99999988569955},
+				{99.99999999, 100.00000000000001},
+				{100, 100.00000011430046},
+				{100.00000001, 100},
+			}},
+		},
+	}.verify(t)
+}
+
 func TestCorruptionResistanceFromFloatingPointImprecision(t *T) {
 	testCases{
 		{
diff --git a/clipper.go b/clipper.go
index 0f9410f..e5dfe56 100644
--- a/clipper.go
+++ b/clipper.go
@@ -303,7 +303,7 @@ func findIntersection(seg0, seg1 segment, tryBothDirections bool) (int, Point, P
 	d0 := Point{seg0.end.X - p0.X, seg0.end.Y - p0.Y}
 	p1 := seg1.start
 	d1 := Point{seg1.end.X - p1.X, seg1.end.Y - p1.Y}
-	sqrEpsilon := 1e-15 // was originally 1e-3, which is very prone to false positives
+	sqrEpsilon := 1e-21 // was originally 1e-3, which is very prone to false positives
 	E := Point{p1.X - p0.X, p1.Y - p0.Y}
 	kross := d0.X*d1.Y - d0.Y*d1.X
 	sqrKross := kross * kross