From 804620d91e737d81b27c1d32dcaca0e860c4c8bb Mon Sep 17 00:00:00 2001
From: Stewart Allen <stewart.allen@gmail.com>
Date: Sat, 20 Feb 2021 23:10:41 -0500
Subject: [PATCH] filter false gap positives. also faster

---
 src/geo/polygons.js | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/geo/polygons.js b/src/geo/polygons.js
index 4e710b0b..e6b389cb 100644
--- a/src/geo/polygons.js
+++ b/src/geo/polygons.js
@@ -555,7 +555,13 @@
             let mid = offset(off, dist / 2, {z});
             let cmp = offset(off, dist, {z});
             let gap = [];
-            subtract(ref, cmp, gap, null, z);
+            let aref = ref.map(p => p.areaDeep()).reduce((a,p) => a +p);
+            let cref = cmp.length ? cmp.map(p => p.areaDeep()).reduce((a,p) => a + p) : 0;
+            // threshold subtraction to area deltas > 0.1 % to filter out false
+            // positives where inset/outset are identical floating point error
+            if (Math.abs(aref - cref) >  1 - (Math.abs(aref / cref) / 1000)) {
+                subtract(ref, cmp, gap, null, z);
+            }
             layers.push({idx: total-count, off, mid, gap});
             ref = off;
         }