Invert order of calc of segment intersections

When the input has two already exactly-overlapping segments with a third that crosses the two, rounding errors can cause slightly different intersection points to be computed. It appears that calculating the intersections in this order avoids these rounding errors from creating disjoint or extraneous segments (at least for this test case, which is the only one I know about now... there could be other circumstances which still fail, in which case a bigger fix will be necessary). Fixes #19
mfogel · May 24, 2018 · 4ab6557 · 4ab6557
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commit 4ab6557
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Showing 7 changed files with 45 additions and 6 deletions.
diff --git a/README.md b/README.md
@@ -80,6 +80,7 @@ The Martinez-Rueda-Feito polygon clipping algorithm is used to compute the resul
 
 ### vNext (in developement)
 
+ * Fix bug with overlapping segments ([#19](https://github.com/mfogel/polygon-clipping/issues/19))
  * Add benchmarks `npm run bench` ([#15](https://github.com/mfogel/polygon-clipping/issues/15))
 
 ### v0.6.1 (2018-04-01)

diff --git a/src/sweep-line.js b/src/sweep-line.js
@@ -38,7 +38,7 @@ class SweepLine {
 
       // Check for intersections against the previous segment in the sweep line
       if (prevSeg) {
-        const prevInters = segment.getIntersections(prevSeg)
+        const prevInters = prevSeg.getIntersections(segment)
         if (prevInters.length > 0) {
           const newEventsFromSplit = this._possibleSplit(prevSeg, prevInters)
           for (let i = 0, iMax = newEventsFromSplit.length; i < iMax; i++) {
@@ -53,7 +53,7 @@ class SweepLine {
 
       // Check for intersections against the next segment in the sweep line
       if (nextSeg) {
-        const nextInters = segment.getIntersections(nextSeg)
+        const nextInters = nextSeg.getIntersections(segment)
         if (nextInters.length > 0) {
           const newEventsFromSplit = this._possibleSplit(nextSeg, nextInters)
           for (let i = 0, iMax = newEventsFromSplit.length; i < iMax; i++) {

diff --git a/test/end-to-end/almost-parrallel-segments/difference.geojson b/test/end-to-end/almost-parrallel-segments/difference.geojson
@@ -10,7 +10,7 @@
           [-69.0043512, 44.296],
           [-69.0043512, 44.2966448],
           [-69.0045759, 44.2970255],
-          [-69.0045298581688, 44.297005473122546],
+          [-69.00452985816878, 44.297005473122546],
           [-69.004, 44.298],
           [-69.006, 44.296]
         ]

diff --git a/test/end-to-end/nearly-vertical-far-right/intersection.geojson b/test/end-to-end/nearly-vertical-far-right/intersection.geojson
@@ -8,7 +8,7 @@
         [
           [-89.42, 30.234475],
           [-89.306147, 30.1781849],
-          [-89.30625006083005, 30.234475],
+          [-89.30625006083004, 30.234475],
           [-89.42, 30.234475]
         ]
       ]

diff --git a/test/end-to-end/parallel-lines-with-crossing/args.geojson b/test/end-to-end/parallel-lines-with-crossing/args.geojson
@@ -0,0 +1,17 @@
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "properties": null,
+      "geometry": {
+        "type": "MultiPolygon",
+        "coordinates": [
+          [[[541, -42], [538.88, -41.92], [540.09, -43.74], [541, -42]]],
+          [[[538, -44], [540.09, -43.74], [538.88, -41.92], [538, -44]]],
+          [[[541, -44], [539.87, -43.37], [538.73, -44.88], [541, -44]]]
+        ]
+      }
+    }
+  ]
+}
diff --git a/test/end-to-end/parallel-lines-with-crossing/union.geojson b/test/end-to-end/parallel-lines-with-crossing/union.geojson
@@ -0,0 +1,21 @@
+{
+  "type": "Feature",
+  "properties": null,
+  "geometry": {
+    "type": "MultiPolygon",
+    "coordinates": [
+      [
+        [
+          [538, -44],
+          [539.538903654485, -43.808557440111905],
+          [538.73, -44.88],
+          [541, -44],
+          [540.1901555424297, -43.548493797991746],
+          [541, -42],
+          [538.88, -41.92],
+          [538, -44]
+        ]
+      ]
+    ]
+  }
+}
diff --git a/test/end-to-end/vertical-intersection-rounding-error/union.geojson b/test/end-to-end/vertical-intersection-rounding-error/union.geojson
@@ -7,10 +7,10 @@
       [
         [
           [-1.1741342, 50.6250111],
-          [-0.1, 49.43659688282012],
+          [-0.1, 49.43659688282011],
           [-0.1, 49],
           [0.1, 49],
-          [0.0001, 49.4995],
+          [0.0001, 49.499500000000005],
           [0.0001, 50.6251],
           [-1.1741342, 50.6250111]
         ]