Commit before attempting to allow for non-convex chasm shapes

src/Geometry/Intersect.hs

@@ -83,14 +83,17 @@ intersectSegSeg (V2 x1 y1) (V2 x2 y2) (V2 x3 y3) (V2 x4 y4)
 
 {- | Intended to intersect a segment with a half-line-segment, ie a segment
  extending infinitely in one direction.
+Will intersect with the first endpoint of the segment, but NOT the second.
+This is to allow sensible intersections with polygons described as lists of points.
+It will also intersect with the point of the ray.
 -}
-intersectSegLineFrom :: Point2 -> Point2 -> Point2 -> Point2 -> Maybe Point2
-{-# INLINE intersectSegLineFrom #-}
-intersectSegLineFrom (V2 x1 y1) (V2 x2 y2) (V2 x3 y3) (V2 x4 y4)
+intersectSegRay :: Point2 -> Point2 -> Point2 -> Point2 -> Maybe Point2
+{-# INLINE intersectSegRay #-}
+intersectSegRay (V2 x1 y1) (V2 x2 y2) (V2 x3 y3) (V2 x4 y4)
     | den == 0 = Nothing
-    | den > 0 && (t' < 0 || u' < 0 || t' > den) =
+    | den > 0 && (t' < 0 || u' < 0 || t' >= den) =
         Nothing
-    | den < 0 && (t' > 0 || u' > 0 || t' < den) =
+    | den < 0 && (t' > 0 || u' > 0 || t' <= den) =
         Nothing
     | otherwise = Just $ V2 (x1 + (x2 - x1) * t' / den) (y1 + (y2 - y1) * t' / den)
   where
@@ -114,18 +117,20 @@ intersectSegLineext (V2 x1 y1) (V2 x2 y2) (V2 x3 y3) (V2 x4 y4)
     u' = (y1 - y2) * (x1 - x3) - (x1 - x2) * (y1 - y3)
 
 -- | Intersect a segment with a line.
+-- the line intersects with the first endpoint of the segment
+-- but NOT the second
 intersectSegLine :: Point2 -> Point2 -> Point2 -> Point2 -> Maybe Point2
 {-# INLINE intersectSegLine #-}
 intersectSegLine (V2 x1 y1) (V2 x2 y2) (V2 x3 y3) (V2 x4 y4)
     | den == 0 = Nothing
-    | den > 0 && (t' < 0 || t' > den) =
+    | den > 0 && (t < 0 || t >= den) =
         Nothing
-    | den < 0 && (t' > 0 || t' < den) =
+    | den < 0 && (t > 0 || t <= den) =
         Nothing
-    | otherwise = Just $ V2 (x1 + (x2 - x1) * t' / den) (y1 + (y2 - y1) * t' / den)
+    | otherwise = Just $ V2 (x1 + (x2 - x1) * t / den) (y1 + (y2 - y1) * t / den)
   where
     den = (x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4)
-    t' = (x1 - x3) * (y3 - y4) - (y1 - y3) * (x3 - x4)
+    t = (x1 - x3) * (y3 - y4) - (y1 - y3) * (x3 - x4)
 
 --u'  = (y1-y2)*(x1-x3) - (x1-x2)*(y1-y3)