Use grid when creating block debris

src/Geometry/Polygon.hs

@@ -3,9 +3,10 @@ module Geometry.Polygon where
 import Geometry.Data
 import Geometry.LHS
 import Geometry.Vector
+import ListHelp
 
 import Data.Maybe
-import Data.List
+--import Data.List
 import qualified Control.Foldl as L
 
 -- | Draw an anticlockwise rectangle based on maximal N E S W values.
@@ -111,3 +112,11 @@ grahamEliminate xs = xs
 
 centroid :: Foldable t => t Point2 -> Point2
 centroid = L.fold $ (/) <$> L.Fold (+.+) (V2 0 0) id <*> L.genericLength
+
+shrinkPolyOnEdges :: Float -> [Point2] -> [Point2]
+shrinkPolyOnEdges x (p:q:ps) = map (shrinkVert x) . slideWindow 3 $ (p:q:ps) ++ [p,q]
+shrinkPolyOnEdges _ _ = error "too few vertices in polygon"
+
+shrinkVert :: Float -> [Point2] -> Point2
+shrinkVert d (x:y:z:[]) = x +.+ (d *.* (normalizeV (x -.- y))) +.+ (d *.* (normalizeV (z -.- y)))
+shrinkVert _ _ = error "wrong number of vertices"