Diagrams intersection bug

README.md

A demonstration of this bug.

The resulting diagram should show all intersection points between an edge and the boundaries of its nodes. As you can see, some are missing. Changing r at line 26 to 7 results in different missed intersections. Most other values of r result in all intersections being found.

vmap and es were originally generated by diagrams-graphviz.

Main.hs

#!/usr/bin/env cabal
{- cabal:
build-depends: base,containers,diagrams-lib,diagrams-svg
-}

module Main where

import Data.Map (Map,(!))
import qualified Data.Map as Map
import Diagrams.Backend.SVG.CmdLine
import Diagrams.Prelude

main :: IO ()
main = mainWith graph

graph :: Diagram B
graph = mconcat edgeArrows <> (mconcat nodeTrails # lc blue)
    where
        nodeTrails = map stroke $ Map.elems nodes
        edgeArrows = map (\(v1,v2,p) -> edgeArrow p (nodes ! v1) (nodes ! v2)) es
        nodes = node <$> vmap

node :: P2 Double -> Located (Trail V2 Double)
node p = (circle r `at` translate (V2 r 0) p)
    where r = 6

edgeArrow :: Located (Trail V2 Double) -> Located (Trail V2 Double) -> Located (Trail V2 Double) -> Diagram B
edgeArrow tr t1 t2 = stroke tr <> mconcat is1 <> mconcat is2
    where
        is1 = map (\p -> circle 0.7 # moveTo p # lc green) $ intersectPoints tr t1
        is2 = map (\p -> circle 1.0 # moveTo p # lc red  ) $ intersectPoints tr t2

vmap :: Map Int (Point V2 Double)
vmap =
    Map.fromList
        [
            ( 2
            , P ( V2 73.0 14.0 )
            )
        ,
            ( 3
            , P ( V2 36.5 14.0 )
            )
        ,
            ( 1
            , P ( V2 0.0 0.0 )
            )
        ]

es :: [(Int, Int, Located (Trail V2 Double))]
es =
    [
        ( 1
        , 3
        , Trail
            ( lineFromSegments
                [ bézier3 ( V2 1.9888 0.795509 ) ( V2 32.876599999999996 13.150839000000001 ) ( V2 34.8056 13.921839 ) ]
            ) `at` P ( V2 1.1004 4.0161e-2 )
        )
    ,
        ( 1
        , 2
        , Trail
            ( lineFromSegments
                [ bézier3 ( V2 1.197 0.11278329999999999 ) ( V2 20.5887 1.9646333 ) ( V2 35.9417 4.9945333 )
                , bézier3 ( V2 15.265 3.012600000000001 ) ( V2 33.801 8.635 ) ( V2 34.944 8.983 )
                ]
            ) `at` P ( V2 1.0583 5.4667e-3 )
        )
    ,
        ( 3
        , 3
        , Trail
            ( lineFromSegments
                [ bézier3
                    ( V2 ( -8.583000000000002 ) 6.086 )
                    ( V2 ( -8.444000000000003 ) 17.714 ) ( V2 0.41499999999999915 17.714 )
                , bézier3 ( V2 8.859000000000002 0.0 )
                    ( V2 8.997999999999998 ( -11.628 ) )
                    ( V2 0.41499999999999915 ( -17.714 ) )
                ]
            ) `at` P ( V2 36.085 14.286 )
        )
    ,
        ( 3
        , 2
        , Trail
            ( lineFromSegments
                [ bézier3
                    ( V2 1.044000000000004 ( -8.4487 ) )
                    ( V2 33.922999999999995 ( -8.4466 ) ) ( V2 34.949999999999996 5.999999999998451e-3 )
                ]
            ) `at` P ( V2 37.025 13.585 )
        )
    ,
        ( 2
        , 3
        , Trail
            ( lineFromSegments
                [ bézier3
                    ( V2 ( -1.027000000000001 ) 8.452999999999998 )
                    ( V2 ( -33.90599999999999 ) 8.455 )
                    ( V2 ( -34.949999999999996 ) 5.999999999998451e-3 )
                ]
            ) `at` P ( V2 71.975 14.409 )
        )
    ,
        ( 2
        , 2
        , Trail
            ( lineFromSegments
                [ bézier3
                    ( V2 ( -8.582999999999998 ) 6.086 )
                    ( V2 ( -8.443999999999988 ) 17.714 ) ( V2 0.41500000000000625 17.714 )
                , bézier3 ( V2 8.858999999999995 0.0 )
                    ( V2 8.998000000000005 ( -11.628 ) )
                    ( V2 0.41500000000000625 ( -17.714 ) )
                ]
            ) `at` P ( V2 72.585 14.286 )
        )
    ]

out.svg