HaskellでOpenGL(GLUT)を使って、ウィンドウのサイズを変えても描画されている図形の大きさが変わらないようにする。

resize.hs

import Graphics.UI.GLUT

display :: DisplayCallback
display = do
    clear [ColorBuffer]
    color (Color3 0 0 0 :: Color3 GLdouble)
    renderPrimitive Polygon $ mapM_ vertex2d [
          Vertex2 (-0.9) (-0.9)
        , Vertex2 0.9 (-0.9)
        , Vertex2 0.9 0.9
        , Vertex2 (-0.9) 0.9
        ]
    flush
        where
            vertex2d = vertex :: Vertex2 GLdouble -> IO ()

resize :: Size -> IO ()
resize s@(Size w h) = do
    viewport $= (Position 0 0,s)
    loadIdentity
    ortho (-w') (w') (-h') (h') (-1.0) (1.0) 
        where
            w' = realToFrac w / 200.0
            h' = realToFrac h / 200.0


main :: IO ()
main = do
    getArgsAndInitialize
    initialDisplayMode $= [RGBAMode]
    createWindow "Sample"
    clearColor $= Color4 1 1 1 1
    displayCallback $= display
    reshapeCallback $= Just resize
    mainLoop