/usr/share/doc/libghc-glut-doc/examples/RedBook/Alpha.hs is in libghc-glut-doc 2.1.2.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Alpha.hs (adapted from alpha.c which is (c) Silicon Graphics, Inc.)
Copyright (c) Sven Panne 2002-2005 <sven.panne@aedion.de>
This file is part of HOpenGL and distributed under a BSD-style license
See the file libraries/GLUT/LICENSE
This program draws several overlapping filled polygons to demonstrate the
effect order has on alpha blending results. Use the 't' key to toggle the
order of drawing polygons.
-}
import Data.Char ( toLower )
import Data.IORef ( IORef, newIORef )
import System.Exit ( exitWith, ExitCode(ExitSuccess) )
import Graphics.UI.GLUT
data State = State { leftFirst :: IORef Bool }
makeState :: IO State
makeState = do
l <- newIORef True
return $ State { leftFirst = l }
-- Initialize alpha blending function.
myInit :: IO ()
myInit = do
blend $= Enabled
blendFunc $= (SrcAlpha, OneMinusSrcAlpha)
shadeModel $= Flat
clearColor $= Color4 0 0 0 0
drawLeftTriangle :: IO ()
drawLeftTriangle =
-- draw yellow triangle on LHS of screen
renderPrimitive Triangles $ do
color (Color4 1 1 0 (0.75 :: GLfloat))
vertex (Vertex3 0.1 0.9 (0 :: GLfloat))
vertex (Vertex3 0.1 0.1 (0 :: GLfloat))
vertex (Vertex3 0.7 0.5 (0 :: GLfloat))
drawRightTriangle :: IO ()
drawRightTriangle =
-- draw cyan triangle on RHS of screen
renderPrimitive Triangles $ do
color (Color4 0 1 1 (0.75 :: GLfloat))
vertex (Vertex3 0.9 0.9 (0 :: GLfloat))
vertex (Vertex3 0.3 0.5 (0 :: GLfloat))
vertex (Vertex3 0.9 0.1 (0 :: GLfloat))
display :: State -> DisplayCallback
display state = do
clear [ ColorBuffer ]
l <- get (leftFirst state)
if l
then do drawLeftTriangle; drawRightTriangle
else do drawRightTriangle; drawLeftTriangle
flush
reshape :: ReshapeCallback
reshape size@(Size w h) = do
viewport $= (Position 0 0, size)
matrixMode $= Projection
loadIdentity
let wf = fromIntegral w
hf = fromIntegral h
if w <= h
then ortho2D 0 1 0 (hf/wf)
else ortho2D 0 (wf/hf) 0 1
keyboard :: State -> KeyboardMouseCallback
keyboard state (Char c) Down _ _ = case toLower c of
't' -> do leftFirst state $~ not; postRedisplay Nothing
'\27' -> exitWith ExitSuccess -- Escape key
_ -> return ()
keyboard _ _ _ _ _ = return ()
-- Main Loop
-- Open window with initial window size, title bar, RGBA display mode, and
-- handle input events.
main :: IO ()
main = do
(progName, _args) <- getArgsAndInitialize
initialDisplayMode $= [ SingleBuffered, RGBMode ]
initialWindowSize $= Size 200 200
createWindow progName
state <- makeState
myInit
reshapeCallback $= Just reshape
keyboardMouseCallback $= Just (keyboard state)
displayCallback $= display state
mainLoop
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