opengl - Haskell OpenGL 绑定(bind)中似乎存在错误

Question

我写了一个程序来可视化氢原子的电子云。

import System.Exit 
import Graphics.UI.GLUT
probDensity :: Double -> Double
probDensity r = abs $ (1 - r) * exp (-r/2.0)

myInit :: IO ()
myInit = clearColor $= Color4 1 1 1 0

grid :: [(GLint,GLint)]
grid = [(x,y) | x <- [-200..200],y <- [-200..200]]

density :: [Double]
density = map (\(i',j') -> probDensity $ sqrt $ (fromIntegral i' ** 2 + fromIntegral j' ** 2 ) / 324) grid

cloud = zip density grid

display :: DisplayCallback
display = do
  clear [ColorBuffer]

  color $ Color4 1 1 1 (0::GLfloat)
  renderPrimitive Points $
    mapM_ (\(c,(x,y)) -> color (Color3 c c 0) >> vertex (Vertex2 x y)) cloud
  flush

idle :: IdleCallback
idle = 
  postRedisplay Nothing


reshape :: ReshapeCallback
reshape (Size _ _) = do
   viewport $= (Position 0 0, Size 400 400)
   matrixMode $= Projection
   loadIdentity
   ortho2D (-200.0) 200.0 (-200.0) 200.0
   matrixMode $= Modelview 0
   loadIdentity

keyboard :: KeyboardMouseCallback
keyboard (Char '\27') Down _ _ = exitSuccess
keyboard _ _ _ _ = return ()


main :: IO ()
main = do
   (_, _args) <- getArgsAndInitialize
   initialDisplayMode $= [  RGBMode ]
   initialWindowSize $= Size 400 400
   initialWindowPosition $= Position 100 100
   _ <- createWindow "Cloud"
   shadeModel $= Smooth
   myInit
   displayCallback $= display 
   reshapeCallback $= Just reshape
   keyboardMouseCallback $= Just keyboard
   idleCallback $= Just idle
   mainLoop 

但是结果在图的右边部分有很多线。

我一遍又一遍地检查我的代码，没有发现任何错误。这是包的错误吗？

Answer 1

我猜这是因为浮点错误导致某些列在光栅化过程中丢失。您有 401 列样本分布在 400 列像素上，并且您的顶点位置作为整数发送。当整数在图形管道中转换为 float 时，they will not be exact .如果您将视口(viewport)和窗口大小更改为其他值，它应该看起来不错:

399x399:

400x400:

401x401(一对一像素采样):

402x402:

请注意，如果您增加所采集的样本数量，这也能正常工作:

grid = [(x,y) | x <- [-400..400],y <- [-400..400]]
density = map (\(i',j') -> probDensity $ sqrt $
                           (fromIntegral i' ** 2 + fromIntegral j' ** 2 ) / 648) grid
renderPrimitive Points $
  mapM_ (\(c,(x,y)) -> do
    color (Color3 c c 0)
    vertex (Vertex2 (fromIntegral x / 2) (fromIntegral y / 2) :: Vertex2 GLfloat)) cloud

另一种解决方法是使用浮点值顶点位置定位像素中心。改变

vertex (Vertex2 x y)

到

vertex (Vertex2 (fromIntegral x + 0.5) (fromIntegral y + 0.5) :: Vertex2 GLfloat)