GLU NURBS错误地渲染

Question

编辑:这是一个库bug.我把它报告给HOpenGL邮件列表.

我使用9点矩形方法将圆/椭圆表示为NURBS.

要点是p1, p2, ..., p9,p9 = p1.它们如图所示:

y2 | p2 p3 p4
y1 | p1    p5
y0 | p8 p7 p6
-------------
   | x0 x1 x2

x1 = (x0 + x2) / 2
y1 = (y0 + y2) / 2

即p1 = (x0, y1), p2 = (x0, y2)等等.

重量是:

• 1中间点(p1,p3,p5,p7)
• sqrt(0.5)角点(p2,p4,p6,p8)

我使用两种方法将权重应用为齐次坐标:

• 对 - (x,y)重量w变成了Vertex4 (w*x) (w*y) 0 w
• 错了 - 它变成了 Vertex4 x y 0 w

结果(右边第一,错误第二,抱歉,如果它们太大):

你看,两个都不是正确的圈子(虽然第二个看起来不错)但我无法理解为什么.

以下是基于Lines.hsGLUT示例的代码:

import System.Exit ( exitWith, ExitCode(ExitSuccess) )
import Graphics.UI.GLUT
import Foreign.Marshal.Array
import Graphics.Rendering.OpenGL.GLU.NURBS

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

display :: DisplayCallback
display = do
  clear [ ColorBuffer, DepthBuffer ]
  color (Color3 1.0 1.0 1.0 :: Color3 GLfloat)

  withNURBSObj () $ \nurbsObj -> do
    nurbsBeginEndCurve nurbsObj $
      withArrayLen knots $ \nKnots knots ->
        withArray controls $ \controls -> do
          nurbsCurve nurbsObj (fromIntegral nKnots) knots stride controls order

  flush

  where
    order = 3
    stride = 4 -- number of floats in Vertex
    controls = zipWith mkControl points weights
    mkControl (x, y) w = Vertex4 (x*w) (y*w) 0 w
    -- mkControl (x, y) w = Vertex4 x y 0 w
    knots = [0, 0, 0, 0.25, 0.25, 0.5, 0.5, 0.75, 0.75, 1, 1, 1]
    weights = let a = sqrt 0.5 in [1, a, 1, a, 1, a, 1, a, 1]
    points = [
      (x0, y1), (x0, y2), (x1, y2),
      (x2, y2), (x2, y1), (x2, y0),
      (x1, y0), (x0, y0), (x0, y1)
      ]

    y1 = (y0 + y2) / 2
    x1 = (x0 + x2) / 2
    (x0, x2) = (50, 450)
    (y0, y2) = (x0, x2)

reshape :: ReshapeCallback
reshape size@(Size w h) = do
   viewport $= (Position 0 0, size)
   matrixMode $= Projection
   loadIdentity
   ortho2D 0 (fromIntegral w) 0 (fromIntegral h)
   -- the following line is not in the original example, but it's good style...
   matrixMode $= Modelview 0

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

--  Request double buffer display mode.
--  Register mouse input callback functions
main :: IO ()
main = do
   (progName, _args) <- getArgsAndInitialize
   initialDisplayMode $= [ SingleBuffered, RGBMode ]
   initialWindowSize $= Size 500 500
   initialWindowPosition $= Position 100 100
   createWindow "Test"
   myInit
   displayCallback $= display
   reshapeCallback $= Just reshape
   keyboardMouseCallback $= Just keyboard
   mainLoop

编辑:我重新检查系数几次,在矩形表示之前我使用7点三角形方法,并且有非常相似的失真.所以它不应该是具体表示的问题.

Answer 1

好的,我的第一个答案是我暂时失误的原因.无论如何,你的代码中根本没有错误:如果我编译并执行给定的代码,我得到:

我怀疑,你的Haskell安装中的某些内容被破坏了.

编辑评论:

现在这很奇怪......

我正在使用Gentoo,因此我的系统通过Portage系统提供Haskell OpenGL模块.因此我明白了:

loki ~ # for p in OpenGL OpenGLRaw GLURaw; do ghc-pkg latest $p; done
OpenGL-2.2.1.1
ghc-pkg: cannot find package OpenGLRaw
ghc-pkg: cannot find package GLURaw

所以我将带有cabal的OpenGL和GLUT模块安装到我的homedir中:

datenwolf@loki ~ $  for p in OpenGL OpenGLRaw GLURaw; do ghc-pkg latest $p; done
OpenGL-2.4.0.1
OpenGLRaw-1.1.0.1
GLURaw-1.1.0.0

安装完成后我可以重现你的第一张照片!