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POGLU NURBS rendered wrongly
 

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    <p>EDIT: This is a library bug. I <a href="http://www.haskell.org/pipermail/hopengl/2011-October/001055.html" rel="nofollow noreferrer">reported</a> it to the HOpenGL mail list.</p> <p>I use 9-point rectangular method to represent a circle/ellipse as a NURBS.</p> <p>The points are <code>p1, p2, ..., p9</code>, <code>p9 = p1</code>. They lay as shown:</p> <pre><code>y2 | p2 p3 p4 y1 | p1 p5 y0 | p8 p7 p6 ------------- | x0 x1 x2 x1 = (x0 + x2) / 2 y1 = (y0 + y2) / 2 </code></pre> <p>I.e. <code>p1 = (x0, y1), p2 = (x0, y2)</code> and so on.</p> <p>And weights are:</p> <ul> <li><code>1</code> for middle points (<code>p1,p3,p5,p7</code>)</li> <li><code>sqrt(0.5)</code> for corner points (<code>p2,p4,p6,p8</code>)</li> </ul> <p>I applied weights as homogeneous coordinates, using two ways:</p> <ul> <li>right - <code>(x,y)</code> with weight <code>w</code> becomes <code>Vertex4 (w*x) (w*y) 0 w</code></li> <li>wrong - it becomes <code>Vertex4 x y 0 w</code></li> </ul> <p>The results (right first, wrong second, sorry if they are too big): <img src="https://i.stack.imgur.com/waoRm.png" alt="first"> <img src="https://i.stack.imgur.com/p2mOb.png" alt="second"></p> <p>You see, both are not proper circles (though the second looks nice) and I can't understand why.</p> <p>Here follows the code, based on <code>Lines.hs</code> from GLUT examples:</p> <pre><code>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 -&gt; do nurbsBeginEndCurve nurbsObj $ withArrayLen knots $ \nKnots knots -&gt; withArray controls $ \controls -&gt; 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) &lt;- getArgsAndInitialize initialDisplayMode $= [ SingleBuffered, RGBMode ] initialWindowSize $= Size 500 500 initialWindowPosition $= Position 100 100 createWindow "Test" myInit displayCallback $= display reshapeCallback $= Just reshape keyboardMouseCallback $= Just keyboard mainLoop </code></pre> <p>EDIT: I rechecked coefficients several times, and before rectangular representation I used 7-point triangle method, and there were very similar distortions. So it shall not be a problem with concrete representation.</p>
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    1. COJust a hint: You should change display to be `Int -> Int -> DisplayCallback`, with the first to parameters being the window size, setting the whole projection in display, too. In reshape use currying `displayCallback $= display w h`; this makes is much more Haskell-ish, by avoiding unnecessarity impurities.
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    2. COgah, so many typos in a single comment, and no way to edit it.
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