Visualizing 2D convex hull using Gtk and OpenGL in Haskell

This note shows how to use OpenGL with Gtk in Haskell. The result is a little visualization to check our implementation of the classic iterative convex hull algorithm.


This post is a valid literate Haskell file so save it to something like ConvexHull.lhs and compile with ghc --make ConvexHull. What you see above is what you’ll get when you run `./ConvexHull`

The best OpenGL tutorial for Haskell that I’ve found is this one from Michi’s blog, using GLUT to interface with X. For this tutorial we are going to use the Gtk GLDrawingArea widget, to illustrate the differences, which can be rather hard to find in the documentation.

The libraries used can be found here:

These are thin bindings, so our code is all going to be pretty imperative.

> import Data.IORef
> import Data.List
> import Graphics.Rendering.OpenGL as GL
> import Graphics.UI.Gtk as Gtk
> import Graphics.UI.Gtk.OpenGL
> import System.Random

I’ll show main first. If you are just looking for the outline of how to initialize everything and make it go, here it is:

> main = do
>   initGUI
>   glConfig    initGL
>   pointRef >= newIORef)
>   canvas     onExpose canvas (_ -> readIORef pointRef >>= drawWithHull canvas)
>   button    Gtk.set button [ buttonLabel := "Generate new points." ]
>   onClicked button (do newPoints                         writeIORef pointRef newPoints
>                        drawWithHull canvas newPoints
>                        return ())
>   vbox    boxPackStart vbox button PackNatural 0
>   boxPackStart vbox canvas PackGrow 0
>   window    Gtk.set window [ containerBorderWidth := 10,
>                    containerChild := vbox ]
>   onDestroy window mainQuit
>   widgetShowAll window
>   mainGUI              

Now, Haskell’s OpenGL binding has some quirks with regards to numeric overloading, so it helps to define some type aliases. Since I want to take cross products I’ll work in three dimensions, and define some basic operations on my points. The OpenGL binding has separate types for points and vectors, but I’m going to abuse the point type to represent both.

> type Point3 = Vertex3 GLfloat

> cross :: Point3 -> Point3 -> Point3
> cross (Vertex3 x0 y0 z0) (Vertex3 x1 y1 z1) = 
>    Vertex3 (y0*z1 - z0*y1) (z1*x0 - x0*z1) (x0*y1 - x1*y0)

> dot :: Point3 -> Point3 -> GLfloat
> dot (Vertex3 x0 y0 z0) (Vertex3 x1 y1 z1) = x0*x1 + y0*y1 + z0*z1

> randomPoints :: Int -> IO [Point3]
> randomPoints 0 = return []
> randomPoints n = do 
>   x    y    rest    return $ Vertex3 x y 0 : rest

Now for the quirks with using Gtk for OpenGL – there are many more setup calls to make. First, you need to explicitly grab a graphics context (glContext) and GL drawing window (glWin). Then, we manage the viewport manually to scale our rendering up to fill the window. Finally, there are Gtk calls to start and end OpenGL rendering calls.
It took me a while to discover them.

> drawWithHull :: GLDrawingArea -> [Point3] -> IO Bool
> drawWithHull canvas points = do
>   -- This is all Gtk code, managing the internal structures
>   glContext    glWin    (w,h) 
>   -- This is again Gtk code
>   glDrawableGLBegin glWin glContext
>   -- These are OpenGL calls to scale up and use the whole canvas
>   (pos, _)    viewport $= (pos, Size (fromIntegral w) (fromIntegral h))
>   renderWithHull points
>   GL.flush -- except this
>   glDrawableSwapBuffers glWin
>   glDrawableGLEnd glWin
>   return True

I use the terminology “draw” to refer to Gtk drawing code, which tends to be bookkeeping, while I use “render” to refer to sequences of OpenGL calls. Here is the code to actually render the points and their convex hull. Note the color3f specialization, to help the type inferencer.

> renderWithHull :: [Point3] -> IO ()
> renderWithHull points = do
>   clear [ColorBuffer]
>   color3f (Color3 1 1 1)
>   renderPrimitive Quads $ mapM_ fatPoint $ points
>   color3f (Color3 1 0 0)
>   renderPrimitive LineStrip $ mapM_ vertex $ hull
>     where hull = convexHull points
>           color3f = color :: Color3 GLfloat -> IO ()

> fatPoint (Vertex3 x y z) = do 
>   vertex $ Vertex3 (x+0.005) (y+0.005) z
>   vertex $ Vertex3 (x-0.005) (y+0.005) z
>   vertex $ Vertex3 (x-0.005) (y-0.005) z
>   vertex $ Vertex3 (x+0.005) (y-0.005) z

From here on, I’m just implementing the convex hull algorithm.

This is an iterative algorithm that computes the upper half-hull by travelling left-to-right across the plane making sure to always make right turns; if ever a left turn occurs, it backtracks as far as necessary, patching up the hull. I defer the obvious helper isLeftOf to the end of the file.

> upperHalfHull points = upperHalfHull' (sort points) []
>    where upperHalfHull' [] hull = hull
>          upperHalfHull' (v:vs) [] = upperHalfHull' vs [v]
>          upperHalfHull' (v:vs) [y] = upperHalfHull' vs [v,y]
>          upperHalfHull' (v:vs) (y:x:zs) = if v `isLeftOf` (x,y)
>                                           then upperHalfHull' (v:vs) (x:zs)
>                                           else upperHalfHull' vs (v:y:x:zs)

Then the lower half of the hull does the same thing right-to-left, and I rather naively combine them into convexHull (I traverse the points maybe three times unneccessarily)

> lowerHalfHull points = map rotate180 $ upperHalfHull $ map rotate180 $ points
> rotate180 (Vertex3 x y z) = Vertex3 (-x) (-y) z

> convexHull :: [Point3] -> [Point3]
> convexHull points = upperHalfHull points ++ lowerHalfHull points

There is a divide-and-conquer algorithm which is probably more idiomatic, and has the same asymptotic complexity (different pathological cases) but this is the one I was trying out.

This last helper function only makes sense when points are all on the z=0 plane. It takes a point and a directed line segment, and indicates whether the point lies to the left of the line defined by that segment.

> isLeftOf :: Point3 -> (Point3, Point3) -> Bool
> isLeftOf (Vertex3 x2 y2 _) (Vertex3 x0 y0 _, Vertex3 x1 y1 _) =
>   let Vertex3 _ _ z = (Vertex3 (x1-x0) (y1-y0) 0) 
>                       `cross` 
>                       (Vertex3 (x2-x0) (y2-y0) 0) 
>   in z > 0

One comment

  1. […] to something like Congestion.lhs and run ghc –make Congestion.lhs. I started with the code from an old post and cut out the bits I didn’t need. The libraries used can be found […]

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