This tutorial  was originally written by Mikael Vejdemo Johansson, and was copied here with permission.
After having failed following the googled tutorial in HOpenGL programming, I thought I'd write down the steps I actually can get to work in a tutorial-like fashion. It may be a good idea to read this in parallell to the tutorial linked, since Panitz actually brings a lot of good explanations, even though his syntax isn't up to speed with the latest HOpenGL at all points.
Note: GHCI interactive shell has problems running these program on some platforms (such as Mac OS X). Compile these programs with ghc, and run the generated executables.
First of all, we'll want to load the OpenGL libraries, throw up a window, and generally get to grips with what needs to be done to get a program running at all.
import Graphics.Rendering.OpenGL import Graphics.UI.GLUT main :: IO () main = do (progname, _) <- getArgsAndInitialize createWindow "Hello World" displayCallback $= flush mainLoop
This code throws up a window, with a given title, and sets the main display function to do nothing but flush the (empty) graphics buffer. This is the skeleton that we'll be building on to.
Save it to HelloWorld.hs and compile it by running
ghc -package GLUT HelloWorld.hs -o HelloWorld.
You will see a window open, with the title "Hello World", with either a blank canvas, or with some garbage graphics content pulled from somewhere in your system's graphics memory.
In either case, this program is profoundly worthless.
At a minimum, let's have our program display a clean blank canvas:
So we modify our code to the following:
import Graphics.Rendering.OpenGL import Graphics.UI.GLUT main :: IO () main = do (progname, _) <- getArgsAndInitialize createWindow "Hello World" displayCallback $= display mainLoop display :: IO () display = do clear [ ColorBuffer ]; flush
This defines a function "display" that calls a few OpenGL functions: "clear" to clear out the graphics color state (so we get a blank canvas), and "flush" to push our OpenGL commands down to the system graphics for actual display.
We don't call "display" directly. (In fact, we don't call any graphics drawing functions directly). Instead, we set a display callback, and then call mainLoop. In mainLoop, OpenGL akes over, handles all the details of interacting with the OS and refreshing our window, calling our displayCallback to draw graphics.
displayCallback is a Data.IORef (mutable state variable), which we set using a call to
Save this to the HelloWorld.hs, recompile, and rerun. This program displays an endless series of blank canvases (a solid blank image).
The displayCallback is a globally defined IORef, which can be accessed through a host of functions defined in Data.IORef. In OpenGL StateVar module, there is a HasSetter type class and an IORef implementation providing functions
($=) (assignment) and
get to fascilitate interactions with these state variables.
height = newIORef 1.0 currentheight <- get height height $= 1.5
Using the drawing canvas
So, we have a window, we have a display callback that clears the canvas. Don't we want more out of it? Sure we do. So let's draw some things.
import Graphics.Rendering.OpenGL import Graphics.UI.GLUT myPoints :: [(GLfloat,GLfloat,GLfloat)] myPoints = map (\k -> (sin(2*pi*k/12),cos(2*pi*k/12),0.0)) [1..12] main = do (progname, _) <- getArgsAndInitialize createWindow "Hello World" displayCallback $= display mainLoop display = do clear [ColorBuffer] renderPrimitive Points $ mapM_ (\(x, y, z)->vertex$Vertex3 x y z) myPoints flush
Now, the important thing to notice in this code extract is that last line. It starts a rendering definition, gives the type to be rendered, and then a sequence of function calls, each of which adds a vertex to the rendering canvas. The statement is basically equivalent to something along the lines of
renderPrimitive Points do vertex Vertex3 ... vertex Vertex3 ...
for appropriate triples of coordinate values at the appropriate places. This results in the rendition here:
We can replace
Points with other primitives, leading to the rendering of:
Each three coordinates following each other define a triangle. The last n mod 3 coordinates are ignored.
TriangleStrip, triangles are drawn according to a “moving window” of size three, so the two last coordinates in the previous triangle become the two first in the next triangle.
When using a
TriangleFan, the first given coordinate is used as a basepoint, and takes each pair of subsequent coordinates to define a triangle together with the first coordinate.
Each pair of coordinates define a line.
LineLoop, each further coordinate defines a line together with the last coordinate used. Once all coordinates are used up, an additional line is drawn back to the beginning.
LineStrip is like a
LineLoop, only without the last link added.
Quads keyword, each four coordinates given define a quadrangle.
QuadStrip works as the
TriangleStrip, only the window is 4 coordinates wide and steps 2 steps each time, so each new pair of coordinates attaches a new quadrangle to the last edge of the last quadrangle.
It is easier to understand what is going on when you see how the window is formed. Giving each coordinate a number, the QuadStrip is rendered as follows: Coordinates 1, 2 and 4 are rendered as a triangle followed by coordinates 1, 3 and 4. Next coordinates 3, 4 and 6 are rendered as a triangle followed by coordinates 3, 5 and 6.
Rendering continues for as many coordinates that can be formed by that pattern.
Polygon is a filled line loop. Simple as that!
There are more things we can do on our canvas than just spreading out coordinates. Within the command list constructed after a renderPrimitive, we can give several different commands that control what things are supposed to look like, so for instance we could use the following:
display = do clear [ColorBuffer] renderPrimitive Quads $ do color $ (Color3 (1.0::GLfloat) 0 0) vertex $ (Vertex3 (0::GLfloat) 0 0) vertex $ (Vertex3 (0::GLfloat) 0.2 0) vertex $ (Vertex3 (0.2::GLfloat) 0.2 0) vertex $ (Vertex3 (0.2::GLfloat) 0 0) color $ (Color3 (0::GLfloat) 1 0) vertex $ (Vertex3 (0::GLfloat) 0 0) vertex $ (Vertex3 (0::GLfloat) (-0.2) 0) vertex $ (Vertex3 (0.2::GLfloat) (-0.2) 0) vertex $ (Vertex3 (0.2::GLfloat) 0 0) color $ (Color3 (0::GLfloat) 0 1) vertex $ (Vertex3 (0::GLfloat) 0 0) vertex $ (Vertex3 (0::GLfloat) (-0.2) 0) vertex $ (Vertex3 ((-0.2)::GLfloat) (-0.2) 0) vertex $ (Vertex3 ((-0.2)::GLfloat) 0 0) color $ (Color3 (1::GLfloat) 0 1) vertex $ (Vertex3 (0::GLfloat) 0 0) vertex $ (Vertex3 (0::GLfloat) 0.2 0) vertex $ (Vertex3 ((-0.2::GLfloat)) 0.2 0) vertex $ (Vertex3 ((-0.2::GLfloat)) 0 0) flush
in order to produce these four coloured squares:
where each color command sets the color for the next item drawn, and the vertex commands give vertices for the four squares.
Callbacks - how we react to changes
We have already seen one callback in action:
displayCallback. The Callbacks are state variables of the HOpenGL system, and are called in order to handle various things that may happen to the place the drawing canvas lives. For a first exercise, go resize the latest window you've used. Go on, do it now.
I bet it looked ugly, didn't it?
This is because we have no code handling what to do if the window should suddenly change. Handling this is done in a callback, residing in the
IORef reshapeCallback. Similarly, repainting is done in
displayCallback, keyboard and mouse input is in
keyboardMouseCallback, and so on. We can refer to the HOpenGL documentation for window callbacks and for global callbacks. Window callbacks are things like display, keyboard and mouse, and reshape. Global callbacks deal with timing issues (for those snazzy animations) and the menu interface systems.
In order for a callback to possibly not be defined, most are typed within the
Maybe monad, so by setting the state variable to
Nothing, a callback can be disabled. Thus, setting callbacks is done using the keyword
Just. We'll add a callback for reshaping the window to our neat code, changing the main function to:
main = do (progname, _) <- getArgsAndInitialize createWindow "Hello World" displayCallback $= display reshapeCallback $= Just reshape mainLoop reshape s@(Size w h) = do viewport $= (Position 0 0, s) postRedisplay Nothing
Here, the code for the reshape function resizes the viewport so that our drawing area contains the entire new window. After setting the new viewport, it also tells the windowing system that something has happened to the window, and that therefore, the display function should be called.
So, in conclusion, so far we can display a window, post basic callbacks to get the windowhandling to run smoothly, and draw in our window. Next installment of the tutorial will bring you 3d drawing, keyboard and mouse interactions, the incredible power of matrices and the ability to rotate 3d objects for your leisure. Possibly, we'll even look into animations.