graphics-drawingcombinators-1.4.0: A functional interface to 2D drawing in OpenGL

Portabilitytested on GHC only
MaintainerLuke Palmer <>




Drawing combinators as a functional interface to 2D graphics using OpenGL.

This module is intended to be imported qualified, as in:

 import qualified Graphics.DrawingCombinators as Draw

Whenever possible, a denotational semantics for operations in this library is given. Read [[x]] as "the meaning of x".

Intuitively, an Image a is an infinite plane of pairs of colors and a's. The colors are what are drawn on the screen when you render, and the a's are what you can recover from coordinates using sample. The latter allows you to tell, for example, what a user clicked on.

The following discussion is about the associated data. If you are only interested in drawing, rather than mapping from coordinates to values, you can ignore the following and just use mappend and mconcat to overlay images.

Wrangling the a's -- the associated data with each "pixel" -- is done using the Functor, Applicative, and Monoid instances.

The primitive Images such as circle and text all return Image Any objects. Any is just a wrapper around Bool, with (||) as its monoid operator. So e.g. the points inside the circle will have the value Any True, and those outside will have the value Any False. Returning Any instead of plain Bool allows you to use Images as a monoid, e.g. mappend to overlay two images. But if you are doing anything with sampling, you probably want to map this to something. Here is a drawing with two circles that reports which one was hit:

 twoCircles :: Image String
 twoCircles = liftA2 test (translate (-1,0) %% circle) (translate (1,0) %% circle)
   test (Any False) (Any False) = "Miss!"
   test (Any False) (Any True)  = "Hit Right!"
   test (Any True)  (Any False) = "Hit Left!"
   test (Any True)  (Any True)  = "Hit Both??!"

The last case would only be possible if the circles were overlapping.

Note, the area-less shapes such as point, line, and bezierCurve always return Any False when sampled, even if the exact same coordinates are given. This is because miniscule floating-point error can make these shapes very brittle under transformations. If you need a point to be clickable, make it, for example, a very small box.



Basic types

data Image a Source

The type of images.

 [[Image a]] = R2 -> (Color, a)

The semantics of the instances are all consistent with type class morphism. I.e. Functor, Applicative, and Monoid act point-wise, using the Color monoid described below.

render :: Image a -> IO ()Source

Draw an Image on the screen in the current OpenGL coordinate system (which, in absense of information, is (-1,-1) in the lower left and (1,1) in the upper right).

clearRender :: Image a -> IO ()Source

Like render, but clears the screen first. This is so you can use this module and pretend that OpenGL doesn't exist at all.


sample :: Image a -> R2 -> aSource

Sample the value of the image at a point.

 [[sample i p]] = snd ([[i]] p)


point :: R2 -> Image AnySource

A single "pixel" at the specified point.

 [[point p]] r | [[r]] == [[p]] = (one, Any True) 
               | otherwise      = (zero, Any False)

line :: R2 -> R2 -> Image AnySource

A line connecting the two given points.

regularPoly :: Int -> Image AnySource

A regular polygon centered at the origin with n sides.

circle :: Image AnySource

An (imperfect) unit circle centered at the origin. Implemented as:

 circle = regularPoly 24

convexPoly :: [R2] -> Image AnySource

A convex polygon given by the list of points.

(%%) :: Affine -> Image a -> Image aSource

Transform an image by an Affine transformation.

 [[tr % im]] = [[im]] . inverse [[tr]]

bezierCurve :: [R2] -> Image AnySource

A Bezier curve given a list of control points. It is a curve that begins at the first point in the list, ends at the last one, and smoothly interpolates between the rest. It is the empty image (mempty) if zero or one points are given.


data Color Source

Color is defined in the usual computer graphics sense: a 4 vector containing red, green, blue, and alpha.

The Monoid instance is given by alpha composition, described at

In the semantcs the values zero and one are used, which are defined as:

 zero = Color 0 0 0 0
 one = Color 1 1 1 1


Color !R !R !R !R 


modulate :: Color -> Color -> ColorSource

Modulate two colors by each other.

 modulate (Color r g b a) (Color r' g' b' a') 
           = Color (r*r') (g*g') (b*b') (a*a')

tint :: Color -> Image a -> Image aSource

Tint an image by a color; i.e. modulate the colors of an image by a color.

 [[tint c im]] = first (modulate c) . [[im]]
    where first f (x,y) = (f x, y)

Sprites (images from files)

data Sprite Source

A Sprite represents a finite bitmap image.

 [[Sprite]] = [-1,1]^2 -> Color

openSprite :: FilePath -> IO SpriteSource

Load an image from a file and create a sprite out of it.

sprite :: Sprite -> Image AnySource

The image of a sprite at the origin.

 [[sprite s]] p | p `elem` [-1,1]^2 = ([[s]] p, Any True) 
                | otherwise         = (zero, Any False)


openFont :: String -> IO FontSource

Load a TTF font from a file.

text :: Font -> String -> Image AnySource

The image representing some text rendered with a font. The baseline is at y=0, the text starts at x=0, and the height of a lowercase x is 1 unit.

textWidth :: Font -> String -> RSource

textWidth font str is the width of the text in text font str.


unsafeOpenGLImage :: (Color -> IO ()) -> (R2 -> a) -> Image aSource

Import an OpenGL action and pure sampler function into an Image. This ought to be a well-behaved, compositional action (make sure it responds to different initial ModelViews, don't change matrix modes or render or anything like that). The color given to the action is the current tint color; modulate all your colors by this before setting them.

class Monoid a where

The class of monoids (types with an associative binary operation that has an identity). Instances should satisfy the following laws:

  • mappend mempty x = x
  • mappend x mempty = x
  • mappend x (mappend y z) = mappend (mappend x y) z
  • mconcat = foldr mappend mempty

The method names refer to the monoid of lists under concatenation, but there are many other instances.

Minimal complete definition: mempty and mappend.

Some types can be viewed as a monoid in more than one way, e.g. both addition and multiplication on numbers. In such cases we often define newtypes and make those instances of Monoid, e.g. Sum and Product.


mempty :: a

Identity of mappend

mappend :: a -> a -> a

An associative operation

mconcat :: [a] -> a

Fold a list using the monoid. For most types, the default definition for mconcat will be used, but the function is included in the class definition so that an optimized version can be provided for specific types.


Monoid Ordering 
Monoid () 
Monoid All 
Monoid Any 
Monoid ByteString 
Monoid Affine 
Monoid Color 
Monoid [a] 
Monoid a => Monoid (Dual a) 
Monoid (Endo a) 
Num a => Monoid (Sum a) 
Num a => Monoid (Product a) 
Monoid (First a) 
Monoid (Last a) 
Monoid a => Monoid (Maybe a) 
Monoid m => Monoid (Image m) 
Monoid b => Monoid (a -> b) 
(Monoid a, Monoid b) => Monoid (a, b) 
(Monoid a, Monoid b, Monoid c) => Monoid (a, b, c) 
(Monoid a, Monoid b, Monoid c, Monoid d) => Monoid (a, b, c, d) 
(Monoid a, Monoid b, Monoid c, Monoid d, Monoid e) => Monoid (a, b, c, d, e) 

newtype Any

Boolean monoid under disjunction.




getAny :: Bool