Colour

This page provides a short introduction to using the colour package on hackage.

The Colour data type

The Colour a data type and its basic operations are found in the Data.Colour module. The type variable a is used to specify the numeric type used for the internal representation of the data. Typically one will use:

Colour Double

You may wish to make a type synonym for this type in your program if you will use it everywhere.

You can always use colourConvert to change to a different internal representation type.

Creating colours

A collections of colours given by name can be found in the Data.Colour.Names module. There is also a readColourName to convert a string with one of these names into a colour. Be aware that the colour tan will conflict with the Prelude function unless you hide the Prelude function or import the module qualified.

Another way to make a colour is by specifying an RGB triple. These functions can be found in the Data.Colour.SRGB library. For example, if you have three Word8s named red, green, and blue, then

sRGB24 red green blue

will create the colour with those sRGB colour coordinates.

If you have three Doubles (or whatever you are using for your internal representation) named red, green, and blue, then

sRGB red green blue

will produce the colour with those colour coordinates. These Double should be in the range [0,1] otherwise the resulting colour would be out of gamut (a colour gamut is a collection of representable colours on a device, such as your monitor).

Lastly, sRGB24read and sRGB24reads can create colour from string specifications of the form "#00aaff" or "00aaff".

Manipulating Colours

The colour operations are found in the Data.Colour module. The most common operation on colours is blend. For example, the function

blend 0.25 red green

will create a new colour that is 25% red, and 75% green. The weight parameter (the first parameter) should be between 0 and 1, otherwise an out-of-gamut colour could result.

If you need to blend more than two colours, you can use multiple applications of blend, or you can use affineCombo. For example,

affineCombo [(0.25,red),(0.5,green)] violet

will create a new colour that is 25% red, 50% green, and 25% violet. Again the weights should all be non-negative and the sum of the weights should be no more than 1, otherwise an out-of-gamut colour could result.

Colour intensity can be changed by using darken. For example,

darken 0.4 turquoise

will produce a turquoise that is only 40% of the intensity of normal turquoise. The weight parameter (the first parameter) should be between 0 and 1, otherwise an out-of-gamut colour could result. However if you know that the intensity is low enough, you may safely "darken" by values greater than 1 (which will actually lighten the colour).

Lastly, colours are instance of a Monoid so colours can be "added" by using mappend (and mempty is a quick way to get black). However, like spotlights, adding colours makes more intense colours. Adding colours could take you out of gamut by making colours too intense. Unless you specifically know you want to be adding colours (for example, when writing a ray-tracer), you probably want to be using blend instead.

Getting colour coordinates out

To retrieve the sRGB coordinates of a colour, use the functions found in the Data.Colour.SRGB module. To get coordinates as Doubles (or whatever your internal representation is) use toSRGB. For example

toSRGB chartreuse

will produce a value of type RGB Double.

RGB triples

The type RGB is special type of (strict) triple used to store colour coordinates. The functions channelRed, channelGreen, and channelBlue can be used to access the three fields. The constructor RGB will create such a triple. For example,

RGB 0.5 0.4 0.6

You might find the functions

curryRGB :: (RGB a -> b) -> a -> a -> a -> b
uncurryRGB :: (a -> a -> a -> b) -> RGB a -> b

useful when working with functions that operate on RGB triples. These functions can be found in Data.Colour.RGBSpace.

Back to colour coordinates

Recall that

toSRGB chartreuse

produces an RGB Double. The coordinates output by toSRGB will all be between 0 and 1 unless the colour is out of gamut.

If you want to retrieve the colour coordinates as Word8s, use toSRGB24

toSRGB24 khaki

will produce an RGB Word8. Out-of-gamut channels will be clamped to the range 0 to 255.

Lastly, the functions sRGB24show and sRGB24shows will produce colour strings of the form "#00aaff".

Transparent Colour

Colours that are semi transparent are represented by the AlphaColour a type found in Data.Colour. Again the a type parameter represents the data type used for the internal representation and would typically be Double. You can use alphaColourConvert to change the internal representation type of a semi-transparent colour.

Opaque AlphaColours are created from Colours using opaque. For example.

opaque goldenrod

creates an opaque goldenrod. Semi transparent colours can be made using withOpacity

moccasin `withOpacity` 0.7

creates a colour that is 70% opaque and hence 30% transparent.

The value transparent is 100% transparent and transparent == anyColour `withOpacity` 0.

Like regular colours, semi-transparent colours can be blended using blend and affineCombo. The function darken will darken a semi-transparent colour without affecting its opacity.

To make an existing semi-transparent colour more transparent use dissolve. For example,

dissolve 0.6 ac

will return a semi-transparent colour that is 60% of the opacity of ac.

One should avoid dissolving with weights (the first parameter) greater than 1, as you may create invalid "super-opaque" colours. If you know the opacity is less than x then you can safely use weights no more than (recip x). Negative weights will also produce invalid "super-transparent" colours.

anyColour `withOpacity` opacity == disolve opacity (opaque anyColour)

Lastly, the key operation on transparent colours is compositing. Given two semitransparent colours acTop and acBottom

acTop `over` acBottom

will produce the semi-transparent colour resulting from acTop being composited over top of acBottom. The bottom layer, acBottom can be a non-transparent colour (of type Colour). In this case the result will also be a non-transparent colour. However, the top layer must be of semi-transparent type (although it could, of course, be opaque).

Compositing is such important operation on semi-transparent colours, that it is the Monoid instance for AlphaColour a. The function mappend is over, and mempty is transparent.

Getting semi-transparent coordinates

The opacity of a semi-transparent colour can be retrieved by the alphaChannel function.

The pure colour of a semi-transparent colour ac can be retrieved by first compositing the colour over black, then darkening by the reciprocal of the alpha channel.

pureColour :: AlphaColour a -> Colour a
pureColour ac | a > 0 = darken (recip a) (ac `over` black)
              | otherwise = error "transparent has no pure colour"
 where
  a = alphaChannel ac

Note however, that transparent has no pure colour, and this case needs to be handled specially.

This operation is not natively provided because it is an operation that should be avoided. It is only really useful for interfacing with libraries that require pure colour components. Ideally it would be these libraries that implement conversion to and from Colour. However, you may find it necessary to implement the conversion functions yourself, in which case you can use the above "trick" to write the conversion function.