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Revision as of 04:40, 10 July 2009 by Roconnor (talk | contribs) (Getting Colour Components)
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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 the 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.

Color 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 safe "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. Unless you specifically know you want to be adding colours, 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 Double<hask>s (or whatever your internal representation is) use <hask>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 created a 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.

Back to colour coordinates

Recall that

toSRGB chartreuse

produces an RGB Double<hask>. The coordinates output by <hask>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 be clamped to either to the range 0 to 255.

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