Datatypes for representing the human perception of colour. Includes common operations for blending and compositing colours. The most common way of creating colours is either by name (see Data.Colour.Names) or by giving an sRGB triple (see Data.Colour.SRGB).
Methods of specifying Colours can be found in
Colours can be specified in a generic Data.Colour.RGBSpace.RGBSpace
by using
 data Colour a
 colourConvert :: (Fractional b, Real a) => Colour a > Colour b
 black :: Num a => Colour a
 data AlphaColour a
 opaque :: Num a => Colour a > AlphaColour a
 withOpacity :: Num a => Colour a > a > AlphaColour a
 transparent :: Num a => AlphaColour a
 alphaColourConvert :: (Fractional b, Real a) => AlphaColour a > AlphaColour b
 alphaChannel :: AlphaColour a > a
 class AffineSpace f where
 affineCombo :: Num a => [(a, f a)] > f a > f a
 blend :: (Num a, AffineSpace f) => a > f a > f a > f a
 class ColourOps f where
 over :: Num a => AlphaColour a > f a > f a
 darken :: Num a => a > f a > f a
 dissolve :: Num a => a > AlphaColour a > AlphaColour a
 atop :: Fractional a => AlphaColour a > AlphaColour a > AlphaColour a
Interfacing with Other Libraries' Colour Spaces
Executive summary: Always use Data.Colour.SRGB when interfacing with
other libraries.
Use toSRGB24
/ sRGB24
when
interfacing with libraries wanting Data.Word.Word8
per channel.
Use toSRGB
/ sRGB
when
interfacing with libraries wanting Double
or Float
per channel.
Interfacing with the colour for other libraries, such as cairo (http://www.haskell.org/gtk2hs/archives/category/cairo/) and OpenGL (http://hackage.haskell.org/cgibin/hackagescripts/package/OpenGL), can be a challenge because these libraries often do not use colour spaces in a consistent way. The problem is that these libraries work in a device dependent colour space and give no indication what the colour space is. For most devices this colours space is implicitly the nonlinear sRGB space. However, to make matters worse, these libraries also do their compositing and blending in the device colour space. Blending and compositing ought to be done in a linear colour space, but since the device space is typically nonlinear sRGB, these libraries typically produce colour blends that are too dark.
(Note that Data.Colour is a device independent colour space, and
produces correct blends.
e.g. compare toSRGB (blend 0.5 lime red)
with RGB 0.5 0.5 0
)
Because these other colour libraries can only blend in device colour spaces, they are fundamentally broken and there is no "right" way to interface with them. For most libraries, the best one can do is assume they are working with an sRGB colour space and doing incorrect blends. In these cases use Data.Colour.SRGB to convert to and from the colour coordinates. This is the best advice for interfacing with cairo.
When using OpenGL, the choice is less clear. Again, OpenGL usually does blending in the device colour space. However, because blending is an important part of proper shading, one may want to consider that OpenGL is working in a linear colour space, and the resulting rasters are improperly displayed. This is born out by the fact that OpenGL extensions that support sRGB do so by converting sRGB input/output to linear colour coordinates for processing by OpenGL.
The best way to use OpenGL, is to use proper sRGB surfaces for textures and rendering. These surfaces will automatically convert to and from OpenGL's linear colour space. In this case, use Data.Colour.SRGB.Linear to interface OpenGL's linear colour space.
If not using proper surfaces with OpenGL, then you have a choice between having OpenGL do improper blending or improper display If you are using OpenGL for 3D shading, I recommend using Data.Colour.SRGB.Linear (thus choosing improper OpenGL display). If you are not using OpenGL for 3D shading, I recommend using Data.Colour.SRGB (thus choosing improper OpenGL blending).
Colour type
colourConvert :: (Fractional b, Real a) => Colour a > Colour bSource
Change the type used to represent the colour coordinates.
data AlphaColour a Source
This type represents a Colour
that may be semitransparent.
The Monoid
instance allows you to composite colours.
x `mappend` y == x `over` y
To get the (premultiplied) colour channel of an AlphaColour
c
,
simply composite c
over black.
c `over` black
ColourOps AlphaColour  
AffineSpace AlphaColour  
Eq a => Eq (AlphaColour a)  
(Fractional a, Read a) => Read (AlphaColour a)  
(Fractional a, Show a) => Show (AlphaColour a)  
Num a => Monoid (AlphaColour a) 

opaque :: Num a => Colour a > AlphaColour aSource
Creates an opaque AlphaColour
from a Colour
.
withOpacity :: Num a => Colour a > a > AlphaColour aSource
Creates an AlphaColour
from a Colour
with a given opacity.
c `withOpacity` o == dissolve o (opaque c)
transparent :: Num a => AlphaColour aSource
This AlphaColour
is entirely transparent and has no associated
colour channel.
alphaColourConvert :: (Fractional b, Real a) => AlphaColour a > AlphaColour bSource
Change the type used to represent the colour coordinates.
alphaChannel :: AlphaColour a > aSource
Returns the opacity of an AlphaColour
.
Colour operations
These operations allow combine and modify existing colours
class AffineSpace f whereSource
affineCombo :: Num a => [(a, f a)] > f a > f aSource
Compute a affine Combination (weightedaverage) of points. The last parameter will get the remaining weight. e.g.
affineCombo [(0.2,a), (0.3,b)] c == 0.2*a + 0.3*b + 0.5*c
Weights can be negative, or greater than 1.0; however, be aware that nonconvex combinations may lead to out of gamut colours.
blend :: (Num a, AffineSpace f) => a > f a > f a > f aSource
Compute the weighted average of two points. e.g.
blend 0.4 a b = 0.4*a + 0.6*b
The weight can be negative, or greater than 1.0; however, be aware that nonconvex combinations may lead to out of gamut colours.
over :: Num a => AlphaColour a > f a > f aSource
c1 `over` c2
returns the Colour
created by compositing the
AlphaColour
c1
over c2
, which may be either a Colour
or
AlphaColour
.
darken :: Num a => a > f a > f aSource
darken s c
blends a colour with black without changing it's opacity.
For Colour
, darken s c = blend s c mempty
dissolve :: Num a => a > AlphaColour a > AlphaColour aSource
Returns an AlphaColour
more transparent by a factor of o
.
atop :: Fractional a => AlphaColour a > AlphaColour a > AlphaColour aSource
c1 `atop` c2
returns the AlphaColour
produced by covering
the portion of c2
visible by c1
.
The resulting alpha channel is always the same as the alpha channel
of c2
.
c1 `atop` (opaque c2) == c1 `over` (opaque c2) AlphaChannel (c1 `atop` c2) == AlphaChannel c2