[cairo] Colour Objects In Qahirah

[Bill Spitzak]

It's odd that colorsys returns vectors but takes 3 numbers as arguments, 
possibly there is an alternative version that takes vectors.

The math really is not that hard, you might be better off making your 
own Colour object that includes all the math rather than depending on 
colorsys, since your version could directly read and create Colour 
objects rather than make intermediate vectors.

On 05/10/2015 11:32 PM, Lawrence D'Oliveiro wrote:
> Qahirah <https://github.com/ldo/qahirah> is a high-level language
> binding for Cairo. By “high-level” I mean it is not restricted to the
> facilities available to someone directly calling Cairo from C; instead,
> it tries to provide conveniences that a Python programmer would find
> useful.
>
> For example, Cairo specifies colours strictly as (r, g, b) or (r, g, b,
> a) components. But Python provides a module called “colorsys”
> <https://docs.python.org/3/library/colorsys.html>, that also allows
> colours to be specified in HSL, HLS and YIQ spaces. So it makes sense
> to take advantage of this.
>
> Thus, the Qahirah “Colour” object allows instances to be constructed
> out of components specified in any of these spaces, and components in
> any of these spaces to be extracted from instances. Of course, the
> underlying representation is always RGBA, and this is what is passed to
> Cairo and retrieved from Cairo.
>
> It is also useful to perform various transformations on colours, like
> mixing two colours, darkening/brightening a colour, determining a
> complementary colour and so on. Thus, I provide “combine” and “mix”
> methods, as well as “replace” methods for selectively replacing
> components in your choice of colour space.
>
> For example, start with some arbitrary colour:
>
>      >>> c = Colour.from_hsva((.3, 1, .5))
>      >>> c
>      Colour(0.10000000000000009, 0.5, 0.0, 1)
>
> The default representation shows the RGBA components, but we can show
> the HSVA components:
>
>      >>> c.to_hsva()
>      hsva(h=0.3, s=1.0, v=0.5, a=1)
>
> Let us create a darker version of the same colour, working in HLS space:
>
>      >>> c.replace_hlsa(l = c.to_hlsa().l / 2)
>      Colour(0.04999999999999999, 0.25, 0.0, 1)
>      >>> c.replace_hlsa(l = c.to_hlsa().l / 2).to_hsva()
>      hsva(h=0.3, s=1.0, v=0.25, a=1)
>
> Or a lighter version, again in HLS space:
>
>      >>> c.replace_hlsa(l = 1 - (1 - c.to_hlsa().l) / 2)
>      Colour(0.39999999999999997, 1.0, 0.25, 1)
>      >>> c.replace_hlsa(l = 1 - (1 - c.to_hlsa().l) / 2).to_hsva()
>      hsva(h=0.30000000000000004, s=0.75, v=1.0, a=1)
>
> See how the saturation is reduced in HLS, to move the colour closer to
> white.
>
> Or how about working out the complementary colour, with the same
> saturation and value, but 180° around the colour wheel:
>
>      >>> c.replace_hsva(h = c.to_hsva().h + 0.5)
>      Colour(0.40000000000000036, 0.0, 0.5, 1)
>      >>> c.replace_hsva(h = c.to_hsva().h + 0.5).to_hsva()
>      hsva(h=0.8000000000000002, s=1.0, v=0.5, a=1)
>