rewrite colour support

[nebulon42]

I found a HuSL implementation in your code, but it is currently not accessible from CartoCSS. Additionally, HSL colour definitions are not supported yet.
Maybe it would be beneficial to rewrite colour support to some generic colour type, which is converted to RGB(A) at the end. Compare mapbox/carto#422, where I defined colours internally as HSLA including a flag if we are in perceptual space or not.

I tried to implement HSLA support, but got stuck at

magnacarto/builder/mapnik/serialize.go

Line 580 in 2537d1e

func fmtColor(v color.RGBA, ok bool) *string {

. My Go knowledge is not good enough to figure out how to input some more generic type to this function and do the necessary adjustments within. Maybe you have a hint.

Another thing: it might be better to move the HuSL implementation to some external library.

[nebulon42]

Currently working on a HUSL port to Go.

[olt]

Currently it's only used by SetHue which can be called from CartoCSS with -mc-set-hue.

[nebulon42]

My Go implementation of HUSL can be found here: https://github.com/husl-colors/husl-go

[nebulon42]

@olt What is the use case of this custom expression (-mc-set-hue)?

[olt]

It sets the hue of A to the hue of B. So that you get a new color with the same saturation/lightness of A, but the hue of B.

[nebulon42]

Solved by merging #15.