[contrast] Add contrast methods

[svgeesus]

Done:

• Add Weber, Michelson, Lstar and APCA contrasts in new /src/contrast folder
• Add and rename contrast to contrastWCAG21, in src/contrast

ToDo:

• more testing
• write tests page to check expected values
• generic "contrast" method that takes a "method" parameter and dispatches to one of these
• default value for "method"
• Contrast documentation

[svgeesus]

@LeaVerou any thoughts so far?

[svgeesus]

I added documentation. Currently using the procedural API as the functions have not been object-oriented

[svgeesus]

Checking for Y=0 is certainly worthwhile as this is likely to be common. These changes would affect Weber and Michelson

Negative Y is unlikely but possible and, as you said, clamping to 0 seems best. That would affect anything luminance-based.

[svgeesus]

So I have

export default function contrastAPCA(foreground, background) {

and then

export function register(Color) {
	Color.defineFunction("contrastAPCA", contrastAPCA);
}

but what I really want is to end up with

Color.contrastAPCAText(background);
Color.contrastAPCABackground(foreground);

which will produce

 contrastAPCA(this, background);
 contrastAPCA(foreground, this);

respectively.

[LeaVerou]

I assume you mean color.contrastAPCAText(background), i.e. instance methods.

I don't think this is a good design. Long method names are hard to read, and if I see color1.contrastAPCAText(color2) I'm still unsure what's the foreground and what's the background.

To borrow from the proposed keywords for color-contrast(), perhaps color1.contrastOver(color2) and color1.contrastUnder(). Or even a single method, that accepts an object literal:

color1.contrast({foreground: color2}), color1.contrast({background: color2})
color1.contrast({over: color2}), color1.contrast({under: color2})

[svgeesus]

To borrow from the proposed keywords for color-contrast(), perhaps color1.contrastOver(color2) and color1.contrastUnder().

It seems pretty important at this stage to include at minimum APCA in the method name as well (and may be too soon to call it that, rather than APCA-0.0.98G)

[svgeesus]

Added tests for the APCA function, based on eight known-good results from the apca-w3 test. Seven are passed (to all significant figures) one is not, and I believe his test is incorrect

[svgeesus]

Eighth APCA test confirmed wrong, corrected

[LeaVerou]

LGTM. We need to add a color.contrast(color, methodname) method though. Could be done in a separate PR.