# Contrast Checker (Gorilla Sun)

> Checks the contrast between two CSS colours against WCAG 2.2 (SC 1.4.3, 1.4.6, 1.4.11), reports APCA Lc (advisory, WCAG 3 draft), simulates colour vision deficiencies, and suggests the nearest passing colour. Free, no key, CORS open.

Tool for people: https://gorillasun.com/other-pages/contrast-checker?fg=%23141210&bg=%23e3ddd4
The page reads ?fg, ?bg, ?size, ?weight, ?target and ?palette from its URL and renders the result on the server, so its HTML already holds the answer (also as JSON in <script type="application/json" id="contrast-result">).

## API

GET https://gorillasun.com/api/contrast?fg=<colour>&bg=<colour>
GET https://gorillasun.com/api/contrast?palette=ink:%23141210,paper:%23e3ddd4,accent:%23d45800
POST https://gorillasun.com/api/contrast with JSON: {"foreground":"…","background":"…"} | {"pairs":[{"fg":"…","bg":"…"}]} | {"palette":{"name":"colour"}}
GET https://gorillasun.com/api/contrast without parameters returns this usage as JSON. OpenAPI: https://gorillasun.com/api/contrast/openapi.json

Parameters:
- fg (or foreground): Text or foreground colour. Any CSS colour: hex, rgb(), hsl(), hwb(), lab(), lch(), oklab(), oklch(), color(), a name. Alpha is allowed.
- bg (or background): Background colour. Any CSS colour.
- base: What a translucent background is painted over. Default white.
- size: Font size in CSS px. With weight, adds a verdict for that text.
- weight: Font weight, 100 to 900.
- target: Ratio the suggested fixes aim for: AA (4.5), AAA (7), large or ui (3), or a number. Default 4.5.
- palette: Instead of fg and bg: every ordered pair of these colours. "name:colour" items, comma-separated. At most 40.
- format: json (default) or text, a one-line summary.

URL-encode # as %23, or leave it off (fg=141210 works).

## Reading the answer

- wcag.ratio is truncated to two decimals, never rounded up; pass/fail is decided on wcag.exactRatio.
- wcag.normalText.AA.pass: body text (4.5:1). wcag.largeText.AA.pass: 24px+, or 18.66px+ bold (3:1). wcag.nonText.AA.pass: icons, borders, focus rings (3:1).
- apca.Lc is signed: positive for dark on light, negative for light on dark. Compare |Lc| with apca.level.
- text (when size or weight is given): the verdict for that exact text, by WCAG and by APCA's Bronze size bands.
- suggestions.foreground.nearest / suggestions.background.nearest: the smallest OKLCH lightness change, hue kept, that reaches suggestions.target. null when none can.
- rendered: both colours as painted, after transparency is flattened. The scores are taken on these.
- vision: the pair under protanopia, deuteranopia, tritanopia and achromatopsia (Machado 2009).

Example: curl "https://gorillasun.com/api/contrast?fg=%23d45800&bg=%23e3ddd4&size=16&weight=400&format=text"