contrastRatio method
The contrast ratio against a background color, as specified by https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef.
Return value will be in the range 1, 21
.
The background must be opaque.
Implementation
num contrastRatio(Color background) {
if (background.alpha != 1) {
throw ArgumentError.value(background, 'background',
'Cannot calculate contrast against non-opaque backgrounds.');
}
final Color a = alpha == 1 ? this : withBackground(background);
final la = a._relativeLuminance;
final lb = background._relativeLuminance;
return (math.max(la, lb) + 0.05) / (math.min(la, lb) + 0.05);
}