// Contrast ratios can range from 1 to 21 (commonly written 1:1 to 21:1).
public static double Contrast(this IColorType thisA, IColorType color)
{
// Formula: http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
var alpha = thisA.Alpha0To1;
if (alpha >= 1)
{
if (color.Alpha0To1 < 1)
{
color = color.OverlayOn(thisA);
}
var l1 = thisA.Luminance0To1() + .05;
var l2 = color.Luminance0To1() + .05;
var ratio = l1 / l2;
if (l2 > l1)
{
ratio = 1 / ratio;
}
ratio = Math.Round(ratio, 1);
return(ratio);
}
// If we’re here, it means we have a semi-transparent background
// The text color may or may not be semi-transparent, but that doesn't matter
var onBlack = thisA.OverlayOn(Color.Black);
var onWhite = thisA.OverlayOn(Color.White);
var contrastOnBlack = onBlack.Contrast(color);
var contrastOnWhite = onWhite.Contrast(color);
var max = Math.Max(contrastOnBlack, contrastOnWhite);
var min = 1.0;
if (onBlack.Luminance0To1() > color.Luminance0To1())
{
min = contrastOnBlack;
}
else if (onWhite.Luminance0To1() < color.Luminance0To1())
{
min = contrastOnWhite;
}
var error = Math.Round((max - min) / 2, 2);
var farthest = contrastOnWhite == max ? Color.White : Color.Black;
return(Math.Round((min + max) / 2, 2));
}
public static IColorType WithContrast(this IColorType colorToAdjust, IColorType fixedColor, double minimumRequiredContrast = 3)
{
var contrast = colorToAdjust.Contrast(fixedColor);
int tries = 0;
while (contrast < minimumRequiredContrast &&
tries++ < 30)
{
if (fixedColor.Luminance0To1() < .5)
{
colorToAdjust = colorToAdjust.WithLightnessAdjustment(1.05).ToColor();
}
else
{
colorToAdjust = colorToAdjust.WithLightnessAdjustment(.95).ToColor();
}
contrast = colorToAdjust.Contrast(fixedColor);
}
return(colorToAdjust);
}
