Lightening a UIColor

Question

I ran into a situation yesterday where I needed to lighten a UIColor so I created a category and added a lighten method. I thought it would be straight forward to just multiply the value by each component of the color but my greens started turning yellow so I knew it had to be more complicated.

The solution I came up with was to convert from sRGB to Linear, multiply the color, and then convert back. This seems to work but I'm not sure if it's "correct". I couldn't find anything in the docs that stated that UIColor was in sRGB space. I'm also no color scientist so I only have a rudimentary knowledge of the math involved.

Anyway here is my code, I'm asking for some peer review and to see if anyone has a better understanding of modifying UIColors.

CGFloat sRGB2Linear(CGFloat x){
    CGFloat a = 0.055;
    if(x <= 0.04045){
        return x * (1.0 / 12.92);
    }else{
        return pow((x + a) * (1.0 / (1 + a)), 2.4);
    }
}

CGFloat linear2sRGB(CGFloat x){
    CGFloat a = 0.055;
    if(x <= 0.0031308){
        return x * 12.92;
    }else{
        return (1 + a) * pow(x, 1 / 2.4) - a;
    }
}

- (UIColor *)lighten:(CGFloat)value{
    const CGFloat *components = CGColorGetComponents([self CGColor]);
    CGFloat newR = (sRGB2Linear(components[0])+1)*value;
    CGFloat newG = (sRGB2Linear(components[1])+1)*value;
    CGFloat newB = (sRGB2Linear(components[2])+1)*value;
    newR = MAX(0, MIN(1, linear2sRGB(newR)));
    newG = MAX(0, MIN(1, linear2sRGB(newG)));
    newB = MAX(0, MIN(1, linear2sRGB(newB)));
    return [UIColor colorWithRed:newR green:newG blue:newB alpha:components[3]];
}

Answer 1

This is my iOS5+ solution to make relative brightness changes easily (lighten and darken all-in-one):

+ (UIColor*)changeBrightness:(UIColor*)color amount:(CGFloat)amount
{

    CGFloat hue, saturation, brightness, alpha;
    if ([color getHue:&hue saturation:&saturation brightness:&brightness alpha:&alpha]) {
        brightness += (amount-1.0);
        brightness = MAX(MIN(brightness, 1.0), 0.0);
        return [UIColor colorWithHue:hue saturation:saturation brightness:brightness alpha:alpha];
    }

    CGFloat white;
    if ([color getWhite:&white alpha:&alpha]) {
        white += (amount-1.0);
        white = MAX(MIN(white, 1.0), 0.0);
        return [UIColor colorWithWhite:white alpha:alpha];
    }

    return nil;
}

It's invoked like so:

[self changeBrightness:someUiColor amount:1.1]

Using 1.1 it will increase the brightness by 10%; 0.9 will decrease the brightness by 10%. Note that the 10% is relative to pure white (i.e., 10% is always a .1 increase in brightness.) This is the expected behavior if you want a percentage increase to lighten or darken the colors consistently regardless of their initial brightness.

Answer 2

You can work with hue, saturation, and brightness.

Get the old values using

- (BOOL)getHue:(CGFloat *)hue saturation:(CGFloat *)saturation brightness:(CGFloat *)brightness alpha:(CGFloat *)alpha

then adjust the brightness, and construct a new color:

- (UIColor *)initWithHue:(CGFloat)hue saturation:(CGFloat)saturation brightness:(CGFloat)brightness alpha:(CGFloat)alpha

Keep in mind that you need to pass pointers to the first function.