How can I change the saturation of an UIImage?

Question

I have an UIImage and want to shift it\'s saturation about +10%. Are there standard methods or functions that can be used for this?

Answer 1

Nothing quite that straightforward. The easiest solution is probably to make an in-memory CGContext with a known pixel format, draw the image into that context, then read/modify the pixels in the known-format buffer.

I don't think CG supports a color space with a separate saturation channel, so you'll have to either convert from RGB to HSV or HSL, or do the calculations directly in the RGB space.

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One way to do the calculation directly in RGB might be something like this:

average = (R + G + B) / 3;
red_delta = (R - average) * 11 / 10;
green_delta = (G - average) * 11 / 10;
blue_delta = (B - average) * 11 / 10;

R = average + red_delta;
G = average + green_delta;
B = average + blue_delta;
// clip R,G,B to be in 0-255 range

This will move the channels that are away from the mean about 10% further away. This is almost like increasing the saturation, though it'll give hue shifts for some colors.