How to convert colors from one color space to another?

Question

Is there a Cocoa Touch way to convert colors from one color space to another?

At the end of this code:

UIColor *grey = [UIColor colorWithWhite: 0.5 alpha: 1.0];
CGColorRef greyRef = [grey CGColor];
int x = CGColorGetNumberOfComponents(greyRef);

...x is 2.

The reason I need this is I'm trying to copy colors to a list of color components for CGGradientCreateWithColorComponents, which needs all colors in a single colorspace. The problem is that grey is in the grayscale colorspace, rather than the RGB one. (The name escapes me at the moment, but it isn't important.)

CGGradientRef createGradient(NSInteger inCount, NSArray* inColors, CGFloat* inLocations) {
 CGColorSpaceRef theColorspace = CGColorSpaceCreateDeviceRGB( );
 size_t numberOfComponents = 4;
 NSInteger colorSize = numberOfComponents * sizeof( CGFloat );
 CGFloat *theComponents = malloc( inCount * colorSize );
 CGFloat *temp = theComponents;
 for ( NSInteger i = 0; i < inCount; i++ ) {
  UIColor *theColor = [inColors objectAtIndex: i];
  CGColorRef cgColor = [theColor CGColor];
  const CGFloat *components = CGColorGetComponents( cgColor );
  memmove( temp, components, colorSize );
  temp += numberOfComponents;
 }
 CGGradientRef gradient = CGGradientCreateWithColorComponents( theColorspace,
                                               theComponents, inLocations, inCount );
 CGColorSpaceRelease( theColorspace );
 free( theComponents );
 return gradient;
}

I know I can look for colors in the greyscale color space and convert them. But that only solves one case. From looking at this question, I think HSB is handled as well. But I'd like to write some code here and never think about it again, which means supporting not just the color spaces that are there now but anything Apple could conceivably add in the future. Am I out of luck?

Answer 1

I'm not sure how to automatically convert them, but to identify different color spaces you can get the CGColorSpaceModel from the color's CGColorSpaceRef:

UIColor* color = some color;
CGColorSpaceRef colorSpace = CGColorGetColorSpace([color CGColor]);
CGColorSpaceModel colorSpaceModel = CGColorSpaceGetModel(colorSpace);

Then you can compare colorSpaceModel with the constants defined in CoreGraphics/CGColorSpace.h. UIColor's getRed:green:blue:alpha works for kCGColorSpaceModelRGB, whereas getWhite:alpha works for kCGColorSpaceModelMonochrome.

Note that a UIColor that was created with colorWithHue:saturation:brightness:alpha: will actually be in the RGB color space.

Answer 2

For colors created using [UIColor colorWithRed:green:blue:alpha:], you can use UIColor getRed:green:blue:alpha:, described in UIColor Class Reference. It's part of iOS 5 and later.

If the color was created with colorWithWhite:alpha: you can use the getWhite:alpha: instead, described in UIColor Class Reference.

To determine which color space is being used, you can use CGColorGetColorSpace([color colorSpace]). But it's probably easier to just check the result of the method call, then fail over to the next attempt. Something like this:

if ([color getRed:&red green:&green blue:&blue alpha:&alpha]) {
    // red, green and blue are all valid
} else if if ([color getWhite:&white alpha:&alpha]) {
    red = white; green = white; blue = white;
} else {
    // can't get it
}

I don't know how to handle color constants such as [UIColor lightGrayColor], other than drawing them to a temporary bitmap and detecting them. Some of these color constants are actually textures; your best bet is probably to avoid them.

If you plan on doing this a lot, it's an appropriate use of a category:

@interface UIColor(sf)
- (BOOL)sfGetRed:(CGFloat *)red green:(CGFloat *)green blue:(CGFloat *)blue alpha:(CGFloat *)alpha;
@end

@implementation UIColor(sf)
- (BOOL)sfGetRed:(CGFloat *)red green:(CGFloat *)green blue:(CGFloat *)blue alpha:(CGFloat *)alpha {
    if ([self getRed:red green:green blue:blue alpha:alpha]) return YES;
    CGFloat white;
    if ([self getWhite:&white alpha:alpha]) {
        if (red) *red = white;
        if (green) *green = white;
        if (blue) *blue = white;
        return YES;
    }
    return NO;
}
@end

Answer 3

In swift 3 we can directly use

 let colorSpace = uiColor.cgColor.colorSpace
 let csModel = colorSpace.model

to get the color space and color space model from a UIColor.

Answer 4

Another way to convert these is by drawing them into contexts and letting core graphics do the conversion for you. See my answer to this question:

Get RGB value from UIColor presets