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

There's a CoreImage filter for this. CIColorControls Just set the inputSaturation to < 1.0 to desaturate or > 1.0 to increase saturation...

eg. Here's a method I've added in a category on UIImage to desaturate an image.

-(UIImage*) imageDesaturated {
    CIContext *context = [CIContext contextWithOptions:nil];
    CIImage *ciimage = [CIImage imageWithCGImage:self.CGImage];
    CIFilter *filter = [CIFilter filterWithName:@"CIColorControls"];
    [filter setValue:ciimage forKey:kCIInputImageKey];
    [filter setValue:@0.0f forKey:kCIInputSaturationKey];
    CIImage *result = [filter valueForKey:kCIOutputImageKey];
    CGImageRef cgImage = [context createCGImage:result fromRect:[result extent]];
    UIImage *image = [UIImage imageWithCGImage:cgImage];
    CGImageRelease(cgImage);
    return image;
}

Answer 2

Starting with a View-based Application Template, create a new subclass of UIView like so:

// header file
@interface DesatView : UIView {
     UIImage *image;
     float saturation;
}
@property (nonatomic, retain) UIImage *image;
@property (nonatomic) float desaturation;
@end  

// implementation file
#import "DesatView.h"
@implementation DesatView
@synthesize image, desaturation;

-(void)setSaturation:(float)sat;
    {
        saturation = sat;
        [self setNeedsDisplay];
    }

- (id)initWithFrame:(CGRect)frame {
     if (self = [super initWithFrame:frame]) {
          self.backgroundColor = [UIColor clearColor]; // else background is black
          desaturation = 0.0; // default is no effect
     }
     return self;
}

- (void)drawRect:(CGRect)rect {
     CGContextRef context = UIGraphicsGetCurrentContext();
     CGContextSaveGState(context);

     CGContextTranslateCTM(context, 0.0, self.bounds.size.height); // flip image right side up
     CGContextScaleCTM(context, 1.0, -1.0);

     CGContextDrawImage(context, rect, self.image.CGImage);
     CGContextSetBlendMode(context, kCGBlendModeSaturation);
     CGContextClipToMask(context, self.bounds, image.CGImage); // restricts drawing to within alpha channel
     CGContextSetRGBFillColor(context, 0.0, 0.0, 0.0, desaturation);
     CGContextFillRect(context, rect);

     CGContextRestoreGState(context); // restore state to reset blend mode
}
@end

Now in your view controller's viewDidLoad method, put the view on screen and set it's saturation like this:

- (void)viewDidLoad {
    [super viewDidLoad];  

    DesatView *dv = [[DesatView alloc] initWithFrame:CGRectZero];
    dv.image = [UIImage imageNamed:@"someImage.png"];
    dv.frame = CGRectMake(0, 0, dv.image.size.width, dv.image.size.height);
    dv.center = CGPointMake(160, 240); // put it mid-screen
    dv.desaturation = 0.2; // desaturate by 20%,

    [self.view addSubview:dv];  // put it on screen   
}  

Change the saturation like this:

 dv.saturation = 0.8; // desaturate by 80%  

Obviously if you want to use it outside of a single method, you should make dv an ivar of the view controller. Hope this helps.

Answer 3

Swift 5

extension UIImage {
    
    func withSaturationAdjustment(byVal: CGFloat) -> UIImage {
        guard let cgImage = self.cgImage else { return self }
        guard let filter = CIFilter(name: "CIColorControls") else { return self }
        filter.setValue(CIImage(cgImage: cgImage), forKey: kCIInputImageKey)
        filter.setValue(byVal, forKey: kCIInputSaturationKey)
        guard let result = filter.value(forKey: kCIOutputImageKey) as? CIImage else { return self }
        guard let newCgImage = CIContext(options: nil).createCGImage(result, from: result.extent) else { return self }
        return UIImage(cgImage: newCgImage, scale: UIScreen.main.scale, orientation: imageOrientation)
    }

}

Answer 4

Here is an implementation of Bessey's hack (put this code in a UIImage category). It ain't fast and it definitely shifts hues, but it sort of works.

+ (CGFloat) clamp:(CGFloat)pixel
{
      if(pixel > 255) return 255;
      else if(pixel < 0) return 0;
      return pixel;
}

- (UIImage*) saturation:(CGFloat)s
{
      CGImageRef inImage = self.CGImage;
      CFDataRef ref = CGDataProviderCopyData(CGImageGetDataProvider(inImage)); 
      UInt8 * buf = (UInt8 *) CFDataGetBytePtr(ref); 
      int length = CFDataGetLength(ref);

      for(int i=0; i<length; i+=4)
      {
            int r = buf[i];
            int g = buf[i+1];
            int b = buf[i+2];

            CGFloat avg = (r + g + b) / 3.0;
            buf[i] = [UIImage clamp:(r - avg) * s + avg];
            buf[i+1] = [UIImage clamp:(g - avg) * s + avg];
            buf[i+2] = [UIImage clamp:(b - avg) * s + avg];
      }

      CGContextRef ctx = CGBitmapContextCreate(buf,
                              CGImageGetWidth(inImage), 
                              CGImageGetHeight(inImage), 
                              CGImageGetBitsPerComponent(inImage),
                              CGImageGetBytesPerRow(inImage), 
                              CGImageGetColorSpace(inImage),
                              CGImageGetAlphaInfo(inImage));

      CGImageRef img = CGBitmapContextCreateImage(ctx);
      CFRelease(ref);
      CGContextRelease(ctx);
      return [UIImage imageWithCGImage:img];
}

Anyone have any ideas on how to improve this without doing a full HSV conversion? Or better yet, a true implementation for:

- (UIImage*) imageWithHueOffset:(CGFloat)h saturation:(CGFloat)s value:(CGFloat)v