How to tint an image/show a colour?

Question

2 things i want to do, which are related:

1. Show a block of any colour. So i could change that colour to something else at any time.
2. Tint a UIImage to be a different colour. An overlay of colour with alpha turned down could work here, but say it was an image which had a transparent background and didn't take up the full square of the image.

Any ideas?

Answer 1

Another option, would be to use category methods on UIImage like this...

// Tint the image, default to half transparency if given an opaque colour.
- (UIImage *)imageWithTint:(UIColor *)tintColor {
    CGFloat white, alpha;
    [tintColor getWhite:&white alpha:&alpha];
    return [self imageWithTint:tintColor alpha:(alpha == 1.0 ? 0.5f : alpha)];
}

// Tint the image
- (UIImage *)imageWithTint:(UIColor *)tintColor alpha:(CGFloat)alpha {

    // Begin drawing
    CGRect aRect = CGRectMake(0.f, 0.f, self.size.width, self.size.height);
    UIGraphicsBeginImageContext(aRect.size);

    // Get the graphic context
    CGContextRef c = UIGraphicsGetCurrentContext(); 

    // Converting a UIImage to a CGImage flips the image, 
    // so apply a upside-down translation
    CGContextTranslateCTM(c, 0, self.size.height); 
    CGContextScaleCTM(c, 1.0, -1.0);

    // Draw the image
    [self drawInRect:aRect];

    // Set the fill color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextSetFillColorSpace(c, colorSpace);

    // Set the mask to only tint non-transparent pixels
    CGContextClipToMask(c, aRect, self.CGImage);

    // Set the fill color
    CGContextSetFillColorWithColor(c, [tintColor colorWithAlphaComponent:alpha].CGColor);

    UIRectFillUsingBlendMode(aRect, kCGBlendModeColor);

    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();    

    // Release memory
    CGColorSpaceRelease(colorSpace);

    return img;
}

Answer 2

The first one is easy. Make a new UIView and set its background color to whatever color you’d like.

The second is more difficult. As you mentioned, you can put a new view on top of it with transparency turned down, but to get it to clip in the same places, you’d want to use a mask. Something like this:

UIImage *myImage = [UIImage imageNamed:@"foo.png"];

UIImageView *originalImageView = [[UIImageView alloc] initWithImage:myImage];
[originalImageView setFrame:CGRectMake(0.0f, 0.0f, 100.0f, 100.0f)];
[parentView addSubview:originalImageView];

UIView *overlay = [[UIView alloc] initWithFrame:[originalImageView frame]];

UIImageView *maskImageView = [[UIImageView alloc] initWithImage:myImage];
[maskImageView setFrame:[overlay bounds]];

[[overlay layer] setMask:[maskImageView layer]];

[overlay setBackgroundColor:[UIColor redColor]];

[parentView addSubview:overlay];

Keep in mind you’ll have to #import <QuartzCore/QuartzCore.h> in the implementation file.