iOS: create an darker version of UIImage and leave transparent pixels unchanged?

Question

I found

Create new UIImage by adding shadow to existing UIImage

and

UIImage, is there an easy way to make it darker or all black

But the selected

Answer 1

This is the class I use to color images even if they are transparent.

+ (UIImage *)colorizeImage:(UIImage *)image withColor:(UIColor *)color {
    UIGraphicsBeginImageContextWithOptions(image.size, NO, image.scale);

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGRect area = CGRectMake(0, 0, image.size.width, image.size.height);

    CGContextScaleCTM(context, 1, -1);
    CGContextTranslateCTM(context, 0, -area.size.height);

    CGContextSaveGState(context);
    CGContextClipToMask(context, area, image.CGImage);

    [color set];
    CGContextFillRect(context, area);

    CGContextRestoreGState(context);

    CGContextSetBlendMode(context, kCGBlendModeMultiply);

    CGContextDrawImage(context, area, image.CGImage);

    UIImage *colorizedImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return colorizedImage;
}

To darken the image you would pass the method a black or gray UIColor with lowered transparency.