How to programmatically change the hue of UIImage?

Question

I am very new to the image processing. I have to implement hue effect in my iPhone application project. Therefore, I need to change the hue of UIImage. Please p

Answer 1

Taking Dondragmer's excellent fixed hue example as a starting point. Here is my modification, to fix the problem of transparent areas being set with the same hue.

- (UIImage*) imageWithImage:(NSString *)name fixedHue:(CGFloat) hue alpha:(CGFloat) alpha;
// Note: the hue input ranges from 0.0 to 1.0, both red.  Values outside this range will be clamped to 0.0 or 1.0.
{
    UIImage *source = [UIImage imageNamed:name];

    // Find the image dimensions.
    CGSize imageSize = [source size];
    CGRect imageExtent = CGRectMake(0,0,imageSize.width,imageSize.height);

    // Create a context containing the image.
    UIGraphicsBeginImageContext(imageSize);
    CGContextRef context = UIGraphicsGetCurrentContext();

    [source drawAtPoint:CGPointMake(0,0)];

    // Setup a clip region using the image
    CGContextSaveGState(context);
    CGContextTranslateCTM(context, 0, source.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);
    CGContextClipToMask(context, imageExtent, source.CGImage);

    //[[UIColor colorWithHue:hue saturation:1.0 brightness:1 alpha:alpha] set];
    CGContextFillRect(context, imageExtent);

    // Draw the hue on top of the image.
    CGContextDrawImage(context, imageExtent, source.CGImage);
    CGContextSetBlendMode(context, kCGBlendModeHue);
    [[UIColor colorWithHue:hue saturation:1.0 brightness:1 alpha:alpha] set];
    UIBezierPath *imagePath = [UIBezierPath bezierPathWithRect:imageExtent];
    [imagePath fill];

    CGContextRestoreGState(context); // remove clip region

    // Retrieve the new image.
    UIImage *result = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return result;
}

Thanks to:

How to change hue of UIIMAGE having transparent area?

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