Gradient Filling a PNG with Quartz - ios

Question

How can I fill the non-transparent areas of a PNG UIImage with a linear gradient? I'd like to reuse a PNG shape for MKAnnotationViews, but change the gradient per annotation's properties.

Answer 1

To use an image as a mask for a gradient (i.e. to have a gradient in the shape of the non-transparent pixels of your image), you can:
create a simple view with a gradient (you can either create a simple UIView and use the addGradientLayerToView shown below to give it a gradient or you can create the gradient PNG in advance and add it to your bundle).
apply your PNG as a mask to that gradient view:
UIImage *mask = [UIImage imageNamed:#"mask.png"];
CALayer *maskLayer = [CALayer layer];
maskLayer.frame = CGRectMake(0, 0, mask.size.width, mask.size.height);
maskLayer.contents = (id)[mask CGImage];
gradientViewToMask.layer.mask = maskLayer;
To apply a gradient to the transparent pixels, you can either:
Create a new image with a gradient:
- (UIImage *)imageWithGradient:(UIImage *)image
{
UIGraphicsBeginImageContextWithOptions(image.size, NO, 1.0);
CGContextRef context = UIGraphicsGetCurrentContext();
size_t locationCount = 2;
CGFloat locations[2] = { 0.0, 1.0 };
CGFloat components[8] = { 0.0, 0.8, 0.8, 1.0, // Start color
0.9, 0.9, 0.9, 1.0 }; // End color
CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradient = CGGradientCreateWithColorComponents (colorspace, components, locations, locationCount):
CGPoint startPoint = CGPointMake(0.0, 0.0);
CGPoint endPoint = CGPointMake(0.0, image.size.height);
CGContextDrawLinearGradient (context, gradient, startPoint, endPoint, 0);
CGContextTranslateCTM(context, 0, image.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextDrawImage(context, CGRectMake(0.0, 0.0, image.size.width, image.size.height), [image CGImage]);
UIImage *gradientImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
CGGradientRelease(gradient);
CGColorSpaceRelease(colorspace);
return gradientImage;
}
You can also add a CAGradientLayer to a view and then add the UIImageView as a subview of that view.
- (void)addGradientLayerToView:(UIView *)view
{
CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = view.bounds;
gradient.colors = #[(id)[[UIColor colorWithRed:0.0 green:0.8 blue:0.8 alpha:1.0] CGColor],
(id)[[UIColor colorWithRed:0.9 green:0.9 blue:0.9 alpha:1.0] CGColor]];
[view.layer insertSublayer:gradient atIndex:0];
}
Note, you have to #import <QuartzCore/QuartzCore.h> as well as add the QuartzCore framework to your project.

Answer 2

I ended up hacking together some bits of Rob's code and an extension to UIImage I found at http://coffeeshopped.com/2010/09/iphone-how-to-dynamically-color-a-uiimage
+ (UIImage *)imageNamed:(NSString *)name withGradient:(CGGradientRef)gradient
{
// load the image
UIImage *img = [UIImage imageNamed:name];
// begin a new image context, to draw our colored image onto
UIGraphicsBeginImageContextWithOptions(img.size, NO, [[UIScreen mainScreen] scale]);
// get a reference to that context we created
CGContextRef context = UIGraphicsGetCurrentContext();
// translate/flip the graphics context (for transforming from CG* coords to UI* coords
CGContextTranslateCTM(context, 0, img.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
// set the blend mode to overlay, and the original image
CGContextSetBlendMode(context, kCGBlendModeOverlay);
CGRect rect = CGRectMake(0, 0, img.size.width, img.size.height);
// set a mask that matches the shape of the image, then draw (overlay) a colored rectangle
CGContextClipToMask(context, rect, img.CGImage);
CGContextAddRect(context, rect);
//gradient
CGPoint startPoint = CGPointMake(0.0, img.size.height);
CGPoint endPoint = CGPointMake(0.0, 0.0);
CGContextDrawLinearGradient (context, gradient, startPoint, endPoint, 0);
// generate a new UIImage from the graphics context we drew onto
UIImage *coloredImg = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
CGGradientRelease(gradient);
//return the color-burned image
return coloredImg;
}