An example of using CIFilters to mess with UIViews on iOS.

gistfile1.m

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    // Great tutorial: http://www.raywenderlich.com/22167/beginning-core-image-in-ios-6
    // Official docs: https://developer.apple.com/library/ios/#documentation/GraphicsImaging/Conceptual/CoreImaging/ci_intro/ci_intro.html#//apple_ref/doc/uid/TP30001185-CH1-TPXREF101
    // Alt-click on function names for more!
    
    // Make any old label, with our frame set to the view so we know it's there.
    UILabel *label = [[UILabel alloc] initWithFrame:self.view.frame];
    label.text = @"HELLO";
    label.textAlignment = NSTextAlignmentCenter;
    // Uncomment to render.
    // [self.view addSubview:label];
    
    // To apply a filter to any UIView, we first need to get a UIImage of it.
    // To do this, we create an offscreen "canvas" to draw our label into.
    // Our "Canvas" in this case is called a UIGraphicsImageContext.    
    // Let's create a canvas, and make it the size of our label.
    UIGraphicsBeginImageContext(label.bounds.size);
    // We then render the label's CALayer into the canvas.
    // To access label.layer, we need to import QuartzCore, at the top.
    // This is because QuartzCore is responsible for the Core Animation classes.
    // Why can't we just get an image from the layer? Because it seems one of iOS's design
    // principles is that "everything is animatable" - every UIView is built on a CALayer.
    // Alt-click "renderInContext" for more.
    [label.layer renderInContext:UIGraphicsGetCurrentContext()];
    // Having drawn the label on the canvas, we get a UIImage "screenshot" of our canvas.
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    // Now we're finished with our canvas, let's free up some memory.
    // As we have a strong reference to our screenshot, that should be safe.
    UIGraphicsEndImageContext();
    
    // It's good to make sure we have an image:
    assert(image);

    // Let's log the list of "blur" filters that are available on the device.
    // From this we see that "CIGaussianBlur" is available.
    // It's best to play with this in GDB. Try:
    // p [CIFilter filterNamesInCategory:kCICategoryBlur]
    // in the debugger.
    NSLog(@"%@", [CIFilter filterNamesInCategory:kCICategoryBlur]);
    // What other categories can you find?
    
    // Now let's create the filter.
    CIFilter *blurFilter = [CIFilter filterWithName:@"CIGaussianBlur"];
    // Logging a CIFilter's attributes gives all of the information on the filter
    // and what values it accepts.
    NSLog(@"%@", [blurFilter attributes]);
    [blurFilter setDefaults];
    
    // This part is weird. Our UIImage does have a CIImage property, but there's something
    // strange about it. Option-click on "CIImage" here:
    assert(image.CIImage == 0);
    // So in some cases we don't get a CIImage from a UIImage.
    // That's ok - we can make a CIImage from the UIImage's CGImage. Hooray!
    CIImage *imageToBlur = [CIImage imageWithCGImage:image.CGImage];
    assert(imageToBlur);
    
    // iOS provides a nice constant for the input image key
    [blurFilter setValue:imageToBlur forKey:kCIInputImageKey];
    // ... but not for anything else
    [blurFilter setValue:@3.0 forKey:@"inputRadius"];
    
    // Now let's get the output image. Hope it works!
    CIImage *outputImage = blurFilter.outputImage;
    assert(outputImage);
    
    UIImageView *blurredImageView = [[UIImageView alloc] initWithFrame:self.view.frame];
    // UIImage imageWithCIImage was added recently. Convenient!
    blurredImageView.image = [UIImage imageWithCIImage:outputImage];
    [self.view addSubview:blurredImageView];
    
    // Apple's documentation give a pretty good explanation as to why this is so complicated.
    // Basically, on iOS, image filters use the GPU, and can be fucked with in real time.
    // Some food for thought: Can you create a slider to dynamically change the blur on the UILabel?
}