Correct crop of CIGaussianBlur

Question

As I noticed when CIGaussianBlur is applied to image, image\'s corners gets blurred so that it looks like being smaller than original. So I figured out that I need to crop i

Answer 1

Try this, let the input's extent be -createCGImage:fromRect:'s parameter:

-(UIImage *)gaussianBlurImageWithRadius:(CGFloat)radius {
    CIContext *context = [CIContext contextWithOptions:nil];
    CIImage *input = [CIImage imageWithCGImage:self.CGImage];
    CIFilter *filter = [CIFilter filterWithName:@"CIGaussianBlur"];
    [filter setValue:input forKey:kCIInputImageKey];
    [filter setValue:@(radius) forKey:kCIInputRadiusKey];
    CIImage *output = [filter valueForKey:kCIOutputImageKey];
    CGImageRef imgRef = [context createCGImage:output
                                      fromRect:input.extent];
    UIImage *outImage = [UIImage imageWithCGImage:imgRef
                                            scale:UIScreen.mainScreen.scale
                                      orientation:UIImageOrientationUp];
    CGImageRelease(imgRef);
    return outImage;
}

Answer 2

Take the following code as an example...

CIContext *context = [CIContext contextWithOptions:nil];

CIImage *inputImage = [[CIImage alloc] initWithImage:image];

CIFilter *filter = [CIFilter filterWithName:@"CIGaussianBlur"];

[filter setValue:inputImage forKey:kCIInputImageKey];

[filter setValue:[NSNumber numberWithFloat:5.0f] forKey:@"inputRadius"];

CIImage *result = [filter valueForKey:kCIOutputImageKey];

CGImageRef cgImage = [context createCGImage:result fromRect:[result extent]];

This results in the images you provided above. But if I instead use the original images rect to create the CGImage off of the context the resulting image is the desired size.

CGImageRef cgImage = [context createCGImage:result fromRect:[inputImage extent]];

Answer 3

See below two implementations for Xamarin (C#).

1) Works for iOS 6

public static UIImage Blur(UIImage image)
{   
    using(var blur = new CIGaussianBlur())
    {
        blur.Image = new CIImage(image);
        blur.Radius = 6.5f;

        using(CIImage output = blur.OutputImage)
        using(CIContext context = CIContext.FromOptions(null))
        using(CGImage cgimage = context.CreateCGImage (output, new RectangleF(0, 0, image.Size.Width, image.Size.Height)))
        {
            return UIImage.FromImage(cgimage);
        }
    }
}

2) Implementation for iOS 7

Using the way shown above isn't working properly on iOS 7 anymore (at least at the moment with Xamarin 7.0.1). So I decided to add cropping another way (measures may depend on the blur radius).

private static UIImage BlurImage(UIImage image)
{   
    using(var blur = new CIGaussianBlur())
    {
        blur.Image = new CIImage(image);
        blur.Radius = 6.5f;

        using(CIImage output = blur.OutputImage)
        using(CIContext context = CIContext.FromOptions(null))
        using(CGImage cgimage = context.CreateCGImage (output, new RectangleF(0, 0, image.Size.Width, image.Size.Height)))
        {
            return UIImage.FromImage(Crop(CIImage.FromCGImage(cgimage), image.Size.Width, image.Size.Height));
        }
    }
}

private static CIImage Crop(CIImage image, float width, float height)
{
    var crop = new CICrop
    { 
        Image = image,
        Rectangle = new CIVector(10, 10, width - 20, height - 20) 
    };

    return crop.OutputImage;   
}