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 it correctly to avoid having transparent edges of image. But how to calculate how much I need to crop in dependence of blur amount?

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Example:

Original image:

Image with 50 inputRadius of CIGaussianBlur (blue color is background of everything):

Answer 1

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]];