Gaussian blur without border artifacts (CoreImage bug) #308

/// A Gaussian blur filter that clamps to extent to avoid the gray border artefact.
struct GaussianBlurClampedToExtent: ImageProcessing, Hashable, CustomStringConvertible {    
    private let radius: Int
    
    /// Initializes the receiver with a blur radius.
    public init(radius: Int = 8) {
        self.radius = radius
    }
    
    /// Applies `CIGaussianBlur` filter to the image.
    public func process(image: Image, context: ImageProcessingContext?) -> Image? {
        
        // Get CI image
        let ciImageOptional: CoreImage.CIImage? = {
            if let image = image.ciImage {
                return image
            }
            if let image = image.cgImage {
                return CoreImage.CIImage(cgImage: image)
            }
            return nil
        }()
        
        // Ensure CI image was retrieved
        guard let ciImage = ciImageOptional else { return nil }
        
        // Remember original image extent
        let extent = ciImage.extent
        
        // Create image with infinitely extended border pixels to prevent gray edges from blur filter
        let inputImage: CIImage = ciImage.clampedToExtent()
        
        // Create blur filter
        let filter = CIFilter(name: "CIGaussianBlur", parameters: [kCIInputRadiusKey: radius, kCIInputImageKey: inputImage])
        
        // Get filtered image
        guard let filteredImage = filter?.outputImage else { return nil }
        
        // Get default context shared between all Core Image filters.
        let context = CIContext(options: [.priorityRequestLow: true])
        
        // Create CI image cropped to original extent
        guard let imageRef: CGImage = context.createCGImage(filteredImage, from: extent) else { return nil }
        return UIImage(cgImage: imageRef, scale: image.scale, orientation: image.imageOrientation)
    }

    public var identifier: String {
        return "com.github.kean/nuke/gaussian_blur_clamped_to_extent?radius=\(radius)"
    }

    public var hashableIdentifier: AnyHashable {
        return self
    }

    public var description: String {
        return "GaussianBlurClampedToExtend(radius: \(radius))"
    }
}