How to make a custom CIFilter in Swift

Here’s an example Swift program which will swap the red and green components of an image, and save the output to a file. It uses a custom CIFilter, which uses a GLSL kernel, the source of which is embedded in the program.

import Foundation
import CoreImage
import AppKit
let kernels = CIKernel.kernels(with:
    "kernel vec4 swapRG(sampler image) { " +
    "  vec4 t = sample(image, samplerCoord(image)); float r = t.r; t.r = t.g; t.g = r; return t;" +
let myKernel = kernels[0]
class SwapRedAndGreenFilter : CIFilter {
    var inputImage:CIImage?
    override var outputImage: CIImage? {
        let src = CISampler(image: self.inputImage!)
        return self.apply(myKernel, arguments: [src], options: nil)
let nsImage = NSImage(contentsOfFile: "/Users/jim/Lenna.png")!
let cgImage = nsImage.cgImage(forProposedRect: nil, context: nil, hints: [:])!
let inputImage = CIImage(cgImage: cgImage)
let filter = SwapRedAndGreenFilter()
filter.setValue(inputImage, forKey: kCIInputImageKey)
let outputImage = filter.outputImage!
let rep = NSBitmapImageRep(ciImage: outputImage.cropping(to: CGRect(x: 0, y: 0, width: cgImage.width, height: cgImage.height)))
let imageData = rep.representation(using: NSBitmapImageFileType.PNG, properties: [:])!
try! imageData.write(to: URL.init(string: "file:/Users/jim/output.png")!, options: NSData.WritingOptions.atomic)

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I wrote this because Future Vidrio requires image filters This post is not associated with my employer. This site is hosted by Netlify (who are great, but I'm not associated with them either).