UnrolledKuwaharaFilter - replace a GLSL loop with a Swift one. Performance is rubbish, this is here for reference only!

gistfile1.txt

class UnrolledKuwaharaFilter: CIFilter
{
    var inputImage: CIImage?
    
    var inputRadius: CGFloat = 15
    {
        didSet
        {
            if Int(inputRadius) != Int(oldValue)
            {
                kuwaharaKernel = nil
            }
        }
    }
    
    override var attributes: [String : AnyObject]
    {
        return [
            kCIAttributeFilterDisplayName: "Kuwahara Filter (Unrolled)",
            
            "inputImage": [kCIAttributeIdentity: 0,
                kCIAttributeClass: "CIImage",
                kCIAttributeDisplayName: "Image",
                kCIAttributeType: kCIAttributeTypeImage],
        
            "inputRadius": [kCIAttributeIdentity: 0,
                kCIAttributeClass: "NSNumber",
                kCIAttributeDefault: 15,
                kCIAttributeDisplayName: "Radius",
                kCIAttributeMin: 0,
                kCIAttributeSliderMin: 0,
                kCIAttributeSliderMax: 30,
                kCIAttributeType: kCIAttributeTypeScalar]
        ]
    }
    
    override func setDefaults()
    {
        inputRadius = 15
    }
    
    var kuwaharaKernel: CIKernel?
    
    func createKuwaharaKernel() -> CIKernel?
    {
        var kernelString: String =
            "kernel vec4 kuwahara(sampler image, float r) \n" +
            "{" +
            "   vec2 d = destCoord();" +
                
            "   int radius = int(r); " +
            "   float n = float((radius + 1) * (radius + 1)); " +
                
            "   vec3 means[4]; " +
            "   vec3 stdDevs[4]; " +
                
            "   for (int i = 0; i < 4; i++) " +
            "   { " +
            "       means[i] = vec3(0.0); " +
            "       stdDevs[i] = vec3(0.0); " +
            "   } "

        for x in -Int(inputRadius) ... Int(inputRadius)
        {
            for y in -Int(inputRadius) ... Int(inputRadius)
            {
                let suffix = "\(Int(inputRadius) + x)_\(Int(inputRadius) + y)"
                    
                kernelString +=
                "vec3 color\(suffix) = sample(image, samplerTransform(image, d + vec2(\(x),\(y)))).rgb; \n" +
                
                "vec3 colorA\(suffix) = vec3(float(\(x) <= 0 && \(y) <= 0)) * color\(suffix); \n" +
                "means[0] += colorA\(suffix); \n" +
                "stdDevs[0] += colorA\(suffix) * colorA\(suffix); \n" +

                "vec3 colorB\(suffix) = vec3(float(\(x) >= 0 && \(y) <= 0)) * color\(suffix); \n" +
                "means[1] +=  colorB\(suffix); \n" +
                "stdDevs[1] += colorB\(suffix) * colorB\(suffix); \n" +

                "vec3 colorC\(suffix) = vec3(float(\(x) <= 0 && \(y) >= 0)) * color\(suffix); \n" +
                "means[2] += colorC\(suffix); \n" +
                "stdDevs[2] += colorC\(suffix) * colorC\(suffix); \n" +

                "vec3 colorD\(suffix) = vec3(float(\(x) >= 0 && \(y) >= 0)) * color\(suffix); \n" +
                "means[3] += colorD\(suffix); \n" +
                "stdDevs[3] += colorD\(suffix) * colorD\(suffix); \n"
            }
        }

        kernelString +=
            "   float minSigma2 = 1e+2;" +
            "   vec3 returnColor = vec3(0.0); " +
            
            "   for (int j = 0; j < 4; j++) " +
            "   { " +
            "       means[j] /= n; " +
            "       stdDevs[j] = abs(stdDevs[j] / n - means[j] * means[j]);" +

            "       float sigma2 = stdDevs[j].r + stdDevs[j].g + stdDevs[j].b;" +

            "       returnColor = (sigma2 < minSigma2) ? means[j] : returnColor; " +
            "       minSigma2 = (sigma2 < minSigma2) ? sigma2 : minSigma2; " +
            "   } " +
            
        "   return vec4(returnColor, 1.0); " +
        "}"
        
        return CIKernel(string: kernelString)
    }

    
    override var outputImage : CIImage!
    {
        if kuwaharaKernel == nil
        {
            kuwaharaKernel = createKuwaharaKernel()
        }
        
        if let inputImage = inputImage
        {
            let arguments = [inputImage, inputRadius]
            let extent = inputImage.extent
            
            return kuwaharaKernel!.applyWithExtent(extent,
                roiCallback:
                {
                    (index, rect) in
                    return rect
                },
                arguments: arguments)
        }
        return nil
    }
}