Colorize a UIImage in Swift

Question

I am trying to write an helper function that applies a color mask to a given image. My function has to set all opaque pixels of an image to the same color.

Here is what I have so far :

extension UIImage {

    func applyColorMask(color: UIColor, context: CIContext) -> UIImage {

        guard let cgImageInput = self.cgImage else {
            print("applyColorMask: \(self) has no cgImage attribute.")
            return self
        }

        // Throw away existing colors, and fill the non transparent pixels with the input color
        // s.r = dot(s, redVector), s.g = dot(s, greenVector), s.b = dot(s, blueVector), s.a = dot(s, alphaVector)
        // s = s + bias
        let colorFilter = CIFilter(name: "CIColorMatrix")!
        let ciColorInput = CIColor(cgColor: color.cgColor)
        colorFilter.setValue(CIVector(x: 0, y: 0, z: 0, w: 0), forKey: "inputRVector")
        colorFilter.setValue(CIVector(x: 0, y: 0, z: 0, w: 0), forKey: "inputGVector")
        colorFilter.setValue(CIVector(x: 0, y: 0, z: 0, w: 0), forKey: "inputBVector")
        colorFilter.setValue(CIVector(x: 0, y: 0, z: 0, w: 1), forKey: "inputAVector")
        colorFilter.setValue(CIVector(x: ciColorInput.red, y: ciColorInput.green, z: ciColorInput.blue, w: 0), forKey: "inputBiasVector")
        colorFilter.setValue(CIImage(cgImage: cgImageInput), forKey: kCIInputImageKey)

        if let cgImageOutput = context.createCGImage(colorFilter.outputImage!, from: colorFilter.outputImage!.extent) {
            return UIImage(cgImage: cgImageOutput)
        } else {
            print("applyColorMask: failed to apply filter to \(self)")
            return self
        }
    }
}

The code works fine for black and white but not what I expected when applying funnier colors. See the original image and the screenshots below: the same color is used for the border and for the image. Though they're different. My function is doing wrong. Did I miss something in the filter matrix ?

The original image (there's white dot at the center):

From top to bottom: The image filtered with UIColor(1.0, 1.0, 1.0, 1.0) inserted into a UIImageView which has borders of the same color. Then the same with UIColor(0.6, 0.5, 0.4, 1.0). And finally with UIColor(0.2, 0.5, 1.0, 1.0)

---

EDIT

Running Filterpedia gives me the same result. My understanding of the CIColorMatrix filter may be wrong then. The documentation says:

This filter performs a matrix multiplication, as follows, to transform the color vector:

• s.r = dot(s, redVector)
• s.g = dot(s, greenVector)
• s.b = dot(s, blueVector)
• s.a = dot(s, alphaVector)
• s = s + bias

Then, let say I throw up all RGB data with (0,0,0,0) vectros, and then pass (0.5, 0, 0, 0) mid-red as the bias vector; I would expect my image to have all its fully opaque pixels to (127, 0, 0). The screenshots below shows that it is slightly lighter (red=186):

Here is some pseudo code I want to do:

// image "im" is a vector of pixels
// pixel "p" is struct of rgba values
// color "col" is the input color or a struct of rgba values
for (p in im) {
    p.r = col.r
    p.g = col.g
    p.b = col.b
    // Nothing to do with the alpha channel
}

Answer 1

I finally wrote a CIColorKernel, as @dfd suggested and it works fine:

class ColorFilter: CIFilter {

    var inputImage: CIImage?
    var inputColor: CIColor?

    let kernel: CIColorKernel = {
        let kernelString = "kernel vec4 colorize(__sample pixel, vec4 color)\n"
            + "{\n"
            + "    pixel.rgb = color.rgb;\n"
            + "    return pixel;\n"
            + "}\n"

        return CIColorKernel(source: kernelString)!
    }()

    override var outputImage: CIImage? {

        guard let inputImage = inputImage else {
            print("\(self) cannot produce output because no input image provided.")
            return nil
        }
        guard let inputColor = inputColor else {
            print("\(self) cannot produce output because no input color provided.")
            return nil
        }

        let inputs = [inputImage, inputColor] as [Any]
        return kernel.apply(extent: inputImage.extent, arguments: inputs)
    }
}

---

To summarize, the CIColorMatrix I used first seems not to be linear (when using the bias vector). Giving a red 0,5 (float) value did not output an image with red 127 color in the [0-255] interval.

Writing a custom filter was my solution.

Answer 2

Glad to have helped. If I may, one thing. You can send a color into a kernel - it's a vec4, with the fourth value being the alpha channel. Just remember that CoreImage uses 0-1, not 0-254.

Here's the kernel code:

kernel vec4 colorize(__sample pixel, vec4 color) {
    pixel.rgb = color.rgb;
    return pixel;
}

It's pretty much the same as your's. But now all you need to do is create a CIColor instead of an image. If you already have a UIColor called inputColor, just do this:

let ciColor = CIColor(color: inputColor)
var inputs = [inputImage, ciColor] as [Any]

A couple of FYIs.

• There's also a vec3 that can be used but since you already have a UIColor `vec4 looks to be the easiest way.
• __sample is the value of the pixel being processed, so it's "base type" is really a vec4.