CODEWITHSUNDEEP
iosimagecifilterciimage
bryanjclark
bryanjclark

Reputation: 6414

How can I isolate a single color in a UIImage / CIImage?

I'd like to isolate an element of a certain color in a UIImage, so that I can then process that image in another tool.

Let's say I've got the following image, and I want to isolate all red elements.

Input:

enter image description here

Desired Output: All the red elements

enter image description here

What steps can I take to accomplish this? I figured it'd be something in CIFilter but haven't found the right filter(s) yet. Is there a straightforward way to achieve this?

Upvotes: 1

Views: 441

Answers (1)

bryanjclark
bryanjclark

Reputation: 6414

Alright, got some help and have it sorted out! Here's how to do this:

// First, turn our UIImage into a CIImage
let ciImage = CIImage(image: image)!

// Next, let's make a CIVector for the color we want.
let colorToIsolate = CIVector(x: 0.239, y: 0.951, z: 0.937) // normalized RGB

// OK, here's where it gets interesting. 
// We're going to write a CIColorKernel that maps every pixel in the image. 
// If it's the color we want, then we turn it white, otherwise, black.
// This is iOS-10 friendly; in iOS 11 there's a non-string-based way,
// which helps with compiler type-checking and non-runtime error detection.
// NOTE: The 0.5 value is a tolerance; adjust and adapt it to your needs for your task. 
// I was surprised that we needed it that large to get the desired effect; 
// had hoped that I could use a tighter value like 0.01

let kernelString = "kernel vec4 filterColorMatch(sampler image, vec3 match) {" +
        "vec2 dc = destCoord();" +
        "vec3 color = sample(image, samplerTransform(image, dc)).rgb; " +
        "return distance(color, match) < 0.5 ? vec4(1,1,1,1) : vec4(0,0,0,1); }"

let colorIsolate = CIColorKernel(string: kernelString)

// Now we can run our kernel, and extract the image!
let after = colorIsolate?.apply(
  withExtent: ciImage.extent, 
  arguments: [ciImage, colorToIsolate]
)

let result = UIImage(ciImage: after)

Upvotes: 2

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