Color cube-style CoreImage filter enabling real-time alpha knock-out of solid colors.

CIFilter+Knockout.swift

//
//  CIFilter+Knockout.swift
//
//
//  Created by Stephan Casas on 10/13/23.
//

import Foundation
import CoreImage;

fileprivate let kCIFilterKnockoutThreshold: Float = 0.8;

fileprivate var __knockoutBlack: CIFilter? = nil;
fileprivate var __knockoutGreen: CIFilter? = nil;

public extension CIFilter {
    
    // MARK: - Black Knockout
    
    /// A color-cube filter which diminishes the alpha value of blacks to 0.
    static var knockoutBlack: CIFilter {
        if let cached = __knockoutBlack {
            return cached;
        }
        
        let _size = 64;
        let capacity = _size * _size * _size * 4;
        var cube: [Float] = .init(repeating: 0.0, count: capacity)
        
        guard let colorSpace = CGColorSpace(
            name: CGColorSpace.displayP3
        ) else {
            fatalError("Could not acquire P3 color space.");
        }
        
        let size = Float(_size);
        for _b in 0..<_size {
            for _g in 0..<_size {
                for _r in 0..<_size {
                    let r = Float(_r);
                    let g = Float(_g);
                    let b = Float(_b);
                    
                    let i = 4 * ((_b * _size * _size) + (_g * _size) + _r);
                    
                    cube[i + 0] = r / size;
                    cube[i + 1] = g / size;
                    cube[i + 2] = b / size;
                    
                    // knockout darker colors below threshold
                    cube[i + 3] = (
                        r / size < kCIFilterKnockoutThreshold
                        &&
                        g / size < kCIFilterKnockoutThreshold
                        &&
                        b / size < kCIFilterKnockoutThreshold
                    ) ? 0 : 1;
                }
            }
        }
        
        guard let ciFilter = CIFilter(name: "CIColorCubeWithColorSpace", parameters: [
            "inputCubeData": Data(bytes: &cube, count: capacity * MemoryLayout<Float>.size),
            "inputCubeDimension": _size,
            "inputColorSpace": colorSpace
        ]) else {
            fatalError("Could not create CIFilter for black knockout.");
        }
        
        __knockoutBlack = ciFilter;
        
        return ciFilter;
    }
    
    // MARK: - Green Knockout
    
    /// A color-cube filter which diminishes the alpha value of greens to 0.
    static var knockoutGreen: CIFilter {
        if let cached = __knockoutGreen {
            return cached;
        }
        
        let _size = 64;
        let capacity = _size * _size * _size * 4;
        var cube: [Float] = .init(repeating: 0.0, count: capacity);
        
        guard let colorSpace = CGColorSpace(
            name: CGColorSpace.displayP3
        ) else {
            fatalError("Could not acquire P3 color space.");
        }
        
        let size = Float(_size);
        for _b in 0..<_size {
            for _g in 0..<_size {
                for _r in 0..<_size {
                    let r = Float(_r);
                    let g = Float(_g);
                    let b = Float(_b);
                    
                    let i = 4 * ((_b * _size * _size) + (_g * _size) + _r);
                    cube[i + 0] = r / size;
                    cube[i + 1] = g / size;
                    cube[i + 2] = b / size;
                    
                    cube[i + 3] = (
                        g / size > kCIFilterKnockoutThreshold
                        &&
                        r / size < kCIFilterKnockoutThreshold
                        &&
                        b / size < kCIFilterKnockoutThreshold
                    ) ? 0 : 1;
                }
            }
        }
        
        guard let ciFilter = CIFilter(name: "CIColorCubeWithColorSpace", parameters: [
            "inputCubeData": Data(bytes: &cube, count: capacity * MemoryLayout<Float>.size),
            "inputCubeDimension": _size,
            "inputColorSpace": colorSpace
        ]) else {
            fatalError("Could not create CIFilter for green knockout.");
        }
        
        __knockoutGreen = ciFilter;
        
        return ciFilter;
    }
    
}