ColorReplacement.swift

//
//  TestView.swift
//  FolderColor
//
//  Created by 黄宝成 on 2022/5/10.
//

import Foundation
import SwiftUI

struct ColorReplacement: View {
		var body: some View {
			
		 Image("Folder")
			
		}
}

// Helper functions.
extension NSImage {
		/**
		 Replaces a color in the image with a different color.
		 - Parameter color: color to be replaced.
		 - Parameter with: the new color to be used.
		 - Parameter tolerance: tolerance, between 0 and 1. 0 won't change any colors,
														1 will change all of them. 0.5 is default.
		 - Returns: image with the replaced color.
		 */
		func replaceColor(_ color: NSColor, with: NSColor, tolerance: CGFloat = 0.5) -> NSImage {
		   
				let imageRef = self
			
				// Get color components from replacement color
				let withColorComponents = with.cgColor.components
				let newRed = UInt8(withColorComponents![0] * 255)
				let newGreen = UInt8(withColorComponents![1] * 255)
				let newBlue = UInt8(withColorComponents![2] * 255)
				let newAlpha = UInt8(withColorComponents![3] * 255)

			let width = Int(imageRef.size.width)
			let height = Int(imageRef.size.height)
				
			let bytesPerPixel: Int = 4
				let bytesPerRow = bytesPerPixel * width
				let bitmapByteCount = bytesPerRow * height
				
				let rawData = UnsafeMutablePointer<UInt8>.allocate(capacity: bitmapByteCount)
				defer {
						rawData.deallocate()
				}
				
				guard let colorSpace = CGColorSpace(name: CGColorSpace.genericRGBLinear) else {
						return self
				}
				
				guard let context = CGContext(
						data: rawData,
						width: width,
						height: height,
						bitsPerComponent: 8,
						bytesPerRow: bytesPerRow,
						space: colorSpace,
						bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue
								| CGBitmapInfo.byteOrder32Big.rawValue
				) else {
						return self
				}
				
				var rc = CGRect(x: 0, y: 0, width: width, height: height)
				// Draw source image on created context.
			context.draw(imageRef.cgImage(forProposedRect: &rc, context: nil, hints: nil)!, in: rc)
			
				var byteIndex = 0
				// Iterate through pixels
				while byteIndex < bitmapByteCount {
						// Get color of current pixel
						let red = CGFloat(rawData[byteIndex + 0]) / 255
						let green = CGFloat(rawData[byteIndex + 1]) / 255
						let blue = CGFloat(rawData[byteIndex + 2]) / 255
						let alpha = CGFloat(rawData[byteIndex + 3]) / 255
					
						let currentColor = NSColor(red: red, green: green, blue: blue, alpha: alpha)
					
						// Replace pixel if the color is close enough to the color being replaced.
						if compareColor(firstColor: color, secondColor: currentColor, tolerance: tolerance) {
								rawData[byteIndex + 0] = newRed
								rawData[byteIndex + 1] = newGreen
								rawData[byteIndex + 2] = newBlue
								rawData[byteIndex + 3] = newAlpha
						}
						byteIndex += 4
				}
				
				// Retrieve image from memory context.
				guard let image = context.makeImage() else {
						return self
				}
			
			return NSImage(cgImage: image, size: NSSize(width: image.width, height: image.height))
		}
		
		/**
		 Check if two colors are the same (or close enough given the tolerance).
		 - Parameter firstColor: first color used in the comparisson.
		 - Parameter secondColor: second color used in the comparisson.
		 - Parameter tolerance: how much variation can there be for the function to return true.
														0 is less sensitive (will always return false),
														1 is more sensitive (will always return true).
		 */
		private func compareColor(
				firstColor: NSColor,
				secondColor: NSColor,
				tolerance: CGFloat
		) -> Bool {
				var r1: CGFloat = 0.0, g1: CGFloat = 0.0, b1: CGFloat = 0.0, a1: CGFloat = 0.0;
				var r2: CGFloat = 0.0, g2: CGFloat = 0.0, b2: CGFloat = 0.0, a2: CGFloat = 0.0;

				firstColor.getRed(&r1, green: &g1, blue: &b1, alpha: &a1)
				secondColor.getRed(&r2, green: &g2, blue: &b2, alpha: &a2)
				
				return abs(r1 - r2) <= tolerance
						&& abs(g1 - g2) <= tolerance
						&& abs(b1 - b2) <= tolerance
						&& abs(a1 - a2) <= tolerance
		}
		
}

struct ColorReplacement_Previews: PreviewProvider {
		static var previews: some View {
			ColorReplacement()
		}
}