Change color of certain pixels in a UIImage

Question

For a given multi-color PNG UIImage (with transparency), what is the best/Swift-idiomatic way to:

1. create a duplicate UIImage
2. find all black pixels in the copy and change them to red
3. (return the modified copy)

There are a few related questions on SO but I haven't been able to find something that works.

Answer 1

You have to extract the pixel buffer of the image, at which point you can loop through, changing pixels as you see fit. At the end, create a new image from the buffer.

In Swift 3, this looks like:

func processPixels(in image: UIImage) -> UIImage? {     guard let inputCGImage = image.cgImage else {         print("unable to get cgImage")         return nil     }     let colorSpace       = CGColorSpaceCreateDeviceRGB()     let width            = inputCGImage.width     let height           = inputCGImage.height     let bytesPerPixel    = 4     let bitsPerComponent = 8     let bytesPerRow      = bytesPerPixel * width     let bitmapInfo       = RGBA32.bitmapInfo      guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {         print("unable to create context")         return nil     }     context.draw(inputCGImage, in: CGRect(x: 0, y: 0, width: width, height: height))      guard let buffer = context.data else {         print("unable to get context data")         return nil     }      let pixelBuffer = buffer.bindMemory(to: RGBA32.self, capacity: width * height)      for row in 0 ..< Int(height) {         for column in 0 ..< Int(width) {             let offset = row * width + column             if pixelBuffer[offset] == .black {                 pixelBuffer[offset] = .red             }         }     }      let outputCGImage = context.makeImage()!     let outputImage = UIImage(cgImage: outputCGImage, scale: image.scale, orientation: image.imageOrientation)      return outputImage }  struct RGBA32: Equatable {     private var color: UInt32      var redComponent: UInt8 {         return UInt8((color >> 24) & 255)     }      var greenComponent: UInt8 {         return UInt8((color >> 16) & 255)     }      var blueComponent: UInt8 {         return UInt8((color >> 8) & 255)     }      var alphaComponent: UInt8 {         return UInt8((color >> 0) & 255)     }              init(red: UInt8, green: UInt8, blue: UInt8, alpha: UInt8) {         let red   = UInt32(red)         let green = UInt32(green)         let blue  = UInt32(blue)         let alpha = UInt32(alpha)         color = (red << 24) | (green << 16) | (blue << 8) | (alpha << 0)     }      static let red     = RGBA32(red: 255, green: 0,   blue: 0,   alpha: 255)     static let green   = RGBA32(red: 0,   green: 255, blue: 0,   alpha: 255)     static let blue    = RGBA32(red: 0,   green: 0,   blue: 255, alpha: 255)     static let white   = RGBA32(red: 255, green: 255, blue: 255, alpha: 255)     static let black   = RGBA32(red: 0,   green: 0,   blue: 0,   alpha: 255)     static let magenta = RGBA32(red: 255, green: 0,   blue: 255, alpha: 255)     static let yellow  = RGBA32(red: 255, green: 255, blue: 0,   alpha: 255)     static let cyan    = RGBA32(red: 0,   green: 255, blue: 255, alpha: 255)      static let bitmapInfo = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Little.rawValue      static func ==(lhs: RGBA32, rhs: RGBA32) -> Bool {         return lhs.color == rhs.color     } } 

For Swift 2 rendition, see previous revision of this answer.

Answer 2

For getting better result we can search color range in image pixels, referring to @Rob answer I made update and now the result is better.

func processByPixel(in image: UIImage) -> UIImage? {      guard let inputCGImage = image.cgImage else { print("unable to get cgImage"); return nil }     let colorSpace       = CGColorSpaceCreateDeviceRGB()     let width            = inputCGImage.width     let height           = inputCGImage.height     let bytesPerPixel    = 4     let bitsPerComponent = 8     let bytesPerRow      = bytesPerPixel * width     let bitmapInfo       = RGBA32.bitmapInfo      guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {         print("Cannot create context!"); return nil     }     context.draw(inputCGImage, in: CGRect(x: 0, y: 0, width: width, height: height))      guard let buffer = context.data else { print("Cannot get context data!"); return nil }      let pixelBuffer = buffer.bindMemory(to: RGBA32.self, capacity: width * height)      for row in 0 ..< Int(height) {         for column in 0 ..< Int(width) {             let offset = row * width + column             /*              * Here I'm looking for color : RGBA32(red: 231, green: 239, blue: 247, alpha: 255)              * and I will convert pixels color that in range of above color to transparent              * so comparetion can done like this (pixelColorRedComp >= ourColorRedComp - 1 && pixelColorRedComp <= ourColorRedComp + 1 && green && blue)              */              if pixelBuffer[offset].redComponent >=  230 && pixelBuffer[offset].redComponent <=  232 &&                 pixelBuffer[offset].greenComponent >=  238 && pixelBuffer[offset].greenComponent <=  240 &&                 pixelBuffer[offset].blueComponent >= 246 && pixelBuffer[offset].blueComponent <= 248 &&                 pixelBuffer[offset].alphaComponent == 255 {                 pixelBuffer[offset] = .transparent             }         }     }      let outputCGImage = context.makeImage()!     let outputImage = UIImage(cgImage: outputCGImage, scale: image.scale, orientation: image.imageOrientation)      return outputImage } 

I hope this helps someone 🎉