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.