Image color bug? - ios

Question

I'd like to generate this cool like on an UIImage using the code I've added below. But there seems to be a crazy bug in relation to the color:
So I'd like my function to draw lines to the image with a given RGB value (in my case r:76, g:255, b:0 to get this green look) as it's color:
But the code I'm using just colors the image kind of magenta instead of green:
Please have a look at my code:
func colorImage(image: UIImage) -> UIImage {
let color = RGBA32(red: 76, green: 255, blue: 0, alpha: 255)
let img: CGImage = image.cgImage!
let context = CGContext(data: nil, width: img.width, height: img.height, bitsPerComponent: 8, bytesPerRow: 4 * img.width, space: CGColorSpaceCreateDeviceRGB(), bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)!
context.draw(img, in: CGRect(x: 0, y: 0, width: img.width, height: img.height))
let binaryData = context.data!.bindMemory(to: RGBA32.self, capacity: img.width * img.height)
for y in stride(from: 0, through: img.height, by: 10) {
for x in 0..<img.width {
let pixel = (y*img.width) + x
binaryData[pixel] = color
}
}
let output = context.makeImage()!
return UIImage(cgImage: output, scale: image.scale, orientation: image.imageOrientation)
}
struct RGBA32: Equatable {
private var color: UInt32
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 func ==(lhs: RGBA32, rhs: RGBA32) -> Bool {
return lhs.color == rhs.color
}
}
Tbh I'm absolutely clueless why this issue appears - so any help how to fix this would be very appreciated. Thanks :)

Answer 1

RGBA is a confusing word. It is not clear if R is the most significant byte in the 32-bit data or the first byte in the 4-byte memory region.
But with your bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue,
R takes the first place and G, B, and then Alpha last.
In little-endian representation of UInt32, R for the least significant byte.
Try this version of RGBA:
struct RGBA32: Equatable {
private var color: (red: UInt8, green: UInt8, blue: UInt8, alpha: UInt8)
init(red: UInt8, green: UInt8, blue: UInt8, alpha: UInt8) {
color = (red: red, green: green, blue: blue, alpha: alpha)
}
static func ==(lhs: RGBA32, rhs: RGBA32) -> Bool {
return lhs.color == rhs.color
}
}
(Memory allocations of Swift structs or tuples are not clearly defined, but the above code works as expected.)