图像颜色错误？

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.)