l.b.dev l.b.dev - 1 month ago 18
iOS Question

How to create an image pixel by pixel

I want to create an UIImage pixel by pixel with swift 3

I have search but i couldn't find a code who actually works

So let me explain, I have an array with characters

var array = ["w", "x", "y", "x", "y", "y", "y", "x", "x", "x", "w", "x", "y", "w", "y"] //there will be something like 26 millions of those


if it's w, the color of the pixel is blue

if it's x, the color of the pixel is red

if it's y, the color of the pixel is green

if it's v, the color of the pixel is black

I want to create an image from those characters and store it in the photos

Any thoughts??

Thanks for your answers

Rob Rob
Answer

You can create a CGContext and then retrieve the data buffer for that image, and then fill that buffer with values corresponding to your string values:

func createImage(width: Int, height: Int, from array: [String], completionHandler: @escaping (UIImage?, String?) -> Void) {
    DispatchQueue.global(qos: .utility).async {
        let colorSpace       = CGColorSpaceCreateDeviceRGB()
        let bytesPerPixel    = 4
        let bitsPerComponent = 8
        let bytesPerRow      = bytesPerPixel * width
        let bitmapInfo       = RGBA32.bitmapInfo

        guard array.count == width * height else {
            completionHandler(nil, "Array size \(array.count) is incorrect given dimensions \(width) x \(height)")
            return
        }

        guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {
            completionHandler(nil, "unable to create context")
            return
        }

        guard let buffer = context.data else {
            completionHandler(nil, "unable to get context data")
            return
        }

        let pixelBuffer = buffer.bindMemory(to: RGBA32.self, capacity: width * height)

        for (index, string) in array.enumerated() {
            switch string {
            case "w": pixelBuffer[index] = .blue
            case "x": pixelBuffer[index] = .red
            case "y": pixelBuffer[index] = .green
            case "v": pixelBuffer[index] = .black
            default: completionHandler(nil, "Unexpected value: \(string)"); return
            }
        }

        let cgImage = context.makeImage()!

        let image = UIImage(cgImage: cgImage)

        // or
        //
        // let image = UIImage(cgImage: cgImage, scale: UIScreen.main.scale, orientation: .up)

        completionHandler(image, nil)
    }

}

If there are 26 million pixels, you probably want to make this asynchronous to avoid blocking the main queue.

By the way, the above uses this struct:

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) {
        color = (UInt32(red) << 24) | (UInt32(green) << 16) | (UInt32(blue) << 8) | (UInt32(alpha) << 0)
    }

    static let bitmapInfo = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Little.rawValue

    static func ==(lhs: RGBA32, rhs: RGBA32) -> Bool {
        return lhs.color == rhs.color
    }

    static let black = RGBA32(red: 0, green: 0, blue: 0, alpha: 255)
    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)
}

To save the image, you can do:

createImage(width: width, height: height, from: array) { image, errorMessage in
    guard let image = image, errorMessage == nil else {
        print(errorMessage!)
        return
    }

    DispatchQueue.main.async {
        self.imageView.image = image
        UIImageWriteToSavedPhotosAlbum(image, self, #selector(self.image(_:didFinishSavingWithError:contextInfo:)), nil)
    }
}

Where

func image(_ image: UIImage, didFinishSavingWithError error: Error?, contextInfo: Any?) {
    guard error == nil else {
        print(error!.localizedDescription)
        return
    }

    print("image saved")
}