PNG/JPEG representation from CIImage always returns nil - swift

Question

I'm currently making a photo editing app.
When a photo is selected by the user, it is automatically converted into black and white using this code:
func blackWhiteImage(image: UIImage) -> Data {
print("Starting black & white")
let orgImg = CIImage(image: image)
let bnwImg = orgImg?.applyingFilter("CIColorControls", withInputParameters: [kCIInputSaturationKey:0.0])
let outputImage = UIImage(ciImage: bnwImg!)
print("Black & white complete")
return UIImagePNGRepresentation(outputImage)!
}
The problem I am having with this code is that I keep getting this error:
fatal error: unexpectedly found nil while unwrapping an Optional value
I have had my code in a slightly different configuration, but it still breaks when it gets to the UIImagePNG/JPEGRepresentation(xx) section.
Are there any ways to get the PNG or JPEG data from a CIImage for use in an image view / just UIImage in general?
Any of the other methods don't go into enough detail for what code should be used.

Answer 1

Just begin a new graphics context and draw your grayscale image there. iOS 10 or later you can use UIGraphicsImageRenderer, for older iOS version syntax please check edit history:
Xcode 11 • Swift 5.1
func blackWhiteImage(image: UIImage, isOpaque: Bool = false) -> Data? {
guard let ciImage = CIImage(image: image)?.applyingFilter("CIColorControls", parameters: [kCIInputSaturationKey: 0]) else { return nil }
let format = image.imageRendererFormat
format.opaque = isOpaque
return UIGraphicsImageRenderer(size: image.size, format: format).image { _ in
UIImage(ciImage: ciImage).draw(in: CGRect(origin: .zero, size: image.size))
}.pngData()
}
You can also extend UIImage to return a grayscale image :
extension UIImage {
var coreImage: CIImage? { CIImage(image: self) }
func grayscale(isOpaque: Bool = false) -> UIImage? {
guard let coreImage = coreImage?.applyingFilter("CIColorControls", parameters: [kCIInputSaturationKey: 0]) else { return nil }
let format = imageRendererFormat
format.opaque = isOpaque
return UIGraphicsImageRenderer(size: size, format: format).image { _ in
UIImage(ciImage: coreImage).draw(in: CGRect(origin: .zero, size: size))
}
}
}
let profilePicture = UIImage(data: try! Data(contentsOf: URL(string:"http://i.stack.imgur.com/Xs4RX.jpg")!))!
if let grayscale = profilePicture.grayscale(), let data = grayscale.pngData() { // or Swift 4.1 or earlier -> let data = UIImagePNGRepresentation(grayscale)
print(data.count) // 689035
}