Convert [UInt8] into white and transparent image - swift

Question

I'm trying to create a white and transparent image in swift from a [Uint8] array. The array has width * height elements and each element correspond to the transparency (alpha value).
So far, I managed to create a black and white image using this :
guard let providerRef = CGDataProvider(data: Data.init(bytes: bitmapArray) as CFData) else { return nil }
guard let cgImage = CGImage(
width: width,
height: height,
bitsPerComponent: 8,
bitsPerPixel: 8,
bytesPerRow: width,
space: CGColorSpaceCreateDeviceGray(),
bitmapInfo: CGBitmapInfo.init(rawValue: CGImageAlphaInfo.none.rawValue),
provider: providerRef,
decode: nil,
shouldInterpolate: true,
intent: .defaultIntent
) else {
return nil
}
let image = UIImage(cgImage: cgImage)
unfortunately, as I said, this gives me a black and white image.
What I would like is to turn every black pixel (0 in my initial array) into a completely transparent pixel (my array contains only either 0 or 255). How do I do that ?
PS: I've tried to use CGImageAlphaInfo.alphaOnlybut I get "CGImageCreate: invalid image alphaInfo: 7"
Any help would be appreciated.

Answer 1

I found a solution that doesn't exactly satisfies me in terms of code elegance but does the job. The solution is to create that black and white full opaque image, and mask all black pixels using a CIFilter.
Here's a working code:
guard let providerRef = CGDataProvider(data: Data.init(bytes: bitmapArray) as CFData) else { return nil }
guard let cgImage = CGImage(
width: width,
height: height,
bitsPerComponent: 8,
bitsPerPixel: 8,
bytesPerRow: width,
space: CGColorSpaceCreateDeviceGray(),
bitmapInfo: CGBitmapInfo.init(rawValue: CGImageAlphaInfo.none.rawValue),
provider: providerRef,
decode: nil,
shouldInterpolate: true,
intent: .defaultIntent
) else {
return nil
}
let context = CIContext(options: nil)
let ciimage = CIImage(cgImage: cgImage)
guard let filter = CIFilter(name: "CIMaskToAlpha") else { return nil }
filter.setDefaults()
filter.setValue(ciimage, forKey: kCIInputImageKey)
guard let result = filter.outputImage else { return nil }
guard let newCgImage = context.createCGImage(result, from: result.extent) else { return nil }
return UIImage(cgImage: newCgImage)
Feel free to provide your own (perhaps more elegant/optimal) solution !

Answer 2

I found a workaround: since kCGAlphaImageOnly is supported by CGBitmapContext, you can create a bitmap context from the data, then create an image from that context. This is Objective-C, but it shouldn't be too hard to translate into Swift:
CGContextRef ctx = CGBitmapContextCreate(
bitmapArray, width, height,
8, width, NULL, (CGBitmapInfo)kCGImageAlphaOnly
);
CGImageRef image = CGBitmapContextCreateImage(ctx);