Question 199070 in IOS Swift Question posted in
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Ios swift – How to apply mask to an image with different grey scale values?

I’m trying to apply this mask to the original image

enter image description here

Notice there are some grey areas, I want to keep everything except PURE black.

I’ve found this code

 func maskImage(image: UIImage, mask: UIImage) -> UIImage {
    let imageReference = (image.cgImage)!
    let maskReference = (mask.cgImage)!

    let imageMask = CGImage(
        maskWidth: maskReference.width
        , height: maskReference.height
        , bitsPerComponent: maskReference.bitsPerComponent
        , bitsPerPixel: maskReference.bitsPerPixel
        , bytesPerRow: maskReference.bytesPerRow
        , provider: maskReference.dataProvider!
        , decode: nil
        , shouldInterpolate: true
    )

    return (UIImage(cgImage: (imageReference.masking(imageMask!))!))
}

But it does the opposite, it removes all white pixels instead.

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2

Answers


  1. 0 
    
func maskImage(image: UIImage, mask: UIImage) -> UIImage {
    let imageReference = image.cgImage!
    let maskReference = mask.cgImage!
    
    let width = maskReference.width
    let height = maskReference.height
    let bitsPerComponent = maskReference.bitsPerComponent
    let bitsPerPixel = maskReference.bitsPerPixel
    let bytesPerRow = maskReference.bytesPerRow
    
    let context = CGContext(
        data: nil,
        width: width,
        height: height,
        bitsPerComponent: bitsPerComponent,
        bytesPerRow: bytesPerRow,
        space: CGColorSpaceCreateDeviceGray(),
        bitmapInfo: CGImageAlphaInfo.none.rawValue
    )
    
    context!.draw(maskReference, in: CGRect(x: 0, y: 0, width: width, height: height))
    
    let maskImage = context!.makeImage()!
    
    let maskedReference = imageReference.masking(maskImage)!
    
    return UIImage(cgImage: maskedReference)
}

    This code creates a new CGContext specifically for the mask image and sets the color space to grayscale (CGColorSpaceCreateDeviceGray()) to ensure that only black pixels are treated as opaque. The CGImageAlphaInfo.none parameter is used to specify that the mask image has no alpha channel.

    By drawing the maskReference into the context, the mask is created with the desired behavior of keeping all non-black pixels. Then, the maskImage is used to mask the imageReference, resulting in an image with the desired effect.

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  2. 0

    Make the mask inverted before applying it like:

    let maskReference = (mask.inverseImage()!.cgImage)!

    Using this simple extension:

    extension UIImage {
    func inverseImage() -> UIImage? {
        let coreImage = CIImage(image: self)
        guard let filter = CIFilter(name: "CIColorInvert") else { return nil }
        filter.setValue(coreImage, forKey: kCIInputImageKey)
        guard let result = filter.value(forKey: kCIOutputImageKey) as? CIImage else { return nil }
        guard let cgImage = CIContext(options: nil).createCGImage(result, from: result.extent) else { return UIImage(ciImage: result) }
        return UIImage(cgImage: cgImage)
    }
}

    By the way, I suggest inverting the mask outside the code for better performance if possible

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