Convert UIImage colored to grayscale using CGColorSpaceCreateDeviceGray()

Question

I am trying to convert a UIImage to GrayScale. All it make is totally a black image.

func convertToGrayScale2(image: UIImage) -> UIImage {
    let imageRect:CGRect = CGRect(x:0, y:0, width:image.size.width, height: image.size.height)
    let colorSpace = CGColorSpaceCreateDeviceGray()
    let width = image.size.width
    let height = image.size.height

    let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue)
    let context = CGContext(data: nil, width: Int(width), height: Int(height), bitsPerComponent: 8, bytesPerRow: 0, space: colorSpace, bitmapInfo: bitmapInfo.rawValue)
    let imageRef = context!.makeImage()
    context?.draw(imageRef!, in: imageRect)
    let newImage = UIImage(cgImage: imageRef!)

    return newImage
}

Is there anything wrong with the code? I need help with existing code. It was working previously. Suddenly its not working.

You probably should draw into the context *before* making an image from it. Also `bytesPerRow: 0` looks wrong. — Martin R, Mar 27 '17 at 07:36
BytesPerRow : 0 is correct for this answer. I should draw before making an image — Rubaiyat Jahan Mumu, Mar 27 '17 at 10:07

score 5 · Accepted Answer · answered Mar 27 '17 at 10:04

func convertToGrayScale(image: UIImage) -> UIImage {
        let imageRect:CGRect = CGRect(x:0, y:0, width:image.size.width, height: image.size.height)
        let colorSpace = CGColorSpaceCreateDeviceGray()
        let width = image.size.width
        let height = image.size.height

        let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue)
        let context = CGContext(data: nil, width: Int(width), height: Int(height), bitsPerComponent: 8, bytesPerRow: 0, space: colorSpace, bitmapInfo: bitmapInfo.rawValue)
        //have to draw before create image

        context?.draw(image.cgImage!, in: imageRect)
        let imageRef = context!.makeImage()
        let newImage = UIImage(cgImage: imageRef!)

        return newImage
    }

Changes the color, but rotates 90 degrees. Very odd. – Adrian Oct 16 '17 at 21:42 — Adrian, Oct 16 '17 at 21:42
You have to add a rotation function. – Rubaiyat Jahan Mumu Oct 17 '17 at 09:02 — Rubaiyat Jahan Mumu, Oct 17 '17 at 09:02

score 2 · Answer 2 · edited Mar 27 '17 at 11:36

You can use this function instead:

func convertImageToDifferentColorScale(with originalImage:UIImage, imageStyle:String) -> UIImage {
    let currentFilter = CIFilter(name: imageStyle)
    currentFilter!.setValue(CIImage(image: originalImage), forKey: kCIInputImageKey)
    let output = currentFilter!.outputImage
    let context = CIContext(options: nil)
    let cgimg = context.createCGImage(output!,from: output!.extent)
    let processedImage = UIImage(cgImage: cgimg!)
    return processedImage
}

And here is the example of how you use:

 let newImage = convertImageToDifferentColorScale(with: UIImage(named: "0318_1")!, imageStyle: "CIPhotoEffectNoir")
 testingImageView.image = newImage

In imageStyle part, there are several different kind of style like below:

 CIColorCrossPolynomial
 CIColorCube
 CIColorCubeWithColorSpace
 CIColorInvert
 CIColorMap
 CIColorMonochrome
 CIColorPosterize
 CIFalseColor
 CIMaskToAlpha
 CIMaximumComponent
 CIMinimumComponent
 CIPhotoEffectChrome
 CIPhotoEffectFade
 CIPhotoEffectInstant
 CIPhotoEffectMono
 CIPhotoEffectNoir

And 'CIPhotoEffectNoir' is one choice that can give you gray scale.

This will not create an image with DeviceGray color space( kcGColorSpaceModelMonochrome), but kCGColorSpaceDeviceRGB. If the output of this solution were to be used as a masking image cgImage.masking() e.g., it would not work, since that would require DeviceGray color space. — joqqy, Aug 18 '22 at 22:42

score 0 · Answer 3 · edited May 23 '17 at 10:30

0

A possible alternative maybe to use CoreImage filters. Please refer to the following SO Post and its Accepted Answer. Hope it helps. Happy Coding!

edited May 23 '17 at 10:30

Community

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answered Mar 27 '17 at 07:38

Md. Ibrahim Hassan

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is that method supposed take forever and a day to execute? – Adrian Oct 16 '17 at 21:46
no it usually does not take any noticeable time for images of normal resolution. What resolution is your image? – Md. Ibrahim Hassan Oct 17 '17 at 18:20
I was just using a picture taken with an iPhone 8. I didn't look at the size, but it wasn't very big. – Adrian Oct 17 '17 at 18:24
Yes the image size would not be very large in that case. Maybe the issue is not due to image resolution – Md. Ibrahim Hassan Oct 17 '17 at 18:25
I can't remember what I did, but I found something a little faster. – Adrian Oct 17 '17 at 20:44

Convert UIImage colored to grayscale using CGColorSpaceCreateDeviceGray()

3 Answers3