Convert UIImage to grayscale keeping image quality

Question

I have this extension (found in obj-c and I converted it to Swift3) to get the same UIImage but grayscaled:

public func getGrayScale() -> UIImage
{
    let imgRect = CGRect(x: 0, y: 0, width: width, height: height)

    let colorSpace = CGColorSpaceCreateDeviceGray()

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

    let imageRef = context!.makeImage()
    let newImg = UIImage(cgImage: imageRef!)

    return newImg
}

I can see the gray image but its quality is pretty bad... The only thing I can see that's related to the quality is bitsPerComponent: 8 in the context contructor. However looking at Apple's doc, here is what I get:

It shows that iOS only supports 8bpc... Thus why can't I improve the quality ?

Answer 1

Try below code:

Note: code Updated and error been fixed...

• Code tested in Swift 3.
• originalImage is the image that you trying to convert.

Answer 1:

     var context = CIContext(options: nil)

Update: CIContext is the Core Image component that handles rendering and All of the processing of a core image is done in a CIContext. This is somewhat similar to a Core Graphics or OpenGL context.For more info available in Apple Doc.

     func Noir() {

        let currentFilter = CIFilter(name: "CIPhotoEffectNoir") 
        currentFilter!.setValue(CIImage(image: originalImage.image!), forKey: kCIInputImageKey)
        let output = currentFilter!.outputImage 
        let cgimg = context.createCGImage(output!,from: output!.extent)
        let processedImage = UIImage(cgImage: cgimg!)
        originalImage.image = processedImage
      }

Also you need to Considered following filter that can produce similar effect

• CIPhotoEffectMono
• CIPhotoEffectTonal

Output from Answer 1:

Output from Answer 2:

Improved answer :

Answer 2: Auto adjusting input image before applying coreImage filter

var context = CIContext(options: nil)

func Noir() {


    //Auto Adjustment to Input Image
    var inputImage = CIImage(image: originalImage.image!)
    let options:[String : AnyObject] = [CIDetectorImageOrientation:1 as AnyObject]
    let filters = inputImage!.autoAdjustmentFilters(options: options)

    for filter: CIFilter in filters {
       filter.setValue(inputImage, forKey: kCIInputImageKey)
   inputImage =  filter.outputImage
      }
    let cgImage = context.createCGImage(inputImage!, from: inputImage!.extent)
    self.originalImage.image =  UIImage(cgImage: cgImage!)

    //Apply noir Filter
    let currentFilter = CIFilter(name: "CIPhotoEffectTonal") 
    currentFilter!.setValue(CIImage(image: UIImage(cgImage: cgImage!)), forKey: kCIInputImageKey)

    let output = currentFilter!.outputImage 
    let cgimg = context.createCGImage(output!, from: output!.extent)
    let processedImage = UIImage(cgImage: cgimg!)
    originalImage.image = processedImage

}

Note: If you want to see the better result.You should be testing your code on real device not in the simulator...