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

All the above solutions rely on CIImage, while UIImage will often have CGImage as its underlying image, not CIImage. So it means you have to convert your underlying image into CIImage in the beginning, and convert it back to CGImage in the end (if you don't, constructing UIImage with CIImage will effectively do it for you).

Although it probably OK for many use cases, the conversion between CGImage and CIImage is not free: it can be slow, and can create a big memory spike while converting.

So I want to mention a completely different solution, that doesn't require converting image back and forth. It's using Accelerate, and it's perfectly described by Apple here.

Here's a playground example that demonstrates both methods.

import UIKit
import Accelerate

extension CIImage {

    func toGrayscale() -> CIImage? {

        guard let output = CIFilter(name: "CIPhotoEffectNoir", parameters: [kCIInputImageKey: self])?.outputImage else {
            return nil
        }

        return output
    }

}

extension CGImage {

    func toGrayscale() -> CGImage {

        guard let format = vImage_CGImageFormat(cgImage: self),
              // The source image bufffer
              var sourceBuffer = try? vImage_Buffer(
                cgImage: self,
                format: format
              ),
              // The 1-channel, 8-bit vImage buffer used as the operation destination.
              var destinationBuffer = try? vImage_Buffer(
                width: Int(sourceBuffer.width),
                height: Int(sourceBuffer.height),
                bitsPerPixel: 8
              ) else {
            return self
        }

        // Declare the three coefficients that model the eye's sensitivity
        // to color.
        let redCoefficient: Float = 0.2126
        let greenCoefficient: Float = 0.7152
        let blueCoefficient: Float = 0.0722

        // Create a 1D matrix containing the three luma coefficients that
        // specify the color-to-grayscale conversion.
        let divisor: Int32 = 0x1000
        let fDivisor = Float(divisor)

        var coefficientsMatrix = [
            Int16(redCoefficient * fDivisor),
            Int16(greenCoefficient * fDivisor),
            Int16(blueCoefficient * fDivisor)
        ]

        // Use the matrix of coefficients to compute the scalar luminance by
        // returning the dot product of each RGB pixel and the coefficients
        // matrix.
        let preBias: [Int16] = [0, 0, 0, 0]
        let postBias: Int32 = 0

        vImageMatrixMultiply_ARGB8888ToPlanar8(
            &sourceBuffer,
            &destinationBuffer,
            &coefficientsMatrix,
            divisor,
            preBias,
            postBias,
            vImage_Flags(kvImageNoFlags)
        )

        // Create a 1-channel, 8-bit grayscale format that's used to
        // generate a displayable image.
        guard let monoFormat = vImage_CGImageFormat(
            bitsPerComponent: 8,
            bitsPerPixel: 8,
            colorSpace: CGColorSpaceCreateDeviceGray(),
            bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue),
            renderingIntent: .defaultIntent
        ) else {
                return self
        }

        // Create a Core Graphics image from the grayscale destination buffer.
        guard let result = try? destinationBuffer.createCGImage(format: monoFormat) else {
            return self
        }

        return result
    }
}

To test, I used a full size of this image.

let start = Date()
var prev = start.timeIntervalSinceNow * -1

func info(_ id: String) {
    print("\(id)\t: \(start.timeIntervalSinceNow * -1 - prev)")
    prev = start.timeIntervalSinceNow * -1
}

info("started")
let original = UIImage(named: "Golden_Gate_Bridge_2021.jpg")!
info("loaded UIImage(named)")

let cgImage = original.cgImage!
info("original.cgImage")
let cgImageToGreyscale = cgImage.toGrayscale()
info("cgImage.toGrayscale()")
let uiImageFromCGImage = UIImage(cgImage: cgImageToGreyscale, scale: original.scale, orientation: original.imageOrientation)
info("UIImage(cgImage)")

let ciImage = CIImage(image: original)!
info("CIImage(image: original)!")
let ciImageToGreyscale = ciImage.toGrayscale()!
info("ciImage.toGrayscale()")
let uiImageFromCIImage = UIImage(ciImage: ciImageToGreyscale, scale: original.scale, orientation: original.imageOrientation)
info("UIImage(ciImage)")

The result (in sec)

CGImage method took about 1 sec. total:

original.cgImage        : 0.5257829427719116
cgImage.toGrayscale()   : 0.46222901344299316
UIImage(cgImage)        : 0.1819549798965454

CIImage method took about 7 sec. total:

CIImage(image: original)!   : 0.6055610179901123
ciImage.toGrayscale()       : 4.969912052154541
UIImage(ciImage)            : 2.395193934440613

When saving images as JPEG to disk, the one created with CGImage was also 3 times smaller than the one created with CIImage (5 MB vs. 17 MB). The quality was good on both images. Here's a small version that fits SO restrictions:

Answer 2

A Swift 4.0 extension that returns an optional UIImage to avoid any potential crashes down the road.

import UIKit

extension UIImage {
    var noir: UIImage? {
        let context = CIContext(options: nil)
        guard let currentFilter = CIFilter(name: "CIPhotoEffectNoir") else { return nil }
        currentFilter.setValue(CIImage(image: self), forKey: kCIInputImageKey)
        if let output = currentFilter.outputImage,
            let cgImage = context.createCGImage(output, from: output.extent) {
            return UIImage(cgImage: cgImage, scale: scale, orientation: imageOrientation)
        }
        return nil
    }
}

To use this:

let image = UIImage(...)
let noirImage = image.noir // noirImage is an optional UIImage (UIImage?)

Answer 3

Joe's answer as an UIImage exension for Swift 4 working correctly for different scales:

extension UIImage {
    var noir: UIImage {
        let context = CIContext(options: nil)
        let currentFilter = CIFilter(name: "CIPhotoEffectNoir")!
        currentFilter.setValue(CIImage(image: self), forKey: kCIInputImageKey)
        let output = currentFilter.outputImage!
        let cgImage = context.createCGImage(output, from: output.extent)!
        let processedImage = UIImage(cgImage: cgImage, scale: scale, orientation: imageOrientation)

        return processedImage
    }
}

Answer 4

As per Joe answer we easily converted Original to B&W . But back to Original image refer these code :

var context = CIContext(options: nil)
var startingImage : UIImage = UIImage()

func Noir() {     
    startingImage = imgView.image!
    var inputImage = CIImage(image: imgView.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.imgView.image =  UIImage(cgImage: cgImage!)

    //Filter Logic
    let currentFilter = CIFilter(name: "CIPhotoEffectNoir")
    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!)
    imgView.image = processedImage
}

func Original(){ 
    imgView.image = startingImage
}

Answer 5

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...

Answer 6

Here's a category in objective c. Note that, critically, this version takes scale into consideration.

- (UIImage *)grayscaleImage{
    return [self imageWithCIFilter:@"CIPhotoEffectMono"];
}

- (UIImage *)imageWithCIFilter:(NSString*)filterName{
    CIImage *unfiltered = [CIImage imageWithCGImage:self.CGImage];
    CIFilter *filter = [CIFilter filterWithName:filterName];
    [filter setValue:unfiltered forKey:kCIInputImageKey];
    CIImage *filtered = [filter outputImage];
    CIContext *context = [CIContext contextWithOptions:nil];
    CGImageRef cgimage = [context createCGImage:filtered fromRect:CGRectMake(0, 0, self.size.width*self.scale, self.size.height*self.scale)];
    // Do not use initWithCIImage because that renders the filter each time the image is displayed.  This causes slow scrolling in tableviews.
    UIImage *image = [[UIImage alloc] initWithCGImage:cgimage scale:self.scale orientation:self.imageOrientation];
    CGImageRelease(cgimage);
    return image;
}

Answer 7

I'd use CoreImage, which may keep the quality.

func convertImageToBW(image:UIImage) -> UIImage {

    let filter = CIFilter(name: "CIPhotoEffectMono")

    // convert UIImage to CIImage and set as input

    let ciInput = CIImage(image: image)
    filter?.setValue(ciInput, forKey: "inputImage")

    // get output CIImage, render as CGImage first to retain proper UIImage scale

    let ciOutput = filter?.outputImage
    let ciContext = CIContext()
    let cgImage = ciContext.createCGImage(ciOutput!, from: (ciOutput?.extent)!)

    return UIImage(cgImage: cgImage!)
}

Depending on how you use this code, you may want to create the CIContext outside of it for performance reasons.