How to read the average color of a UIImage using CIAreaAverage

Swift version: 5.4

Paul Hudson @twostraws July 7th 2020

Core Image has a filter that resamples an image down to 1x1 pixels so you can read the most dominant color in an image, although it’s weirdly hard to use.

To simplify things, here’s an extension on UIImage that returns an optional UIColor – it will be nil only if something went wrong while reading your image, but otherwise it will contain the average color for the entire image:

extension UIImage {
    var averageColor: UIColor? {
        guard let inputImage = CIImage(image: self) else { return nil }
        let extentVector = CIVector(x: inputImage.extent.origin.x, y: inputImage.extent.origin.y, z: inputImage.extent.size.width, w: inputImage.extent.size.height)

        guard let filter = CIFilter(name: "CIAreaAverage", parameters: [kCIInputImageKey: inputImage, kCIInputExtentKey: extentVector]) else { return nil }
        guard let outputImage = filter.outputImage else { return nil }

        var bitmap = [UInt8](repeating: 0, count: 4)
        let context = CIContext(options: [.workingColorSpace: kCFNull])
        context.render(outputImage, toBitmap: &bitmap, rowBytes: 4, bounds: CGRect(x: 0, y: 0, width: 1, height: 1), format: .RGBA8, colorSpace: nil)

        return UIColor(red: CGFloat(bitmap[0]) / 255, green: CGFloat(bitmap[1]) / 255, blue: CGFloat(bitmap[2]) / 255, alpha: CGFloat(bitmap[3]) / 255)
    }
}

As you can see, that reads in the source image and creates an extent for the full image. It then uses the “CIAreaAverage” filter to do the actual work, then renders the average color to a 1x1 image. Finally, it reads each of the color values into a UIColor, and sends it back.