https://stackoverflow.com/q/56966883/1271826

AccelerateAverageColor.swift

import Accelerate

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 as Any])
        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: 1)
    }

    var averageColorS: UIColor? {
        guard let inputCGImage = cgImage else {
            print("unable to get cgImage")
            return nil
        }

        let colorSpace       = CGColorSpaceCreateDeviceRGB()
        let width            = inputCGImage.width
        let height           = inputCGImage.height
        let bytesPerPixel    = 4
        let bitsPerComponent = 8
        let bytesPerRow      = bytesPerPixel * width
        let bitmapInfo       = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Big.rawValue

        guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {
            print("unable to create context")
            return nil
        }
        context.draw(inputCGImage, in: CGRect(x: 0, y: 0, width: width, height: height))

        guard let buffer = context.data else {
            print("unable to get context data")
            return nil
        }

        let pixelBuffer = buffer.bindMemory(to: UInt8.self, capacity: width * height)

        let pixelCount = width * height
        let channelsPerPixel: Int = 4

        let m: Int32 = Int32(channelsPerPixel)
        let n: Int32 = Int32(pixelCount)
        let lda = m

        var a = [Float](repeating: 0, count: pixelCount * channelsPerPixel)

        vDSP_vfltu8(pixelBuffer, vDSP_Stride(1), &a, vDSP_Stride(1), vDSP_Length(pixelCount * channelsPerPixel))

        var x = [Float](repeating: 1 / Float(pixelCount) / 255, count: pixelCount)

        var y = [Float](repeating: 0, count: channelsPerPixel)

        cblas_sgemv(CblasColMajor, CblasNoTrans, m, n, 1, &a, lda, &x, 1, 1, &y, 1)

        return UIColor(red: CGFloat(y[0]), green: CGFloat(y[1]), blue: CGFloat(y[2]), alpha: 1)
    }

    var averageColorD: UIColor? {
        guard let inputCGImage = cgImage else {
            print("unable to get cgImage")
            return nil
        }

        let colorSpace       = CGColorSpaceCreateDeviceRGB()
        let width            = inputCGImage.width
        let height           = inputCGImage.height
        let bytesPerPixel    = 4
        let bitsPerComponent = 8
        let bytesPerRow      = bytesPerPixel * width
        let bitmapInfo       = CGImageAlphaInfo.premultipliedLast.rawValue | CGBitmapInfo.byteOrder32Big.rawValue

        guard let context = CGContext(data: nil, width: width, height: height, bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: colorSpace, bitmapInfo: bitmapInfo) else {
            print("unable to create context")
            return nil
        }
        context.draw(inputCGImage, in: CGRect(x: 0, y: 0, width: width, height: height))

        guard let buffer = context.data else {
            print("unable to get context data")
            return nil
        }

        let pixelBuffer = buffer.bindMemory(to: UInt8.self, capacity: width * height)

        let pixelCount = width * height
        let channelsPerPixel: Int = 4

        let m: Int32 = Int32(channelsPerPixel)
        let n: Int32 = Int32(pixelCount)
        let lda = m

        var a = [Double](repeating: 0, count: pixelCount * channelsPerPixel)

        vDSP_vfltu8D(pixelBuffer, vDSP_Stride(1), &a, vDSP_Stride(1), vDSP_Length(pixelCount * channelsPerPixel))

        var x = [Double](repeating: 1 / Double(pixelCount) / 255, count: pixelCount)

        var y = [Double](repeating: 0, count: channelsPerPixel)

        cblas_dgemv(CblasColMajor, CblasNoTrans, m, n, 1, &a, lda, &x, 1, 1, &y, 1)

        return UIColor(red: CGFloat(y[0]), green: CGFloat(y[1]), blue: CGFloat(y[2]), alpha: 1)
    }

}