CGimage+GrayscalePixelData.swift

import CoreGraphics

extension CGImage {

    func grayscalePixelData() -> [UInt8] {
        let startTime = CACurrentMediaTime()
        let size = CGSize(width: self.width,
                          height: self.height)
        let dataSize = size.width * size.height
        var pixelData = [UInt8](repeating: 0,
                                count: Int(dataSize))
        let colorSpace = CGColorSpaceCreateDeviceGray()
        let context = CGContext(data: &pixelData,
                                width: Int(size.width),
                                height: Int(size.height),
                                bitsPerComponent: 8,
                                bytesPerRow: 1 * Int(size.width),
                                space: colorSpace,
                                bitmapInfo: CGImageAlphaInfo.none.rawValue)!
        context.draw(self,
                     in: CGRect(origin: .zero,
                                size: .init(width: size.width,
                                            height: size.height)))
        #if DEBUG
        print("Grayscale time: \(CACurrentMediaTime() - startTime)")
        #endif

        return pixelData
    }

}

#if os(macOS)
extension NSImage {

    var cgImage: CGImage? {
        return self.cgImage(forProposedRect: nil,
                            context: nil,
                            hints: nil)
    }

}
#endif