ConvertImageToGrayScale.swift

func grayScale() -> UIImage {
    let width = self.size.width
    let height = self.size.height
    let imageRect:CGRect = CGRectMake(0, 0, width, height)
    let colorSpace = CGColorSpaceCreateDeviceGray()
    
    let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.None.rawValue)
    let context = CGBitmapContextCreate(nil, Int(width), Int(height), 8, 0, colorSpace, bitmapInfo.rawValue)
    
    CGContextDrawImage(context, imageRect, self.CGImage)
    let imageRef = CGBitmapContextCreateImage(context)
    let grayScaleImage = UIImage(CGImage: imageRef!)
    
    return grayScaleImage
  }