UIImage+Grayscale is an Objective-C category for converting a UIImage to 
grayscale taking into account retina displays.

The usage is quite simple, import the header file where needed and use it to 
produce a grayscale version of an UIImage, like so:

UIImage *grayscaleImg = [UIImage imageWithContentsOfFile:img] convertToGrayscale]

Please have a look at COPYRIGHT for original credits and to the comments in 
the .m file for interesting insights on grayscale conversion.