-(UIImage *)grayImage:(UIImage *)sourceImage{ int bitmapInfo = kCGImageAlphaNone; int width = sourceImage.size.width; int height = sourceImage.size.height; CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray(); CGContextRef context = CGBitmapContextCreate (nil, width, height, 8, // bits per component 0, colorSpace, bitmapInfo); CGColorSpaceRelease(colorSpace); if (context == NULL) { return nil; } CGContextDrawImage(context, CGRectMake(0, 0, width, height), sourceImage.CGImage); UIImage *grayImage = [UIImage imageWithCGImage:CGBitmapContextCreateImage(context)]; CGContextRelease(context); return grayImage;}原文：http://www.cnblogs.com/guochaoboke/p/5257350.html