UIImage+Grayscale

UIImage+Grayscale.m

@implementation UIImage (Grayscale)

UIImage *grayscaleImageFromCIImage(CIImage *image, CGFloat scale) {
    CIImage *blackAndWhite = [CIFilter filterWithName:@"CIColorControls" keysAndValues:kCIInputImageKey, image, @"inputBrightness", @0.0, @"inputContrast", @1.1, @"inputSaturation", @0.0, nil].outputImage;
    CIImage *output = [CIFilter filterWithName:@"CIExposureAdjust" keysAndValues:kCIInputImageKey, blackAndWhite, @"inputEV", @0.7, nil].outputImage;
    CGImageRef ref = [[CIContext contextWithOptions:nil] createCGImage:output fromRect:output.extent];
    UIImage *result = [UIImage imageWithCGImage:ref scale:scale orientation:UIImageOrientationUp];
    CGImageRelease(ref);
    return result;
}

UIImage *grayscaleImageFromCGImage(CGImageRef imageRef, CGFloat scale) {
    NSInteger width = CGImageGetWidth(imageRef) * scale;
    NSInteger height = CGImageGetHeight(imageRef) * scale;
    
    NSMutableData *pixels = [NSMutableData dataWithLength:width*height];
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    CGContextRef context = CGBitmapContextCreate(pixels.mutableBytes, width, height, 8, width, colorSpace, 0);
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
    CGImageRef ref = CGBitmapContextCreateImage(context);
    UIImage *result = [UIImage imageWithCGImage:ref scale:scale orientation:UIImageOrientationUp];
    
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(ref);
    
    return result;
}

- (UIImage *)grayscaleImage {
    if (self.CIImage)
        return grayscaleImageFromCIImage(self.CIImage, self.scale);
    if (self.CGImage)
        return grayscaleImageFromCGImage(self.CGImage, self.scale);
    return nil;
}

@end