UIImage category for inverting the image

UIImage+NegativeImage.h

// Retina-ready version of this StackOverflow answer
// http://stackoverflow.com/a/6672628/123781

#import <UIKit/UIKit.h>

@interface UIImage (NegativeImage)

- (UIImage *)negativeImage;

@end

UIImage+NegativeImage.m

// Retina-ready version of this StackOverflow answer
// http://stackoverflow.com/a/6672628/123781

#import "UIImage+NegativeImage.h"

@implementation UIImage (NegativeImage)

- (UIImage *)negativeImage
{
    // get width and height as integers, since we'll be using them as
    // array subscripts, etc, and this'll save a whole lot of casting
    CGSize size = self.size;
    int width = size.width * self.scale;
    int height = size.height * self.scale;
    
    // Create a suitable RGB+alpha bitmap context in BGRA colour space
    CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *memoryPool = (unsigned char *)calloc(width*height*4, 1);
    CGContextRef context = CGBitmapContextCreate(memoryPool, width, height, 8, width * 4, colourSpace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colourSpace);
    
    // draw the current image to the newly created context
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), [self CGImage]);
    
    // run through every pixel, a scan line at a time...
    for(int y = 0; y < height; y++)
    {
        // get a pointer to the start of this scan line
        unsigned char *linePointer = &memoryPool[y * width * 4];
        
        // step through the pixels one by one...
        for(int x = 0; x < width; x++)
        {
            // get RGB values. We're dealing with premultiplied alpha
            // here, so we need to divide by the alpha channel (if it
            // isn't zero, of course) to get uninflected RGB. We
            // multiply by 255 to keep precision while still using
            // integers
            int r, g, b;
            if(linePointer[3])
            {
                r = linePointer[0] * 255 / linePointer[3];
                g = linePointer[1] * 255 / linePointer[3];
                b = linePointer[2] * 255 / linePointer[3];
            }
            else
            {
                r = g = b = 0;
            }
            
            // perform the colour inversion
            r = 255 - r;
            g = 255 - g;
            b = 255 - b;
            
            // multiply by alpha again, divide by 255 to undo the
            // scaling before, store the new values and advance
            // the pointer we're reading pixel data from
            linePointer[0] = r * linePointer[3] / 255;
            linePointer[1] = g * linePointer[3] / 255;
            linePointer[2] = b * linePointer[3] / 255;
            linePointer += 4;
        }
    }
    
    // get a CG image from the context, wrap that into a
    // UIImage
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    UIImage *returnImage = [UIImage imageWithCGImage:cgImage scale:self.scale orientation:self.imageOrientation];
    
    // clean up
    CGImageRelease(cgImage);
    CGContextRelease(context);
    free(memoryPool);
    
    // and return
    return returnImage;
}


@end