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    <p>To expand on quixoto's answer and because I have relevant source code from a project of my own, if you were to need to drop to on-CPU pixel manipulation then the following, which I've added exposition to, should do the trick:</p> <pre><code>@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; int height = size.height; // 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 &lt; height; y++) { // get a pointer to the start of this scan line unsigned char *linePointer = &amp;memoryPool[y * width * 4]; // step through the pixels one by one... for(int x = 0; x &lt; 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]; // clean up CGImageRelease(cgImage); CGContextRelease(context); free(memoryPool); // and return return returnImage; } @end </code></pre> <p>So that adds a category method to UIImage that:</p> <ol> <li>creates a clear CoreGraphics bitmap context that it can access the memory of</li> <li>draws the UIImage to it</li> <li>runs through every pixel, converting from premultiplied alpha to uninflected RGB, inverting each channel separately, multiplying by alpha again and storing back</li> <li>gets an image from the context and wraps it into a UIImage</li> <li>cleans up after itself, and returns the UIImage</li> </ol>
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    1. POIs programmatically inverting the colors of an image possible?
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    1. COlooks good, I will give it a try
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    2. COWorked awesome, thanks
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    3. COThanks for supplying this code. How could I get this code to work with retina devices? e.g. if (retinaScale>0.0) { UIGraphicsBeginImageContextWithOptions(image.size, NO, retinaScale); } else { UIGraphicsBeginImageContext(image.size); }
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