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copied!<p>Based on Cam's code with the ability to deal with the scale for Retina displays.</p> <pre><code>- (UIImage *) toGrayscale { const int RED = 1; const int GREEN = 2; const int BLUE = 3; // Create image rectangle with current image width/height CGRect imageRect = CGRectMake(0, 0, self.size.width * self.scale, self.size.height * self.scale); int width = imageRect.size.width; int height = imageRect.size.height; // the pixels will be painted to this array uint32_t *pixels = (uint32_t *) malloc(width * height * sizeof(uint32_t)); // clear the pixels so any transparency is preserved memset(pixels, 0, width * height * sizeof(uint32_t)); CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); // create a context with RGBA pixels CGContextRef context = CGBitmapContextCreate(pixels, width, height, 8, width * sizeof(uint32_t), colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast); // paint the bitmap to our context which will fill in the pixels array CGContextDrawImage(context, CGRectMake(0, 0, width, height), [self CGImage]); for(int y = 0; y < height; y++) { for(int x = 0; x < width; x++) { uint8_t *rgbaPixel = (uint8_t *) &pixels[y * width + x]; // convert to grayscale using recommended method: http://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale uint8_t gray = (uint8_t) ((30 * rgbaPixel[RED] + 59 * rgbaPixel[GREEN] + 11 * rgbaPixel[BLUE]) / 100); // set the pixels to gray rgbaPixel[RED] = gray; rgbaPixel[GREEN] = gray; rgbaPixel[BLUE] = gray; } } // create a new CGImageRef from our context with the modified pixels CGImageRef image = CGBitmapContextCreateImage(context); // we're done with the context, color space, and pixels CGContextRelease(context); CGColorSpaceRelease(colorSpace); free(pixels); // make a new UIImage to return UIImage *resultUIImage = [UIImage imageWithCGImage:image scale:self.scale orientation:UIImageOrientationUp]; // we're done with image now too CGImageRelease(image); return resultUIImage; } </code></pre>

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