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    <p>I needed a version that preserved the alpha channel, so I modified the code posted by Dutchie432:</p> <pre><code>@implementation UIImage (grayscale) typedef enum { ALPHA = 0, BLUE = 1, GREEN = 2, RED = 3 } PIXELS; - (UIImage *)convertToGrayscale { CGSize size = [self size]; int width = size.width; int height = 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 &lt; height; y++) { for(int x = 0; x &lt; width; x++) { uint8_t *rgbaPixel = (uint8_t *) &amp;pixels[y * width + x]; // convert to grayscale using recommended method: http://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale uint32_t gray = 0.3 * rgbaPixel[RED] + 0.59 * rgbaPixel[GREEN] + 0.11 * rgbaPixel[BLUE]; // 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]; // we're done with image now too CGImageRelease(image); return resultUIImage; } @end </code></pre>
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    1. POConvert image to grayscale
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    1. COIt works, but not support Retina display, ruralcoder(below) updates this.
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