Given an UIImage return a new UIImage with the colors in gray scale

gistfile1.txt

- (UIImage *)convertToGrayscale:(UIImage*)img {
    
    CGImageRef imageRef = [img CGImage];
    int widthI = CGImageGetWidth(imageRef);
    int heightI = CGImageGetHeight(imageRef);
    
    // the pixels will be painted to this array
    uint32_t *pixels = (uint32_t *) malloc(widthI * heightI * 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, widthI, heightI, 8, widthI * sizeof(uint32_t), colorSpace,
                                                 kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast);
    
    // paint the bitmap to our context which will fill in the pixels array
    CGContextDrawImage(context, CGRectMake(0, 0, widthI, heightI), [img CGImage]);
    
    for(int y = 0; y < heightI; y++) {
        for(int x = 0; x < widthI; x++) {
            uint8_t *rgbaPixel = (uint8_t *) &pixels[y * widthI + 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 scale:1.0 orientation:img.imageOrientation];
    
    // we're done with image now too
    CGImageRelease(image);
    
    return resultUIImage;
}