How to convert a UIImage to Grayscale?

Please do follow this post along with the post here

Note that I have already shown you the coding on how to pick an image from the UIImagePickerViewController and display it in the UIImageView we have in the screen. So, I assume you have already assigned the selected image's value using imageDisplay.image to the sourceImage variable. 

OK, let's discuss the code to convert an image to Grayscale and display the output.

//this button click even twill call the toGrayscale() method 

-(IBAction) convertGrayScale:(id)sender

{    

    if(sourceImage==nil)//sourceImage  is the user selected image from the Image Picker. I simply display it on a UIImageView and for the processing  i get the image that is currently viewed in the UIImageView

    {

        NSLog(@"Image Not selected");

    }

    

    else

    {   

        UIImage *greyScaledImage=[self toGrayscale:sourceImage];

        resultImageView.image=greyScaledImage;

    } 

}

//method is used to convert an image to its grayscale image and view in the UIImageView on te screen

- (UIImage *) toGrayscale:(UIImage *)initialImage

{

    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, initialImage.size.width * initialImage.scale, initialImage.size.height * initialImage.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), [initialImage 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

            

            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:initialImage.scale 

                                           orientation:UIImageOrientationUp];

    

    return resultUIImage;

}