Convert RGB Image to Grayscale and Grayscale to RGB Image?

Question

I have successfully converted an Image to Grayscale, I want to revert the Grayscale Image back to RGB Image. Please help. Thanks in advance.

-(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;
}

Answer 1

To answer the question about converting, your code for grayscale:

uint8_t gray = (uint8_t) ((30 * rgbaPixel[RED] + 59 * rgbaPixel[GREEN] + 11 * rgbaPixel[BLUE]) / 100);

gives the relationship:

S = 0.3R + 0.59G + 0.11B

Go go from RGB to S involves solving for one unknown (S) with one equation (fine!). To convert back is like trying for three unknowns (RGB) given one equation which isn't possible.

One hack to doing a grayscale colorisation is to consider grayscale as just intensity, and set

R = G = B = S - but this isn't going to restore your colors correctly (obviously). So in short, conversion to grayscale is an irreversible function, like squaring a number (was it positive or negative?) - information is lost and can't be retrieved.