Note how the writing is dark against light backgrounds and light against dark backgrounds and hardly visible against gray backgrounds. Compare the result with that described under OR. In fact XORing is not particularly good for producing easy to read text on gray backgrounds --- we might do better just to add a constant offset to the image pixels that we wish to highlight (assuming wraparound under addition overflow) --- but it is often used to quickly produce highlighted pixels where the background is just black and white or where legibility is not too important.