Have you ever laser engraved a bitmap and a second pass was needed on only a small portion of the image, perhaps a very large word with thick strokes? Or maybe the laser stopped in the middle of the job and you have to restart the laser and the job. You don't want to laser the whole image again or you don't want to edit the bitmap or risk moving it?
Just make a shape such as a rectangle, fill it with pure white without any outline, and place it over the portion of the bitmap you don't want to re-engrave. Now send the job to the laser and only the portion left uncovered will be lasered. Test this solution out on your own laser to make sure it works before you really need it.
—Bob Hagel, Eagle's Mark