For color-coated tumblers, every time black text in a drawing is rastered onto a dark color-coated tumbler, a negative image in stainless steel or aluminum is created. With text, that usually isn’t a problem; it’s the contrast that is important. If, however, a photograph, grayscale image, or other graphic where relative colors are important is engraved without inverting the image in the drawing, a negative image will be the result.
To get a positive image, you must use a negative image. The exception is when the color of the tumbler is lighter than the exposed metal, as in some of the can coolers in the photo above. You can see that the engravings appear dark on the light-colored coolers and light on the dark-colored coolers. It comes down to judgment when deciding what to do with these exceptions.
—Jim Puentes, COOLaserCraft
Read more about how to invert graphics for laser engraving.