There are a few ways to make marks on fabrics. The two most common methods are burning and fading the color. Burning is usually done on lighter colored material. While not recommended if the integrity of the material is to be preserved, it is acceptable for display pieces. In this case, a mark is made that is darker than the material being marked, so the graphics in the drawing do not need to be inverted. A positive image should be used.
Fading the color on material is usually done with a low power, high speed raster on dark material, like blue jeans or cotton duck canvas. A lighter mark is being made, so the graphics – at least any grayscale or photographic images – in the drawing should be inverted.
-Jim Puentes, COOLaserCraft