The bitmap files are imported into 3D routing programs. The routing program reads the values of the bitmaps and automatically assigns routing depths to the project based on the bitmap file. Black does nothing, White areas are raised to a numeric amount entered via the keyboard. The various shades of grey are raised according to their value.
The end result are some fabulous woodgrains, bumpy surfaces, weaves or anyting else we can imagine - all done with a few clicks of a mouse.