I've seen tutorials for roads, using Ecosystems, controlled by grayscale image maps. But roads and trails don't have much depth, not sure how this would work for a tall structure like a wall.

Try a searches on "bitmap", "ecosystem", "map", "mapping"?

_ jc...'Art Head Start' e-book
.......'Art Head Start.com site Digital Art skills. Free lighting chapter, tutes, Vue models, tex pix.