Presence means somthing like "How many of this object to i want in my Ecosystem Vs these other objects?" The proportion of each object Vs the total objects.

As mentioned above, the scale and presence affect the number in the population. That is, if you make them too big, not many will fit - depending on the size of your underlying (base) object.

Look in the Back Room and do a search for "ecosystem" and "map" and such, to find one of the several tutorials on using a B&W image to control an Ecosystem. Too complex for a single post to cover.

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