The following is an example of extruding circles with its depth controlled by a bitmap image.

surface7.3dm

The input bitmap used in the example is this.

add_library('igeo')

size(480, 360, IG.GL)

IG.open("surface7.3dm")
surfaces = IG.surfaces()

map = IImageMap("map4.jpg")

for surf in surfaces :
    unum = 100
    vnum = 50
    uinc = 1.0/unum
    vinc = 1.0/vnum
    for i in range(unum) :
        for j in range(vnum) : 
            val = map.get( i*uinc, j*vinc )
            
            pt = surf.pt( i*uinc, j*vinc )
            nml = surf.nml( i*uinc, j*vinc )
            cir = ICircle(pt, nml, 0.1)
            
            IG.extrude(cir, val*-2).clr(0)
    surf.del()

Then the generation of circles and surfaces are also controlled by a probabilistic switch responding to the bitmap input.

add_library('igeo')

size(480, 360, IG.GL)

IG.open("surface7.3dm")
surfaces = IG.surfaces()

map = IImageMap("map4.jpg")

for surf in surfaces :
    unum = 200
    vnum = 100
    uinc = 1.0/unum
    vinc = 1.0/vnum
    for i in range(unum) :
        for j in range(vnum) : 
            val = map.get( i*uinc, j*vinc )
            
            if IRand.pct(val*100) : 
                pt = surf.pt( i*uinc, j*vinc )
                nml = surf.nml( i*uinc, j*vinc )
                cir = ICircle(pt, nml, 0.05)
                
                IG.extrude(cir, -2*val).clr(0)
    surf.del()