dGeomPlaneGetParams ((DPlane)g,n4);
dRFromZAxis (R,n4.get0(),n4.get1(),n4.get2());
for (j=0; j<3; j++) pos.set(j, n4.get(j)*n4.get3());
pos.add(2, Z_OFFSET );
for (j=0; j<3; j++) pos2.set(j, pos.get(j) + 0.1*n4.get(j));
dsDrawLine (pos,pos2);
dsSetColorAlpha (1f,0f,1f,0.8f);