Turtle turtle = (Turtle) agent;
boolean riding_agent = (world.observer().perspective() == PerspectiveJ.RIDE())
&& (world.observer().targetAgent() == turtle);
return !riding_agent && !turtle.hidden()
&& (turtle.alpha() > 0.0 || turtle.hasLabel());
} else if (agent instanceof Link) {
Link link = (Link) agent;
return !link.hidden() && (link.alpha() > 0.0 || link.hasLabel());
} else if (agent instanceof Patch3D) {