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) {
// Patch3D supports the alpha variable, so check Patch3D
// before checking the regular Patch.
Patch3D patch = (Patch3D) agent;