Thanks for your reply and links. After trying some more angles, it seems "up" and "down" lighting makes the difference between "innies" and "outies", which hin

Thanks for your reply and links. After trying some more angles, it seems "up" and "down" lighting makes the difference between "innies" and "outies", which hints the brain's interpretation of lights and shadows is in play here. "Left" or "right" doesn't seem to make a big difference.