What is a SIRDS/Stereogram/Hollusion/SIS?

Have you walked through a mall lately? These days, as you wander past most of the poster shops, there will be a large group of people staring at the same poster with surprisingly weird expressions on their faces. Some will be in the initial stages of denial or rejection---they will be concentrating, some slowly rocking their heads backwards and forwards, searching for an image that they have never seen before. Others will be grinning from ear to ear, pointing at the poster, chuckling with their friends that a member of their group hasn't seem them yet. "Come on Bill, come on!", they cry and as Bill gets increasingly more frustrated he concentrates harder and harder, until finally (if he's lucky) he sees a true 3D image, without the need for special glasses or equipment.

These pictures are known as Single Image Random Dot Stereograms (SIRDS), or Single Image Stereograms (SIS) depending on whether the picture contains random dots as a base for the 3D effect, or a repetitive pattern. Unfortunately, each commercial company has labelled them differently. Shop owners generally don't know what you mean, unless you say "Hollusion" or one of the many other specific names.