The above image is what is known as a parallel stereo pair. Believe it or not, you probably have seen something like this before advertised under the name ViewMaster
. ViewMasters are a modern incarnation of what is officially called a stereoscope. Stereoscopes have been around for over a hundred years and are widely used today in the field of microscopy. The Wikipedia article on stereoscopes is quite informative.The trick to "free-viewing" image pairs like this is to pick a common point on each and try focusing "through" it. The two images will merge and you will begin to see three. While the outer two will be semi-transparent the middle one will be fully visible and 3D! This technique is different from crossing your eyes; that requires what is known as a cross-eyed stereo-pair.
There is another type of stereogram called Anaglyphs. These most commonly have one color blocked out for one of the images and another color blocked out for the other (usually red and blue). These are what most "3D Movies" usually are. Unfortunately, viewing this type of stereogram requires special color filtering glasses, like this:
The downside to the common red/cyan stereogram is that you lose quite a bit of color of the original images and your brain has to fill in the blanks. One way stereo technology has improved in this area are what's known as linearly polarized anaglyphs -- that is the method they employed in the movie "Avatar
".The final type of stereogram I want to mention is what is sometimes referred to as a wigglegram. This is done by taking two images and rapidly switching between the two, like this:
