The most simple method of presenting 3D imagery is by showing two pictures or films side by side, each of which is shot from a slightly different angle. A mechanical device ensures that each image is seen by only one eye and the viewer's brain interprets this as three dimensional. This technique was first developed in the 1850s, and has since been used in toys like the stereoscope and View-Master to present still photography in three dimensions. Parallel projection is rarely used with motion pictures.
Imagery presented in anaglyph format is also shot from two slightly differing angles, but these two images are then overlaid and projected in either two different colors (red and blue, usually) or polarizations. Colored or polarized glasses must be worn to correctly perceive the anaglyph in 3D. Red and blue anaglyphs work best with black-and-white imagery, while the newer polarized technology allows viewers to perceive 3D images in full color. This method is what is generally used to present 3D film in movie theaters.
This type of 3D display involves a solid screen with vertical slits in it placed directly in front of another screen with two overlapping images projected onto it in vertical stripes. The barrier makes each eye see a slightly different image, allowing the viewing of 3D imagery without any glasses or headset required. However, the viewer cannot be seated too far to the left or right or the effect will be lost. This technique is used in Sharp televisions and some billboards.
Using a rotating mirror, 3D images can actually be projected into three dimensions, allowing the image to be seen from every angle without special equipment. This method can be used to produce genuine 3D animated images, and can be used to project color if a multi-sided mirror is used. This method is primarily experimental, and as of June 2010, is not yet in public use.