An anaglyph is an image designed to be viewed through special glasses. The technique to make these images is over a hundred years old, and has been used in everything from comic books - each stereo pair was drawn by hand - to movies to the space program.

The methods used to make them can seem intimidating to novices and can be tricky to master, especially when one is presented with modern graphics programs like Photoshop and Paint Shop Pro. This tutorial is intended to help you understand how we make these images in our lab.

This tutorial was written using Adobe Photoshop; the general principles are applicable to other image editors as well. Consult your software's documentation.

We start with a stereo pair - two images taken from separate viewpoints: one for the right eye and one for the left. The camera is translated horizontally but not vertically - that is, both points are along the same imaginary line drawn parallel to the ground. In the case of our example, we use two screenshots from a fully-3D action game. This allows tighter control over the viewpoint since the movement of the "camera" can be much more tightly controlled than in the real world or (unfortunately) in our lab.

These two images were made by snapping a screenshot using the software's built-in screen capture function. The viewpoint was then moved a small distance to one side in order to replicate the disparity introduced by distance between our eyes - using only the sideways-movement controls so that vertical offset will not be introduced - and a second image recorded. In this way, we re-create the two simultaneous images that the human brain processes to create a 3-D rendering of the world in our minds. Notice that there are noticeable differences in the two images, such as the relative placement of the distant container to the break in the window on the far wall; this is the parallax effect.

Because color in stereo pair images can interfere with the 3-D effect, the images are converted to greyscale in an image editor. Only in rare cases where hues in source images are neutral in tone (greys, tans) can this step be skipped.

The left image ... and the right.

While not immediately apparent from the screenshots, these two images differ slightly from one another. The location of the containers, for example, appears to be slightly different when compared against the background wall. This effect, called parallax, is useful for measuring the distance between yourself and distant objects. This technique is often used by astronomers in measuring the distance to other galaxies.

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