What is an anaglyph ?
I won’t explain it on a very technical way. Others already did and I am only a simple “amateur”.
We see the relief, and therefore the third dimension, because we have got two eyes seeing two different view of the same scene. So, our vision of the relief is stereoscopic. Cyclops, with their single eye, see a flat world whereas flies with their loads of eyes should see a very different relief of the World. Ask one of these about it next time you cross their way!
Left eye image Right eye image
Parallel stereoscopic pair (L-R)
Crossed stereoscopic pair (R-L)
Left view through red filter Right view through cyan filter
Mixing both = anaglyph
For more details, see Rafael Mullor’s website (Spanish) explaining the conversion of a stereoscopic pair to an anaglyph in a colorimetric point of view. See French stereoclub’s Website (French) for history of 3D shooting. That page (English) describes stereoscopic composition and shooting rules.
Note: If you look at anaglyphs for too long, everything will appear in 3D (even a flat blank sheet of paper) when removing the bicoloured glasses from your nose. It is scary but not dangerous. Colour vision comes back to normal after a while.