Starting with the famous John Trumbull painting of the signing of the Declaration of Independence, I turned it (or a detail from it, anyway) into a 3D anaglyph by using a “depth map” as a displacement map in Photoshop.

The “depth map” is a separate file the same size as the flat image, with various shades of gray to indicate the depth of every part of the image. White is closest, black is farthest. Using this as a displacement map in Photoshop on the flat image — with a negative X offset for the red channel and a positive X offset for the green and blue channels — turns it into a 3D anaglyph.