Mary Ella – this statue of a child who died in 1875 is still the most visited monument at one of the largest cemeteries in the United States. She is never seen without fresh flowers in her arms. The stereo pair was originally photographed in infrared, using a Harrison & Harrison 89B glass filter on a Nikon Coolpix 990.

If you gently cross your eyes until you see a third image in the middle, the middle image will be three-dimensional. For the subtractive illusion, I extrapolated from the concept of subtractive color used in printing, postulating that complementary magenta and cyan tints would cancel each other out in the brain. Therefore, the 3D image you see in between will be perceived as black and white. This effect is to my knowledge original.

 Frequently, the first response I get when I show someone a cross-eye stereo pair, is, “I can’t cross my eyes!” Of course you can – the key is to do it gently, without forcing. Try this experiment:

Now try this: With the Mary Ella stereo pair centered on your screen, again touch your nose with your index finger. Slowly move your finger out in front of you, staying focused on the finger.

Behind your finger, you will notice the middle image forming on the screen – you should already be able to see it in black and white. Keep focused on your finger while you move it out slowly – on the screen, you will notice the image of Mary Ella converging slowly as you keep moving your finger out.

When she is aligned, drop your finger and you should be able to see the 3D image, in black and white, clearly and without eye strain.

The subtractive illusion is intriguing because, while it is perfectly reasonable conceptually, it makes no sense experientially; like adding two positive integers and ending up with zero.

And, while it apparently works equally well on color and black and white photos, as a filter overlay, if the pair of filters is applied to a white background, the results perceptually (while viewing as a cross-eye stereo pair) are a gray patch between the two tints, or, alternatively, a magenta and cyan mix.

In the 3D image, something interesting appears to be happening in the brain – at the same time the 2D halves are being combined to produce the perception of depth, the colors of the tints are being subtracted from our final perception.

If you take two exact copies of the same image, combine them into a stereo pair and apply the tints, the subtractive illusion still persists – the tints vanish – but the combined image, while now flat, is by some people perceived as concave, like a photo held by the sides and bowed inward.

Most 3D aficionados I’ve corresponded with report a slight shift in state of consciousness while viewing in 3D, whether the materials are stereo pairs, anaglyphs or Magic Eye(TM) stereograms.

This “felt-sense” change in awareness is generally pleasant, and indicative of increased cooperation between the left and right hemispheres of the brain.

The color subtraction illusion also requires hemispheric cooperation, and the combination of the two effects may involve more dynamic perceptual processing in the brain than either would separately.

When applied with the proper sequencing of attention and appropriate metaphoric content, illusions such as this that stimulate perceptual processing can be integral in a program to heighten one’s creativity and problem solving abilities.

                Interview with John on Corel.com