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A Word on Stereoscopy

We seldom knock ourselves against obstacles while moving around, unless we are lured by attractive objects or hit a too-clean plane of glass. Thanks to our pair of eyes, binocular vision enables us to discriminate depth. Because of the two eye's different positions, each actually reads a slightly different image in the real world. Our biological computer will make sense out of them. By knowing this mechanism, we can simulate three-dimensional effect when looking at some two-dimensional images. This is the interesting and fun field of stereoscopy.

There are quite a few techniques developed. The simplest form is using a pair of stereo images. People can enjoy the fun free just by looking at them with their bare eyes, with some skill easily learnt. Or with a little help of a pair of color glasses, we can play with anaglyphs.

It is necessary to state the methods I use here since there are many variations involved in both techniques.

For anaglyphic stereograms, I use the red/blue version. Left eye reads the red image and the right blue. You cannot capture the full color spectrum using this method but it is another choice if you cannot get hold of the free viewing technique.

I will explain a bit more on free viewing of stereo pairs because it is my preferred practice. I use parallel viewing which seems more natural to me. Left image of the pair is viewed with the left eye. Small dots above the image pairs are often present to help but are not obligatory.

*A little experiment*
We can do a little experiment to help us grasp the concept.
Face a background, say, a meter away.
Put your two arms about 25 cm in front of your eyes.
Hold only the index fingers up and about 2.5 cm apart.
Focus at the fingers.
You should see the two fingers clearly but the background blur.
Now switch your focus to the background.
If everything goes right, the background is sharp this time while you can find three blur finger images instead of two.

From a photo or a two dimensional image enhanced with perspective, we can get some sense of relative position of objects from hinting. As in below, we have enough suggestions that the blue figure is closest to us, and the green one farthest. Still it is just something like halfway between two and three dimensional.

To achieve true depth perception, we need two images like the following.

The technique in parallel viewing is to relax our focus. The left eye reads the left image and the right right. The two converge into a single one some distance further back as if we are looking behind the stereo pairs. The feeling is like staring blank. Your vision blurs. The pair seems to move sideways giving room for a new centre one to grow. And if you are using the accessory dots, you notice them blurring and moving towards each other.

Get use to this feeling. You might find the dots oscillate when you are not used to controlling them. Very likely, it would be fine after a few trials. Relax your focus and try to bring the two dots closer to the middle.

Once the two dots meet, that is the right focus point. Hold that and try to slowly side-glance down from the dot to the main image. You notice three images, clear one in the middle and blur on both sides. And in this centre image, you should be able to clearly see the spatial relationship between the objects.
(Note: not shown on this simulation image)

Now, I will give you a larger version of the stereo pair to try.

Not all stereo pairs are taken under the same circumstances and conditions. Some might be easier to read than the others. But it helps if you look from a distance, say, at least 50 centimeters; and straight on the images without tilting in any planes. Never over-strain or over-stress yourself. If you feel tired or dizzy, take a rest.

Fun stereoscoping!

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