Continued from page 1
A pigeon, to land, needs stereopsis in a triangular region in front and below. The cheeks of pigeon are hollow to allow optical axis of each eye to "sneak" past beak, allowing a small area of overlap for precision landing.
Otherwise, pigeon has eyes that give a panoramic view with little overlap.
This leads to obvious danger. What happens when pigeon is on ground? Is it vulnerable to attack by cats?
No. It JERKS ITS HEAD!
From one position it takes a "SNAPSHOT" of scene. Then it moves its head to a position that is removed by a distance remarkably similar to interocular separation of human eyes.
From new position, it takes a new "SNAPSHOT". It "knows" which image is front one, and which is back because its brain not only commanded movement but also received gnosisceptor confirmation.
So it needs only REMEMBER first image for a split-second, long enough to combine it with second image for a "Wheatsone glimpse", and it has perceived stereoscopic depth of its environment.
Does this only apply to pigeons?
Apparently not. When a human closes one eye, limbic system will no longer have two images for stereoscopic evaluation of environment. It will become dependent upon "TEMPORAL" data in one-eye image. That is, upon how perspective changes over TIME.
I have made some tools for stereoscopists available at http://www.wehner.org/3d/ or http://www.wehner.org ools .
Of interest in this context is LEN.COM lenticular stereogram maker. This creates an image of interleaved stripes for use with lenticular plastic.
However, suppose we make stripes HORIZONTAL instead of vertical. Suppose also, that we make a mask in GIF process where each odd row of pixels is black and each even row transparent. As mask slides over striped image, left-eye and right-eye images will be seen alternately.
Using Internet Explorer, trick works quite well. So you can see Professor Wheatstone rocking back and forth if you slide scroll-bar whilst viewing introductory page at http://www.wehner.org/3d/ .
Look closely, and it becomes quite obvious that tuft of hair to left of his head is moving forward and backward in space.
You can see effect EVEN WITH ONE EYE CLOSED.
Charles Douglas Wehner
Charles Wehner is an electronics engineer and technical author born in 1944. He was involved with radar, nucleonics and measurement-and-control systems - and was for many years a member of the Stereoscopic Society in London, England.