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For crossed-eyed autostereograms, a different approach needs to be taken. The viewer may hold one finger between their eyes and move it slowly towards the picture, maintaining focus on the finger at all times, until they are correctly focused on the spot that will allow them to view the illusion.
Stereoscopy creates the impression of three-dimensional depth from a pair of two-dimensional images. [5] Human vision, including the perception of depth, is a complex process, which only begins with the acquisition of visual information taken in through the eyes; much processing ensues within the brain, as it strives to make sense of the raw information.
There are two ways an autostereogram can be viewed: wall-eyed and cross-eyed. [a] Most autostereograms (including those in this article) are designed to be viewed in only one way, which is usually wall-eyed. Wall-eyed viewing requires that the two eyes adopt a relatively parallel angle, while cross-eyed viewing requires a relatively convergent ...
Wiggle stereoscopy offers the advantages that no glasses or special hardware is required; most people can perceive the effect more quickly than when using cross-eyed and parallel viewing techniques. Furthermore, it offers stereo-like depth to people with limited or no vision in one eye.
To view the stereogram, use a stereoscope to present the left image to the left eye and the right image to the right eye or focus on a point behind the image to achieve the same thing. (How to achieve this wall-eyed position of the eyes is described in Autostereogram). The shifted region of random dots will appear as a small, central, square ...
A third aid is that the viewer goes cross-eyed, and when this is done, the two images appear as three images, due to the lines of sight when cross-eyed. If the viewer focuses on the middle of the three images, it will come into focus as stereoscopic, without any viewing apparatus or use of a finger to get the eyes correctly focused.
A stereo transparency viewer is a type of stereoscope that offers similar advantages, e.g. the View-Master. Disadvantages of stereo cards, slides or any other hard copy or print are that the two images are likely to receive differing wear, scratches and other decay. This results in stereo artifacts when the images are viewed.
At one time, Underwood & Underwood was the largest publisher of stereoviews (also known as stereographs or stereoscopic cards), in the world, producing 10 million views a year. The Underwood brothers developed a selling system of thorough canvassing using college students.