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by Gene Levine

If you will be using stereograms for a project targeting a large audience or people with little or no viewing experience your primary concern will be with ease of viewing.  There are multiple factors controlling this.

Stereograms are created by repeating vertical patterns, textures, and/or objects. The horizontal distance between these repeats determines a parallax (visual displacement). If the width of this parallax is too wide it will strain a viewers eyes. If parallax is too narrow, the viewer will fuse too many vertical displacements at once and see extra layers that may fly in & out of focus: called double fusion.

For experienced practiced viewers there are advantages to a parallax that is very wide or narrow, but for ease of viewing, things must fall within a standard range.  This standard range is in turn strongly affected by different factors.

The most important factor is how distant the image is from the viewers face.  Simply moving an image with broader parallax further away from your eyes effectively makes parallax narrower.  Conversely, standard parallax coming closer to the viewers face will demand wider viewing technique.

For the same reason, image size is a critical part of the equation when printing or posting a stereogram.  The same image printed larger will have wider parallax than if printed smaller.  Again, the viewer simply adjust the distance between their eyes and the image. 

In practice, an image that views properly and is easy to see at a large size can be problematic if reproduced at a smaller size where effective parallax becomes so small that double fusing is unavoidable.  You might adjust this by bringing the image closer to your face, but there is a practical limit to this that may be determined by the quality of your screen or printed page, or your vision and eyeglasses, as well as ambient lighting.

As with everything else, no two people are alike.  Aside from corrective lenses that may include progressive, bifocal, or trifocal compensation, there is subtle, but different spacing between individuals' eyes as well as differences in peripheral visions.  Some people see the stereo 3D right away, others require time.  This means basic viewing ability falls within the mean of a curve at best, rather than being alike for everybody.  The reality is ease of viewing is seldom predictable when people view stereogram images.

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