Subject: Small question about RDS theory

From: [email-masked] (Ken Hornstein)
Date: 4 Nov 92 22:58:32 GMT

I've written a small C program to output random-dot stereograms as PostScript
files.  It works quite well for me, but I have one small question pertaining
to the images.

How much is the the percieved depth change per pixel shift?  Like, if I have
my focusing trianges 90 pixels apart, and I make a 5 pixel shift, how much
will I perceive the change in depth to be?

If anyone wants this program, they are welcome to it.  However, it's not
very robust .... most of the constants are hard coded, for example :-)  I
plan on making the program more general eventually.

Thanks for any replies.

--Ken

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From: [email-masked] (Ian Collier)
Date: Sun, 8 Nov 1992 18:26:13 GMT

[Apologies if you see this twice - it didn't seem to make it the first time]
In article <[email-masked]>, [email-masked] (Ken Hornstein) wrote:
>the focusing trianges 90 pixels apart, and I make a 5 pixel shift, how much

That is difficult to say with any degree of accuracy.  It depends on several
parameters; moreover the eyes tend to interpret the depths of RDS images in
a relative, rather than an absolute way - and so you cannot usually estimate
very well the depth of a particular part of the image.

That said, you can calculate the theoretical depth of any pixel by
triangulation, and thus work out the difference between two adjacent pixels
which are at different depths.  The answer is approximately

    v/(e-d/res)

where v is the viewing distance (i.e. how far you are away from the image),
e is the separation of your eyes (average approx. 2.5 inches), and d/res is
how far apart the viewing triangles are on the image.  (All three are in the
same measurement units, obviously; d/res is expressed like that because d is
the distance in pixels and res is the number of pixels per measurement
unit). 

The answer you get will be a number (probably in the range 5-50) which
indicates how much larger the z pixels are than the (x,y) pixels.

Ian Collier
[email-masked] | [email-masked]