Subject: SIRDS (single image random dot stereograms) how do they work?

From: [email-masked] (WILLIAMSON, JOHN EDWARD)
Date: 9 Feb 93 17:55:00 GMT

I am looking for a lucid, short description of how SIRDS work,
how they are created (an explanation of the code)
and how they differ from the original random dot stereograms
(which used side-by-side) pairs.
Also, did Julesz discover SIRDS as well.

In related news, a company based in Texas is doing very well
selling SIRD posters through novelty and poster/frame shops
at malls across the country.  They have sold over $2,000 worth
here in my small town alone.  The Simpsons people have been
rumored to have contacted them, so a Bart SIRD may not be to far
away.  The company is Nvision graphics based in Dallas Tx.
I have no address or phone number.

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From: [email-masked] (Ken Hornstein)
Date: 10 Feb 93 07:14:29 GMT

>(which used side-by-side) pairs.

You could always look at my "sirds" program ... it's only one page of C
code and the algorithm is so idioticly simple you'll never believe it :-)

Available for anonymous ftp on dev-null.phys.psu.edu, in /pub/rds.

--Ken

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From: [email-masked]
Date: 10 Feb 93 22:45:58 GMT

> (which used side-by-side) pairs.

They work because your brain determines depth (with SIRDS) by differences in
how much you cross your eyes from one nearly identical pattern to the next.
The slight variations in distance cause the depth perception.

Assuming diverging eyes,        L      R
This  ->                        *      *
is "closer" than this.  ->      *       *

Crossing your eyes gives you a depth negative, making the close parts
far and vice versa.

Off the top of my head, the goes something like this.

Per scan line:
  Generate N random pixels.
  For each pixel:
    I = altitude (generally determined by color index)
    Write pixel from (current location - N) to (current location - I)
    Fill  random pixels from (current location -I + 1) to (current location)

Your resulting image is N pixels wider than your source image, but is the
same height.

> Also, did Julesz discover SIRDS as well.

He may have, but at least I and two or three others independently discovered
the technique back around the end of '88 and first of '89.  I sent some info
on it to A.K. Dewdney when he was doing the computer recreations column in
Scientific American, but he didn't cover the technique before the column 
was replaced.  :^(

John Olsen
[email-masked]