hi Andrew, hi Jaap (this is your first name, i guess?)

Jaap Frank wrote:
> Your are mixing up two different Stereograms systems.
> 1. "magic eye pictures"
>     For this you must stare at the picture, that will say you look behind the
> picture.

[...]

> 2. Crossing eye method.
>     By crossing your eyes you look before the picture and your left eye now
>     looks at the right object and your right eye looks at the left object.

which is basically the same method to obtain the same effect,
the only difference is the "point of view" (POV.. duh ;), which
is either in front or behind the image. this difference in turn
results in the "image" (that you perceive) being flipped
back-to-front, for the reasons Jaap explained well already.

but there is another main difference you did not make clear:
there are (real) stereograms consisting of two separate images
to be viewed by either eye, and "Single Image [Random] Dot
Stereograms" (SI[R]DS), which, as the name implies, result
from a single picture - which contains information for both
of your eyes.

the pictures that Andrew posted, including the ascii example,
are SIDS, whereas Jaap posted a (double image) stereogram
combined in a single picture file..

i like your pictures very much, no matter if i have to cross
my eyes or look thru them ;) , and i was reminded of the many
experiments i have done. but i must admit that it did not occur
to me that i could also try this using povray..

i have written a SIRDS generator a long time ago (in AmigaBasic),
i think i don't have that .bas file anymore. but it is not very
hard to do. - if someone wants it, and if no "HOWTO" can be found
on google, i will be happy to re-write it in povray or so. ;)


> Tip: Put the identical objects at a considerable distance and use a camera
>         with an angle of about 6 - 8 degrees, else the effect is exaggerated.

so you approximate a parallel projection for either of the two
objects in order to avoid the "curvature" that andrew mentioned.
- did you try using orthographic projection instead?
i think that it would work very well. you just have to rotate
the two objects a little to get different angles for each eye;
then you can get both "crossing eye" and "magic eye" pictures,
as you call them, depending on which way you rotate them.


g.

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