Re: CHAT: Ultraviolet (was: Orange)

Christophe Grandsire writes:
 > En réponse à Tim May <butsuri@...>:
 >
 > >
 > > Are you crossing your eyes, that is focusing on a point between you
 > > and the image, rather than on a point beyond the image?  That would
 > > result in a reversed 3D effect, or so I've read.  Let's think...  gah,
 > > thinking's too hard.  Tell me if that's what you're doing first, then
 > > I'll work out how it does it.
 > >
 >
 > I do try to focus beyond the image, as is explained, but I probably focus on
 > the image instead... I really don't know how to change that...
 >

Hmmm... if you were focusing _on_ the image (image here meaning the
printed autostereogram on the paper (or whatever), not the 3d image
produced by looking at it the right way) you shouldn't see anything at
all (well, you should see a mess, but not a sailboat or whatever).

What an autostereogram is is two images superimposed on one another.
Each one is a monoscopic view of the scene you're meant to see, taken
from a slightly different angle.  What's necessary for it to work is
for the two images to line up in your field of vision, so each
supplies the view from each eye that simulate the experience of
viewing a solid object some distance away.  The right way to do this
focus beyond the image, but you can get the same effect only backwards
by focusing inward from the image and letting the lines of view cross.

Here, I think I've come up with a good way of explaining this.

Hold up both your index fingers such that they're vertical, positioned
in front of your face, and one is about 15cm from your face and the
other is about 30cm beyond it.  Switch your focus from one to the
other, and you'll see that the one you're not focusing on splits into
two images.  Focus on the near finger.  Now, imagine that the double
image of the far finger isn't an optical illusion, but is actually a
double picture of a finger _which is at the same depth as the finger
you're focusing on_.  If this were the case, then if you were to focus
on the far finger, the pictures of the finger would coalesce into a 3d
image of the far finger*.  This is essentially how a stereogram works.
But, what you're*** maybe doing is the equivalent of taking that
picture back to where the far finger is and focusing on the near
finger**, which would result in an image of the near finger, but
because it's really a picture designed to be seen with the eyes
swapped over, the 3d effect'll be inverted***.


Well, it would be a good explanation if it were better written.
Anyhow, I don't know if this helps anyone, but it might be something
to bear in mind the next time you look at one of these things.


*   It'd be blurry though, because focusing involves both the angle of
    orientation of the eyeballs and the focusing of the lens in the
    eye, and stereograms are going to put them out of whack.

**  Only you're going to have to move the near finger away and focus on
    where it was, because otherwise it'll be in the way.

*** Christophe, that is.

****This is kind of a hand-waving explanation really - I find it
    difficult to really understand the inversion rigorously, and
    harder to put it into words.