Re: [photo-3d] Stereo Shadow Trick?

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

Michael Georgoff asks:

>Can someone explain the stereo shadow trick to me?   Something in my foggy
>memory suggests maybe the scene is flat but the shadows are in stereo?  Am I
>hallucinating again?

Nope!  You are not hallucinating this time! :)

>Something like this: Take left shot of a chair on the patio on a sunny day.
>Let time elapse, so sun moves, hence shadow moves.  Take right shot from
>exact same camera position as left shot was done before time elapsed.  Now:
>scene is flat but shadow is in stereo?  This is from memory.  Elapse = how
>long in time or shadow inches? Do I need to get out my slide rule to
>calculate MAOFD of shadow?

You've got it buddy!

Here is my personal involvement with this:  Last year (or 
was it two years ago...? Maybe three?  Boy, time flies!!!)
there was a competition called "Shadows" in Detroit.  I 
mentioned in photo-3d that it is a big challenge to take a 
3d picture of a 2d shadow.  Several people responded with 
ideas, like taking a picture of a shadow forming over a
3dimensional object, etc.

Ron Labbe reminded me of a little book (sorry I forgot the
author!) which had an anaglyph picture of flat chair with a 
3dimensional shadow coming right out of the window (very
impressive!)  This effect is created by shifting the light
source and leaving the camera stationery.

Armed with these ideas I did some experimentation of my
own.  First, I set the camera on a tripod and took pictures
of a tree at different times, to capture the moving shadow
in a stereo pair.  A few things I learned:  Shadows move
must faster than expected!  It is important to do some
calculations/experimentation to get the shadow moving
in the horizontal direction and for a reasonable distance
(not too long).  But then there were other things moving
like bugs, leaves, etc., so I was not happy with this
approach.

I then shifted my attention to tabletop photography where
I can take two pictures with a flash, by moving the flash
between the pictures.  I finally settled to taking a picture
of a small Greek statue (horse rider).  So the statue was
flat but the shadow was in 3d!  Of course, I procrastinated
till the last second so when I got the pictures back, I did
not have the time to project them, only view them in a viewer.
Through the viewer it seemed that mounting the picture upside
down (so the shadow is at the top) was creating a better
effect.  As it turns out, it was not effective in projection.
The judges were confused with my "bike rider" (that's what
they thought the subject was).  Still got some award but
not much. :)

I had better luck with the other picture... During lunch
I got out of my work building and the sun was very bright.
I took a picture of my very black shadow forming on the steps
of the building.  Then I reversed Right and Left, creating
a very interesting effect, which also confused the judges
but for the better.  John Vala posted his impressions in
photo-3d, back then.

Anyway... there is a lot of room for interesting
experimentation with shadows.  Good luck!

George Themelis