P3D Re: Parallax tech

[Larry Berlin <lberlin@xxxxxxxxx>]

> Date: Tue, 30 Nov 1999 
> From: "Gregory J. Wageman" <gjw@xxxxxxxxxx>
> >....(I wrote).... yet I'm unaware if there is an official
> >unit of measurement for parallax in stereoscopic images? I know of a
> >parallax unit used in Astronomy, .............
> 
> If you're referring to the "parsec", that's a unit of distance, not
> a parallax measurement: a parsec is defined as the distance at which the
> PARallax (of a star) would be one SECond of arc, with the diameter of the
> earth's orbit as the base.  (It's not a unit of time, either, as misused
> by Han Solo in the original "Star Wars" movie.)
> 
> Parallax is an angular measurement, and can be expressed in any convenient
> angular unit (degrees, radians, etc.).  As such, there's no difference between
> astronomical parallax and stereo parallax, other than the sizes of the angles
> involved.

*****   This is true, and the parallax data apparently contained in
certain star charts are in ArcSeconds. One ArcSecond being listed as a
parallax of one (labelled as parallax in the data). Since the nearest
stars are more than one parsec distant, all the stars in the charts have
a parallax factor of less than 1. So yes it is an angular measure.

I'm pretty sure photographers seldom have occasion to measure parallax
in ArcSeconds unless they are astronomers. You've suggested appropriate
other terms. Thanks.


> ..........
> Parallax angle becomes a distance in stereophotography when the two
> perspectives are projected onto the film plane.  We can then measure the
> apparent shift of nearby objects with respect to the distant background.

****  Or from the stereo window. It is this comparative distance factor
that I was interested in a unit for. Once you have an image you could
measure it in mm, as is done with slides. Or on the computer screen you
might measure it in pixels out of sheer convenience or necessity. If you
work with Barrier Strip panels or lenticulars the unit lengths can get
pretty small in either english or metric terms. I guess the best way to
work with this factor across different mediums and scale factors is to
work with the *parallax angle.*

Larry Berlin

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