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         (from Robert Cohen <[e-mail]>)
         (at Wed, 25 Oct 1995 17:18:24 -0700)
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From: [e-mail] (Mark Thompson)
To: [e-mail]
Subject: Re: GLOW and FIELD RENDERING QUESTION
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Robert Cohen <[e-mail]> writes:
> >  DP> Why would field rendering reduce your resolution by half, you get the
> >  DP> same number of horizontal lines per frame, field rendering or not?
> >
> > Not true . You get 2 fields . One with the even and one with the
> > uneven lines . This changes every 1/60 th of a second ....
> > That is why one field frame has only half of vertical resolution .
>
> But since these images are meant to view at 30 frames per second, and the
> perceptual persistance of the human eye is one tenth of a second, all of
> this is quite moot.  The "virtual" resolution is the same.

Not true, and I think you are missing the point. For example, consider a
ball that is rapidly bouncing up and down against a black background in
camera view such that the ball is in 2 completely seperate positions in
each video field. In each field, the ball will be resolved with only 240
lines of vertical resolution. The other 240 lines will be black because the
ball is in another position in time. The net result is a ball that is
resolved with 1/2 the spatial resolution and in some instances, this can
become quite visible. A perfect example is a rotating blade that turns
at a rate of 15rps (or 900rpm). If it is nearly vertical in one field,
it will be nearly horizontal in the next. Both blade positions will appear
jagged and be extremely visible. This is a worst case scenario, but there
are many other cases where it can manifest itself in a slightly less
obvious way. And persistance of vision has nothing to do with it.

Sorry to continue on this thread, but it seemed to have drifted off on
a tangent that was incorrectly dealing with the issue. I'll let it die
in piece now :-)
     *~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*
     *           Mark Thompson                  [phone]    *
     *         Fusion Films, Inc.             [e-mail]  *
     * Director of Animation and Special FX                       *
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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