Subject: Rendering Backwards
From: Ernie Wright <[email-masked]>
To: [email-masked]
Date: Mon, 19 Jun 1995 00:23:58 -0400 (EDT)

On rendering the first half of a periodic anim and appending a reversed
copy to finish it, Eric Chard wrote:

> Just to make sure, I just did it again, and no, for whatever reason
> the field flip does not solve this particular problem.  Matter of fact
> (henceforth, "MOF") it makes it look worse.
>
> Time for Ernie to step in and tell us why!  :^)

Like I have any idea.

Okay, I have -one-.  It's specifically a field rendering problem, and
not something that flipping fields will solve.  This is a little hard
to diagram in ASCII-Vision, but consider the following:

   (1 2) (3 4) (5 6)

Each pair of numbers represents a frame with two fields.  The fields
are 1/60 of a second apart, so whatever it is we're counting with those
numbers, it advances by one per field and two per frame.  (Buying this
so far?)  If you're ready to reverse at this point, what's the next
frame?

If we use (3 4) we get a jump from 6 to 3:

   ... (5 6) (3 4) ...

If we reverse fields and use (4 3), we get a jump from 6 to 4:

   ... (5 6) (4 3) ...

See?  What we really want is

   ... (5 6) (5 4) ...

and I'm not sure how to do that.

- Ernie

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