Subject:
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Re: CW/CCW, vertex sequence, co-planar, convex
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Newsgroups:
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lugnet.cad.dev
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Date:
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Thu, 14 Oct 1999 12:49:11 GMT
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Viewed:
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828 times
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Steve Bliss wrote...
> On Mon, 11 Oct 1999 15:23:26 GMT, Lars C. Hassing wrote:
>
> > Will you really require all existing models to go through a new good
> > editing program, before they could benefit from backface-culling?
>
> Sure. But I also feel that it would make sense for rendering programs to
> give the user control, so that clipping can be set to default to off, or
> default to on, or to be totally disabled.
Yes, rendering programs should have these reasonable options. But I think
it is safe to start clipping from opaque certified parts.
> > I don't think that is necessary. Once an old model references a certified
> > part, clipping can be done for that part and its subfiles. Referencing an
> > uncertified part would turn clipping off for that part and its subfiles.
>
> If a certified part-file can turn clipping on, then any certified file can
> use clipping. Where's the difference between a certified part, and any
> other certified file?
Parts are objects with obvious orientation. You are not in doubt what is
inside/outside of a part!
However, consider the 4-4cyli.dat. Even if its orientation was defined
and it was certified, you could not use that info for clipping
if it was not referenced from a certified part, i.e. a part that have
gone through considerations whether to use INVERT or not.
Because the orientation of parts is natural and intuitive, certified
parts would be the right place for enabling clipping.
And it would be safe and legal to do so, because rendering programs
keep track of transformation-inversions.
You can of course enable clipping in your model, but it wouldn't
have any effect unless you have tris or quads in your model.
> > Hm, isn't WINDING a local directive, that shouldn't be passed on?
>
> Ergg, you're right. That seems to be a shortcoming (for us) of the
> CLIPPING/WINDING approach -- there's nothing to clearly indicate that a
> file is certified. The presence of a WINDING meta-statement *could* be
> taken as a clear indication that the file has been checked (or not-checked,
> in the case of 0 WINDING UNKNOWN).
Yes, and since it seems that most people want to split certification in
two tasks (winding of tris/quads in current file, and possible inversion
of subfiles) we would perhaps even need two certified-tags!
I agree with Gary Williams that the CLIPPING is somewhat redundant.
CLIPPING ON/OFF could be accomplished by WINDING CW/UNKNOWN.
The CLIPPING is "syntax sugar", not a functional requirement.
But we should find a way that is easy and logical for part authors
(because FACE CW/CCW/OFF would be enough from the program's view!)
I believe the presence of WINDING/CLIPPING makes a part certified.
If some sections of the part has not been checked yet, it can be
indicated by WINDING UNKNOWN or CLIPPING OFF around the sections.
/Lars
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Message has 1 Reply:
Message is in Reply To:
 | | Re: CW/CCW, vertex sequence, co-planar, convex
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| (...) Sure. But I also feel that it would make sense for rendering programs to give the user control, so that clipping can be set to default to off, or default to on, or to be totally disabled. (...) If a certified part-file can turn clipping on, (...) (25 years ago, 13-Oct-99, to lugnet.cad.dev)
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