Subject:
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Re: Hidden surface removal, and vertex order in part/primitive DAT files
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Newsgroups:
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lugnet.cad.dev
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Date:
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Mon, 27 Sep 1999 11:44:44 GMT
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Viewed:
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464 times
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In lugnet.cad.dev, John VanZwieten writes:
> In lugnet.cad.dev, Gary Williams writes:
> > I performed some hidden surface removal tests today (I should have been
> > working on other things, like the inventory file cleanup utility...) and it
> > appears that there isn't a standard order for listing vertices in DAT files
> > (I only tested a few elements, and my code may be flawed, so I may be
> > wrong).
> >
> > A common technique in the rendering world is to always specify vertices of a
> > polygon in clockwise order (clockwise as viewed from the outside of the
> > polyhedra that contains it). This cuts the rendering time in half by
> > allowing the rendering algorithm to ignore the polygons that face away from
> > the observer (assuming there are no transparent polygons through which the
> > observer might see the backside of another polygon).
> >
> > This technique can also cut in half the amount of time required to determine
> > which polygon the user clicked on when selecting elements in a model, since
> > you can't click on a polygon that's facing away from you, so half the
> > polygons don't need to be tested.
> >
> > So...I've got half a mind to write a utility that will load every existing
> > part and primitive file and analyze them and automatically tweak the vertex
> > orders of all the polygons. But I thought I'd mention the idea first.
> >
> > -Gary
>
> If you run a LUGNET search on backface culling, you'll find extensive
> discussion of this. Some parts (mostly done by Steve Bliss) have a "0 CW-
> Compliant" tag, which means you can use backface culling on them.
>
> It would be pretty amazing if you could create a utility to automatically fix
> other parts. Even if you fix the vertex order of polygons, you have to worry
> about the transformations applied to primitives and subparts in a part file.
> And then you'd have to adjust for transformations applied to parts and
> submodels in a model file.
>
> Would be great to save 1/2 the rendering time, though.
>
> -John Van
I'd be mighty impressed, too, if you could do this automatically. For solid
polyhedra, it's not too difficult as you can assume that the origin of the
solid is within the material, but most Lego parts are re-entrent (ie. partly
hollow), so there is not a single reference point which is inside the plastic.
In the simplest case, imagine a 1x1 brick. For each outer side face there is a
parallel inner face. These would need complmentary vertex sequences.
Good luck.
Chris
chris_w_dee@hotmail.com
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