RE: [Algorithms] VIPM and UV texturing

[Sim D. <SDi...@nv...>]

This mirroring is a huge problem when doing per-pixel lighting with DOT3
bump mapping, because texels don't have a facing.  In some cases, you can
detect backwards facing textures and just flip the local coordinate system,
but not in the case you mention below.

-----Original Message-----
From: Jeff Lander [mailto:je...@di...]
Sent: Wednesday, August 09, 2000 10:06 AM
To: gda...@li...
Subject: Re: [Algorithms] VIPM and UV texturing


Hi all.  New to this discussion but I have recently had a couple of projects
where this issue has been very important.

The issue you bring up about texture coordinates and any sort of mesh LOD
system is critical.  That is why I insist that the LOD tools allow artists
to hand designate edges that will not collapse.  Any algorithmic method you
try and find will fail in certain cases.  The algorithms handle texture
projections like sphere and cylinder wraps pretty well but arbitrary UV
texturing is a real problem.

One particular problem I will share that comes up often is when texturing
characters.  My system automatically will preserve vertices that have
texture coordinate discontinuities.  However, artists will often model a
character with only half of the texture (particularly in the face) then just
reflect the texture coords to save texture space.

Now the problem as you would guess is you have the face with coordinates
like (1,0) -----  (0,0) ---- (1,0).  Obviously more granular then that.
There are no texture coordinate discontinuities so the algorithm says it is
ok.  The surface of the face is roughly flat so the internal vertex and
edges are good candidates for collapse.  However, you then get the nice
situation of  (1,0) ------ (1,0).  Doesn't look so hot.  I have the artists
tag reflected seam vertices as uncollapsable until very late in the order.

Let me paraphrase you by saying the algorithms are most efficient when
working on trivially textured objects.  Anything else requires good artist
tools and some TLC.

-Jeff

At 06:44 PM 8/9/2000 +1000, you wrote:
> >     Well, I think the thing you missed is that VIPM is mainly aimed
>towards
> > meshes that are more like skinned meshes (ie, all points only have a
>single
> > vertex associated with them - Hence each set of adjacent triangles must
be
> > specifiable as a strip). I like to call these sort of meshes, highly
> > connected meshes (HCM), for the simple purpose that with appearance to
the
> > renderer, they share vertices, and are therefore connected.
>
>I cite different vertices because the actual UV coords are different
>(eg. a vertex is NOT just gemoetry, as in the D3D definition)
>
>Doesn't anyone use large meshes that are textured using multiple textures
>with VIPM,
>in that you would want the mesh as a whole to reduce, not the similarly
>textured "pieces" ?
>
>What I'm tending to now assume is that most users of VIPM are using it on
>trivially textured
>objects ... would this be a fair assumption, or once again have I missed
>something ?


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