RE: [Algorithms] VIPM and UV texturing

[Tom F. <to...@mu...>]

> From: Mark Wayland [mailto:mwa...@to...]
[snip]
> 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" ?

Ugh - no. Who would want to do that? :-)

Actually, yes. Edges where two textures meet (or indeed any discontinuous
data - colours, normals, etc) are what I call "feature" edges. The way I
deal with them is that you find all your feature eges, and then only allow
collapses _along_ edges and _onto_ edges. But I never allow collapses of a
vertex on a feature edge onto a another vertex that isn't on the same edge
(remember to disallow collapses of one feature vertex to another feature
vertex that _isn't_ on the same edge, e.g. between two parallel feature
edges).

But the fewer feature edges you have on your model, the better VIPM can cope
with it, and the better it collapses.

> 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 ?

I dunno about "trivially" - mapping textures onto humanoid models (and
indeed the not-so-humanoid - we have slugs, two-headed creatures and ones
with four arms in Startopia) can get quite tricky.

But yes, VIPM only works well when there are comparatively few feature
edges.

There are some schemes that try to perform collapses across feature edges.
One is Jan Svarovsky's (follow the GDC link from the ultra-minimalist
homepage: http://www.svarovsky.com/), which has a discussion of these
collapses. But they're not pretty, and they complicate the collapse process.
By far the best way to proceed is to use friendly persuasion and Pavlovian
conditioning (a big stick is handy for this) until the artists to create
models with few feature edges.

Tom Forsyth - Muckyfoot bloke.
Whizzing and pasting and pooting through the day.