RE: [Algorithms] UV texturing and DOT3 (was:VIPM and UV texturing )

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

Curious - I never had any problem with it, and our artists mirror things all
over the place. I suspect we're using slightly different equations to find
our surface-local basis matrices. I'll have to check the code to see exactly
what equations I'm using, but I'm pretty sure I calculate the N, U and V
basis vectors independently of each other (for each, you just use vertex
positions and the factor you are interested in, so U is never used to
calculate V's vector and vice versa), which means that the whole
"handedness" thing just isn't an issue as far as texture coords. As long as
the triangle indices are ordered clockwise, it all just works.

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

> -----Original Message-----
> From: Sim Dietrich [mailto:SDi...@nv...]
> Sent: 09 August 2000 18:29
> To: 'gda...@li...'
> Subject: RE: [Algorithms] VIPM and UV texturing
> 
> 
> 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