Sascha Ledinsky <sas### [at] userssourceforgenet> wrote:
> Is there anything that can be done about it (e.g. are OpenGL quads meant 
> to somehow overcome this kind of problem)?

  OpenGL quads show this exact problem when the quad is not rectangular
nor rhomboidal (ie opposite sides are not parallel). This (d)effect
can be clearly seen in many cases, for example if you tesselate a
torus with quads (the quads at the upper and lower side of the torus
will exhibit this problem).

  Theoretically there exists a method for avoiding this problem, but
I have not heard any 3D hardware (nor even software) using it:
Render quads as quads, not two triangles, and scale the texture
non-uniformly to adjust to the shape of the quad (as in the
to-be-expected illustration in the image I gave a link to).
  In fact, you can even render the quad as two triangles given
that there's a proper correction factor applied to the texture
of the triangles.
  However, as I said, I have never heard of any 3D hardware
supporting this (nor know of any software which would support it).

  Often you can avoid the problem with better modelling. For example,
in the case of tesselating a torus, you could use triangles placed
so that this effect does not present itself (ie. in a kind of
hexagonal grid on the surface of the torus). However, I'm sure there
are instances where there's no good workaround.

  (Btw, one could hastily think that dividing the quad into four
triangles could solve or at least alleviate the problem, but if you
think about how it would be textured you will realize it will not
help too much.)

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

Post a reply to this message