John VanSickle <evi### [at] hotmailcom> wrote:
> First, incorporate quadrilaterals into the mesh object.  This would
> make polygon modelling simpler.

  What would be the advantage of this over using two triangles?

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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Warp wrote:

> John VanSickle <evi### [at] hotmailcom> wrote:
> 
>>First, incorporate quadrilaterals into the mesh object.  This would
>>make polygon modelling simpler.
>>
> 
>   What would be the advantage of this over using two triangles?


None for native pov, but when converting other 3D mesh formats, some
of them do not stick to the triangles and have either quadrilaterals,
or simple polygon ("simple", because not like the Pov polygon object).

So supporting the polygon in mesh would allows an easier writing of the
conversion programs.

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Warp wrote:
> 
> John VanSickle <evi### [at] hotmailcom> wrote:
> > First, incorporate quadrilaterals into the mesh object.  This would
> > make polygon modelling simpler.
> 
>   What would be the advantage of this over using two triangles?

When something is extruded, it makes more sense to extrude edges as
quads rather than a pair of triangles.

It also makes it easier to deal with a problem involving the Loop
method of surface subdivision.

Regards,
John

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I have another advantage: UV-mapping a quadrilateral consisting of two
triangles simply doesn't work right. A true quadrilateral with properly
implemented UV-mapping would work right.
  Of course this poses a problem: How do you texture an UV-mapped
quadrilateral in patological cases (eg. when it's concave)?

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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On 9 Aug 2002 16:39:31 -0400, Warp <war### [at] tagpovrayorg> wrote:

>  Of course this poses a problem: How do you texture an UV-mapped
>quadrilateral in patological cases (eg. when it's concave)?

I can tell you how to do the forward operation (i.e. from uv to xyz),
but I gave up trying to reverse it, although I am dead sure it has a
unique solution.


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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By the way, this scene demonstrates the problem with uv-mapping when
using two triangles to make a quadrilateral (AFAIK there's no way to
avoid this effect):

camera { location -z*4 look_at 0 }
mesh2
{ vertex_vectors { 4, <-2,-1.5,0>, <2,-1.5,0>, <1,1.5,0>, <-1,1.5,0> }
  uv_vectors { 4, <0,0>, <1,0>, <1,1>, <0,1> }
  face_indices { 2, <0,1,2>, <0,2,3> }
  uv_mapping
  texture
  { pigment { gradient x color_map { [0 rgb 0][1 rgb 1] } scale .1 }
    finish { ambient 1 }
  }
}

  A true quadrilateral could interpolate the texture along the whole
quadrilateral instead of the two separate triangles, giving a more logical
result.

-- 
#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 -

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Warp wrote:
> 
>   I have another advantage: UV-mapping a quadrilateral consisting of
> two triangles simply doesn't work right. A true quadrilateral with
> properly implemented UV-mapping would work right.
>   Of course this poses a problem: How do you texture an UV-mapped
> quadrilateral in patological cases (eg. when it's concave)?

Put a warning in the docs that it may not work right...

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