mesa3d-users

[Mesa3d-users] Where or where have my polygons gone?

[Evan M. C. <le...@ca...>]

I tried looking in the archives and the FAQ but didn't find an answer to
my question so forgive me if this has been answered already.

I'm making a 2D map.  Some of the polygons in this map are very very
small.  It appears that if a polygon is sufficiently small Mesa throws
it out.  I've got sections of my map which are always going to be
composed of _lots_ of little polygons and when they are not rendered I
end up with a big hole in the map.  If I change my orthographic
projection to look closer in at the hole I see that the geometry for the
polygons is valid cause they'll show up at close inspection.

In my application I don't mind that while rendering these small polygons
that the pixel used to represent them will be repeatedly updated until
I've scanned over far enough to start in on a new pixel.  I.E. last
polygon rendered gets it.  That's actually desirable for my domain.

Any suggestions?

Thanks in advance!

--
//  Evan M. Carey                       Camber Corp.
//  ev...@ca...                   4885 Alpha Road, Suite 110
//  972-991-5322 x145                   Dallas, TX  75244-4633
//  972-991-5352 fax.                   www.cambertx.com

Re: [Mesa3d-users] Where or where have my polygons gone?

[Stephen J B. <sj...@ht...>]

On Wed, 12 Apr 2000, Evan M. Carey wrote:

> I'm making a 2D map.  Some of the polygons in this map are very very
> small.  It appears that if a polygon is sufficiently small Mesa throws
> it out.  I've got sections of my map which are always going to be
> composed of _lots_ of little polygons and when they are not rendered I
> end up with a big hole in the map.  If I change my orthographic
> projection to look closer in at the hole I see that the geometry for the
> polygons is valid cause they'll show up at close inspection.
 
I agree that Mesa should not throw out polygons no matter how
small - for precisely the reason you say...and the OpenGL specification
does not permit such a thing to be done.  However, I'm pretty sure I
heard of such an "optimisation" being added to Mesa (and I protested at
the time).

Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
L3Com/Link Simulation & Training (817)619-2466 (Fax)
Work: sj...@ht...            http://www.hti.com
Home: sjb...@ai...       http://web2.airmail.net/sjbaker1

Re: [Mesa3d-users] Where or where have my polygons gone?

[Brian P. <br...@pr...>]

Stephen J Baker wrote:
> 
> On Wed, 12 Apr 2000, Evan M. Carey wrote:
> 
> > I'm making a 2D map.  Some of the polygons in this map are very very
> > small.  It appears that if a polygon is sufficiently small Mesa throws
> > it out.  I've got sections of my map which are always going to be
> > composed of _lots_ of little polygons and when they are not rendered I
> > end up with a big hole in the map.  If I change my orthographic
> > projection to look closer in at the hole I see that the geometry for the
> > polygons is valid cause they'll show up at close inspection.
> 
> I agree that Mesa should not throw out polygons no matter how
> small - for precisely the reason you say...and the OpenGL specification
> does not permit such a thing to be done.  However, I'm pretty sure I
> heard of such an "optimisation" being added to Mesa (and I protested at
> the time).

There was a problem with FP over/underflow when rendering very small
triangles.  It wasn't an optimization.

Evan, in src/tritemp.h you can try changing the area test for small
triangles and see what happens.  The code in question is:

      if (area * bf < 0 || area * area < .0025)
	 return;

Try replacing the .0025 with a smaller value, or zero.

-Brian

Re: [Mesa3d-users] Where or where have my polygons gone?

[Evan M. C. <le...@ca...>]

Brian Paul wrote:

>       if (area * bf < 0 || area * area < .0025)
>          return;
>
> Try replacing the .0025 with a smaller value, or zero.
>
> -Brian

Brian,

I made the predicate like so:

    if (area * bf < 0 || area * area <= 0)
        return;

I realize there might still be problems with excessively small areas but for my
app this seems to work OK...so far.  Thanks for the reply.  I _really_
appreciate the quick solution.  If I do end up having problems I know where to
go to play with the limit.

Thanks,

--
//  Evan M. Carey                       Camber Corp.
//  ev...@ca...                   4885 Alpha Road, Suite 110
//  972-991-5322 x145                   Dallas, TX  75244-4633
//  972-991-5352 fax.                   www.cambertx.com

Re: [Mesa3d-users] Where or where have my polygons gone?

[Stephen J B. <sj...@ht...>]

On Thu, 13 Apr 2000, Evan M. Carey wrote:

> I made the predicate like so:
> 
>     if (area * bf < 0 || area * area <= 0)
                           ^^^^^^^^^^^^^^^^
                          A number times itself can't be
                          less than zero, so '== 0' might
                          be better...then you might as
                          well save the multiply and go
                          with 'area==0'.

Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
L3Com/Link Simulation & Training (817)619-2466 (Fax)
Work: sj...@ht...            http://www.hti.com
Home: sjb...@ai...       http://web2.airmail.net/sjbaker1

Re: [Mesa3d-users] Where or where have my polygons gone?

[Evan M. C. <le...@ca...>]

Stephen J Baker wrote:

> On Thu, 13 Apr 2000, Evan M. Carey wrote:
>
> > I made the predicate like so:
> >
> >     if (area * bf < 0 || area * area <= 0)
>                            ^^^^^^^^^^^^^^^^
>                           A number times itself can't be
>                           less than zero, so '== 0' might
>                           be better...then you might as
>                           well save the multiply and go
>                           with 'area==0'.
>
> Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
> Work: sj...@ht...           http://www.hti.com
> Home: sjb...@ai...       http://web2.airmail.net/sjbaker1

Duh.  What was i thinking.  Your right.  Save that one for you stupid
code page Steve.

--
//  Evan M. Carey                       Camber Corp.
//  ev...@ca...                   4885 Alpha Road, Suite 110
//  972-991-5322 x145                   Dallas, TX  75244-4633
//  972-991-5352 fax.                   www.cambertx.com

Re: [Mesa3d-users] Where or where have my polygons gone?

[Brian P. <br...@pr...>]

"Evan M. Carey" wrote:
> 
> Brian Paul wrote:
> 
> >       if (area * bf < 0 || area * area < .0025)
> >          return;
> >
> > Try replacing the .0025 with a smaller value, or zero.
> >
> > -Brian
> 
> Brian,
> 
> I made the predicate like so:
> 
>     if (area * bf < 0 || area * area <= 0)
>         return;
> 
> I realize there might still be problems with excessively small areas but for my
> app this seems to work OK...so far.  Thanks for the reply.  I _really_
> appreciate the quick solution.  If I do end up having problems I know where to
> go to play with the limit.

Great.

Understand that I'm still hesitant to put this change into Mesa
by default.  I distinctly remember problems with FP over/underflow
and tiny triangles.  I don't have time right now to reexamine this
problem in any detail now.

-Brian