Re: [Plib-devel] How many triangles in a leaf?

[Fay J. F Dr C. U. 46 S. <joh...@eg...>]

Paolo,

	SSG is not my forte, but here is my reaction.

(1)  I don't think we want to keep the limitation on the number of
vertices.  We will need a method that returns unsigned int rather than
short.

(2)  We definitely do not want to break existing applications that are
expecting a "short *" argument type.

(3)  I don't think we want two functions of the same name, one public
and one private, with different argument types.

	Something I don't know (my C++ is a little rusty) is whether
applications will be able to distinguish between two public functions

ssgVtxTable::getTriangle( int n, short *v1, short *v2, short *v3)

and

ssgVtxTable::getTriangle( int n, unsigned int *v1, unsigned int *v2,
unsigned int *v3)

Since the arguments are pointers, I think C++ can make the distinction.
If this is the case, I would suggest that we add a new public function
with "unsigned int *" arguments and that we deprecate the present
function.

John F. Fay
Technical Fellow
Jacobs Technology TEAS Group
850-883-1294=20

-----Original Message-----
From: pli...@li...
[mailto:pli...@li...] On Behalf Of Paolo
Leoncini
Sent: Tuesday, June 19, 2007 3:31 AM
To: pli...@li...
Subject: [Plib-devel] How many triangles in a leaf?

Dear Plib friends,

I'd faced again (see my post on the list "Large geometry objects and
crash
in ssgLOS" on 2005-11-24) with the unclear limit on the number of
primitives
the IndexArray can index.

Even if we accept filling the VertexArray with no more than
sizeof(short)
vertices, the problem is that we can't index more than sizeof(short)/3
triangles per ssgVtxTable (and derived leaf types), and it only arises
when
doing LOS or other queries on the geometry.

Look at the code in ssgVtxArray.cxx:

void ssgVtxArray::getTriangle ( int n, short *v1, short *v2, short *v3 )
{
  short vv1, vv2, vv3 ;

  ssgVtxTable::getTriangle ( n, &vv1, &vv2, &vv3 ) ;

  *v1 =3D *( indices -> get ( vv1 ) ) ;
  *v2 =3D *( indices -> get ( vv2 ) ) ;
  *v3 =3D *( indices -> get ( vv3 ) ) ;
}

ssgIndexArray::get would take unsigned int argument type, but it is fed
with
the results from ssgVtxTable::getTriangle, which does return short
indices.

Thus, by summarizing: max number of triangles in LOS-compatible
ssgVtxTable
and ssgVtxArray: 10922.

For overcoming the limit we could either

- leave such limitaion for ssgVtxTable, yet making
ssgVtxArray::getTriangle
independent of the ssgVtxTable::getTriangle call;

Or

- change the return type for ssgVtxTable::getTriangle indices to
unsigned
int;

Or even

- introduce a private method ssgVtxTable::getTriangle( int n, unsigned
int
*v1, unsigned int *v2, unsigned int *v3 ), and call this from both the
official ssgVtxTable::getTriangle ( int n, short *v1, short *v2, short
*v3 )
and from the ssgVtxArray::getTriangle above.

Waiting for reactions -

Paolo


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