Re: [Plib-users] bug in ssgOptimiser?

[Steve B. <sjb...@ai...>]

lo...@sl... wrote:
> hello all,
> 	In ssgOptimiser.cxx from CVS Sept. 30, at the end of the 
> OptVertexList::makeNormals function there is the following chunk of code 
> (roughly line 323):
> 
>   for ( i = 0 ; i < vnum ; i++ )
>     if ( sgScalarProductVec2 ( vlist[i]->normal, vlist[i]->normal ) < 
> 0.001 )
>       sgSetVec3 ( vlist[i]->normal, 0.0f, 0.0f, 1.0f ) ;
>     else
>       sgNormaliseVec3 ( vlist [ i ] -> normal ) ;
> 
> 
> My belief is that the call to sgScalarProductVec2 should really be a call 
> to sgScalarProductVec3.  Please correct me if I'm wrong.

I guess so - but this is somewhat cryptic code anyway.

The scalar product of a vector against itself is just the length of the
vector...so sgLengthVecN(vlist[i].normal) would be more readable here.

As to whether it should be Vec2 or Vec3, I'm not 100% certain what was
originally intended here.

If we assume the code is *WRONG* and it should be Vec3 - then this reads:

     If the normal is vastly too short and the normalise function might fail,
     then make the vector point straight up instead of maybe getting a divide
     by zero.

If we assume the code is *RIGHT* then it reads:

     If the normal's X/Y projection is very short - then make it point straight
     up.

In fact, both interpretations have the same consequences.   If the vector is
seriously de-normalised then it's X/Y projection will also be very short - so
we'll set the vector to (0,0,1) in either case.

If the vector is short in the X/Y plane - but has significant Z-direction length,
then setting it to (0,0,1) has a very similar effect to normalising it.

Hence, whilst I'm pretty sure this should be:

      if ( sgLengthVec3 ( vlist[i]->normal ) < 0.001 )

...it won't make *any* difference to any programs out there - except to slow
them down a teeny-tiny bit.

Anyway - I've fixed it and committed to CVS.

----------------------------- Steve Baker -------------------------------
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