OK, so I was making an environment with modular set peices and thought I'd be clever by just negating the scale on certain objects to mirror them.

Only I now noticed that this causes my engine to flip the face culling on them. With back face culling, the opposite side of the object is visible when I negate one of the axes.

I guess I can try to determine what face culling mode to enable based on scale. I'm trying to figure out though, do most game engines try to allow for this and switch face culling state based on object scale or should I just make mirrored versions of my objects?

Also how would I reliably tell if I should front face cull or back face cull? I see that one axis being flipped causes this face culling flippage. Is face culling flipped with odd axis numbers, or any flipped axes? I tried searching for the pattern for this, and I can't find a reliable way to check the face culling mode based on scale. If no one has an answer I can try to spend some time experimenting.

Let's say we have the scaling matrix :

a 0 0 0

0 b 0 0

0 0 c 0

0 0 0 1

My guess is that if a*b*c<0 then the culling order must be inverted.

Let's say we have the scaling matrix :

a 0 0 0

0 b 0 0

0 0 c 0

0 0 0 1

My guess is that if a*b*c<0 then the culling order must be inverted.

Correct. More generally, it needs to be inverted whenever det(M) < 0.

You can handle that on the exporter :

int VertexIndex[ 3 ];
if( TMNegParity( tm ) )
{
  VertexIndex[ 0 ] = 1;
  VertexIndex[ 1 ] = 2;
  VertexIndex[ 2 ] = 0;
}
else
{
  VertexIndex[ 0 ] = 0;
  VertexIndex[ 1 ] = 2;
  VertexIndex[ 2 ] = 1;
}
...
for( int j = 0; j < 3; ++j )
{
  ...
  Pos = mesh->verts[ f->v[ VertexIndex[ j ] ] ];
  ...
}

The problem is that this usually happens *after* export. The OP mentioned using 'modular set pieces', so I'm picturing stamping down multiple copies of a simple object (tree, bench, whatever). In that situation, negative scale is a good way to add variety, but you have the culling problem. In any case, the methods suggested by Hodgman are probably the best.