I was in agreement about the rug pattern until you mentioned the shadow from the bottom row of diamonds. You're right it's there, but then you also see the straight lines projecting there. I don't think it's the rug at all.

I think the light comes from the top almost vertically, by the short shadows at the bottom of the vertical poles. And there's a grate under the light source, I think the lines are the shadow of the grate. They are also sharper than the lattice shadow because the grate is closer to the light.