Oh, it is the shoelace formula, but for 3D? OP's description vaguely triggered memories of the shoelace formula from a decade ago, but geometry was never my st

Oh, it is the shoelace formula, but for 3D? OP's description vaguely triggered memories of the shoelace formula from a decade ago, but geometry was never my strong suit. All I remembered was positive/negative triangles (or was it trapezoids)? make magic happen for area calculation.