Computing Cup Products in $$\mathbb {Z}_2$$-Cohomology of 3D Polyhedral Complexes

Let \(I=(\mathbb {Z}^3,26,6,B)\) be a three-dimensional (3D) digital image, let \(Q(I)\) be an associated cubical complex, and let \(\partial Q(I)\) be a subcomplex of \(Q(I)\) whose maximal cells are the quadrangles of \(Q(I)\) shared by a voxel of \(B\) in the foreground—the object under study—and by a voxel of \(\mathbb {Z}^3\backslash B\) in the background—the ambient space. We show how to simplify the combinatorial structure of \(\partial Q(I)\) and obtain a 3D polyhedral complex \(P(I)\) homeomorphic to \(\partial Q(I)\) but with fewer cells. We introduce an algorithm that computes cup products in \(H^*(P(I);\mathbb {Z}_2)\) directly from the combinatorics. The computational method introduced here can be effectively applied to any polyhedral complex embedded in \(\mathbb {R}^3\).