Effective opacity (black) for a homologously expanding medium with clump properties similar to the core of the Type IIb SN models compared with full radiative transfer calculations using the virtual grid method (yellow) and actual 3D grids containing spherical (solid cyan), elongated (dotted cyan), and flat (dashed cyan) clumps. We also show the optically thin (blue), optically thick (red), and geometrical (dotted black) limits. The opacity is given relative to the thin limit, and the geometrical optical depth in the compressed and expanded regions is shown at the top and bottom, respectively.