Top: dependence of view factor, F₂₁, in the case of shared-edge facets as a function of the opening angle between these facets (case C) in Fig. 2). As in previous cases, M₄ is the reference value. Bottom: relative deviation from M₄. The M₃-Linear and M₃-R2 methods show a reasonable accuracy even for small angles (around 10%). Both methods become more accurate with larger opening angles, although never get to less than 1%.