Schematic graph of the entirely inner part of the flaring disk grid in radial-vertical (R-θ) plane with marked ray-tracing for calculation of the disk optical depth. All the rays come from one point (from the “north” pole of a critically rotating star where R_eq = 1.5 R_⋆). The grid cells are depicted as black lines, the blue rays intersect the vertical cell interfaces while the red lines enter the grid cells through their upper flaring interface (or the lower flaring interface for the not plotted rays that emerge from the stellar “south” pole). See Sect. A.5 for the description.