Boundary conditions for two different points, r₁, r₂, and different directions, n₁, n₂. Green: a ray originating from the stellar photosphere. To calculate the intensity at r₁ in direction n₁, the intensities at the neighbouring grid points, and , need to be known. A boundary condition is required for grid point , while the intensity at results within the “normal” RT-scheme. Red: a ray originating from outside the photosphere. For the grid point r₂, a boundary condition has to be specified at the corresponding phantom point, .