Left: region of the [u ₁, u ₂] parameter space where conditions A and B are respected. Center: reparametrization of the limb-darkening parameter space such that the two triangular regions forming the parameter space following conditions A and B are right-angled triangles. Right: parameter space joining the limb-darkening and limb-brightening scenarios, obtained by rotating the limb-brightening scenarios about the u ₁/2 axis.