I’ve watched your excellent tutorial many times and I’m having trouble with one spot: you refer to a box without top and bottom with surface normals pointing inwards as allowing you to “seal your interiors so the light rays don’t fly away.” You referred to it as a portal. I’m wondering why you called it that and how does it keep the light rays from flying away if we have windows with a view of the lightmap? What happens to this "box" that seals the light rays in, when our rendering in its daytime setting has window openings that allow us to see out?
There must be something here that I'm missing
Your assistance with this would be appreciated.