I always get confused by the inverted image on pinhole cameras and this series by a Pixar animator on Khan Academy served as a great resource for me.
I am confused by how a pinhole camera samples radiance. According to the definition, radiance is a quantity associated with a light ray. Does this mean that a pinhole camera can sample the average radiance across a certain area? And how is this used in practice to ensure that there is only one ray pointed at one direction?
I think it is because the hole for the light is so small, only one "ray" is allowed through from each direction, as shown in the diagram - very little light is allowed to enter parallel.