You have stated the problem exactly. However, in practice a painter employs a fair amount of 'bodging' (favoring appearance over exactitude). For example, it i

You have stated the problem exactly. However, in practice a painter employs a fair amount of 'bodging' (favoring appearance over exactitude). For example, it is not entirely unusual to discover that a landscape features more than one implied sun. In 3D rendering this would be done with light-linking, whereby different objects are exclusively light by different lights. Physically impossible, but visually convincing.