Photograph: Linear or Non-Linear?
Seeing the line of light shining through a window, the lines of shadows. Keeping a large perceptual distance, we may hardly notice the penumbra adjacent to umbra of the shadows, and they may appear to behave linearly in the usual perspective projection. So we might establish the intuition that light behaves linearly on a macroscopic scale.
Looking both closer and globally, in the sense of a macroscopic steady-state or constancy, we might conclude that everything depends on everything and the photography is a projection of this non-linear state. Even small perturbations may yield large changes. And this situation is reflected by the exponential behaviour of the global illumination transport theory.
Although some shafts of light appear to be linear in shape, the whole ensemble viewed from the transport mechanism might appear non-linear. And one possible conclusion: this is rather a matter of reference, of perspective, of projection. A perspective, which can be distorted, shifted.