I was driving my boss to the other building at work, and we went past some trees on a hill by the road. The leaves from the trees had fallen on the ground, forming yellow leaf-clouds in the grass. These leaf-clouds were offset from the trees to which they pertained, implying that there must have been a steady eastern wind when these leaves were falling. Nature has a way of beautifully demonstrating simple mathematical principles. The on-tree leaf-density, terrain height, and wind current, mappings from R3 to R, R3 to R and R3 to R3, respectively, combined to form the on-ground leaf density mapping from R2 to R.
Or, as Adam put it, it kinda looked like the trees had shadows made of leaves.