Shadow question

[Ofnuts]

If you look at the geometry of things the shadow (from the sun) of an object on a vertical wall is just the shape of the object, where verticals remain vertical. If the wall isn't perpendicular to the light rays, you get a deformation which is what you would get with the perspective tool. The ground shadow is also a perspective transform. In the following example:

• The shadow on the wall is just the shape of the object, shifted down and with a perspective to match the wall. The bottom is cut where the wall touches the ground  (at half the trunk height)
  
• The shadow on the ground is another perspective transform where: 1) the base of the tree is kept, and 2) it intersects the base of the wall where the wall shadow is cut, and in a place which is roughly at half the trunk height. As you can see from the branches in the top right, this is a very elongated perspective (in a real image you would cut these off of course.
  
• You can probably do the two perspectives in the other order, for instance if you have strong constraints on the ground shadow.