Shading in Photoshop

This next post is probably my favourite assignment of all, mostly because it involved problem solving with perspective drawing.  We were given a scene of four basic geometric forms, and our assignment was to pick a light source and use perspective construction to find the shadows and shade the forms.  The first image here is the final product, and the second is all the construction I used to find the shadows.

One curious thing about this set up was that I discovered that the sphere intersects both the box.  I was able to find where it did this, and attempted to shade it appropriately.