billboard, flat card  Terms used to describe the actual geometry
used for impostors that are based on rendering camera-aligned
shapes.   Any object who's geometry does not even superficially
resemble the final rendered product. In these cases, the mesh
geometry is usually just a way to designate an area of the
screen to draw to, while the fragment shader does the real work.
ray tracing  For the purposes of this book, ray tracing is a
technique whereby a mathematical object is tested against a ray
(direction + position), to see if the ray intersects the object.
At the point of intersection, one can generate a normal. With a
position and normal in hand, one can use lighting equations to
produce an image.  early depth test, early-z  An optimization in
the depth test, where the incoming fragment's depth value is
tested before the fragment shader executes. If the fragment
shader is long, this can save a great deal of time. If the
fragment shader exercises the option to modify or replace the
fragment's depth, then