and all 5 lights are casting
shadows then your entire scene will be drawn 6 times. All 20 objects
will be drawn for light #1, then all 20 objects will be drawn for
light #2, then #3, etc and finally the actual scene will be drawn
using data from the first 5 renders.</p>
<p>It gets worse, if you have a point light casting shadows the scene
has to be drawn 6 times just for that light!</p>
<p>For these reasons it's common to find other solutions than to have
a bunch of lights all generating shadows. One common