Class DotArea2D

         object --+        
                  |        
ClassWithParameters --+    
                      |    
               Stimulus --+
                          |
                         DotArea2D

Random dots of constant velocity

Every dot has the same velocity. Some fraction of the dots all
move in the one direction, while the rest move in random
directions. Dots wrap around edges. Each dot has a lifespan.

This is just one example of the endless variations on drawing random dots.

Parameters
==========
anchor                  -- (String)
                           Default: center
anti_aliasing           -- (Boolean)
                           Default: True
color                   -- (AnyOf(Sequence3 of Real or Sequence4 of Real))
                           Default: (1.0, 1.0, 1.0)
depth                   -- (Real)
                           Default: (determined at instantiation)
dot_lifespan_sec        -- (Real)
                           Default: 5.0
dot_size                -- (Real)
                           Default: 4.0
on                      -- (Boolean)
                           Default: True
position                -- (Sequence2 of Real)
                           Default: (320.0, 240.0)
signal_direction_deg    -- (Real)
                           Default: 90.0
signal_fraction         -- (Real)
                           Default: 0.5
size                    -- (Sequence2 of Real)
                           Default: (300.0, 300.0)
velocity_pixels_per_sec -- (Real)
                           Default: 10.0

Constant Parameters
===================
num_dots -- (UnsignedInteger)
            Default: 100

Instantiate and get ready to draw.

Set parameter values and create anything needed to draw the
stimulus including OpenGL state variables such display lists
and texture objects.

Draw the stimulus. (Called by Viewport instance.)

This method is called every frame.  This method actually
performs the OpenGL calls to draw the stimulus.