Population size and geometry
----------------------------

A population, defined by the ``<pop>`` object, consists of a set of
units typically arranged on a cartesian grid.  The geometry of the population
is specified using the following object::
 
	<geometry>
		xn  16           # Number of cells in x-dimension [Area xn?]
  		yn  16           # Number of cells in y-dimension [Area yn?]
  		zn   4           # Number of cells in z-dimension [1]
  		x0   8           # Origin w.r.t. Area origin [0]
  		y0   8           # Origin w.r.t. Area origin [0]
  		xf   1.0         # Scaling w.r.t. Area (or stimulus grid?) [1.0]
  		yf   1.0         # Scaling w.r.t. Area (or stimulus grid?) [1.0]
  		oddxoff 0.0      # x-offset of odd row numbers [0.0] (grid units)
	</geometry>


Values that are not specified default to those given for the
``area`` to which the population is assigned, or to fixed values,
as specified above.

An ``<area>`` is defined as follows::

   <area>
	name  v1           # Name to identify this area [exit]
  	xn    16           # Size of area in x-dimension [exit]
  	yn    16           # Size of area in y-dimension [exit]
  	x0     8           # Origin w.r.t. stimulus grid [exit]
  	y0     8           # Origin w.r.t. stimulus grid [exit]
  	xf     1.0         # Scaling w.r.t. stimulus grid [1.0]
  	yf     1.0         # Scaling w.r.t. stimulus grid [1.0]
  	umx   59.0 (um)    # micons per Area grid unit [0.0]
  	umy   59.0 (um)    # micons per Area grid unit [umx]
   </area>


The stimulus grid is defined by the following top-level parameters::

    xn      16           # (pix) Size of stimulus in x-dimension [exit]
    yn      16           # (pix) Size of stimulus in y-dimension [exit]
    sscale   0.04        # (deg/pix) spatial scaling of stimulus grid [exit]