ge&#39;</code> on the raster source, operations are
  called with an <code>ImageData</code> object for each of the input sources.
  Operations are also called with a general purpose <code>data</code> object.
  In this example, the sun elevation and azimuth data from the inputs above
  are assigned to this <code>data</code> object and accessed in the shading
  operation.  The shading operation returns an array of <code>ImageData</code>
  objects.  When the raster source is used by an image layer, the first
 