This demo generates a raster layer based on modified data from another layer.
This example generates a OpenLayers.Raster.Composite
from a WMS layer. The composite data is a luminosity calculation
from a OpenLayers.Raster.Operation that works with
the source RGBA data. The resulting luminosity data is displayed
on a second map with a OpenLayers.Layer.Raster.
See the raster-grid-layer.js source for details on how this is done.