Value indicates a percentage between the minimum and maximum values in
the range shall be used as the minimum / midpoint / maximum value for the
gradient.
process current cell.
Note: In consideration of performance, the LightCell object and the object field of it
maybe reused for all cells, if it is needed to use current cell's data later out of current
processing scope, the implementation should create one copy of current cell's data by LightCell.copy()
and keep the cloned one for later using.