[vtkusers] what does the "SetLevel" (I mean the level value) do in vtk?

[kdsfinger at gmail.com]

hi, all
The SetWindow and SetLeve often come together (or SetWindowLevel). I
understand the window gives a range of the color that vtk is going to
deal with and cast those out of range to the two ends. Level value is
usually set in the middle of that range. But I don't understand what
does the level value stands for. If I adjust the level value lower
then the image looks brighter. What does the level value do? Thanks
for help.
zl2k