[vtkusers] cell picker tolerance zoom dependence

[Gary L]

I'm trying to set the tolerance of a vtkCellPicker. I would like to
control precisely how many pixels away from a given cell
are sensitive to a pick. According to the documentation the tolerance
is related to the rendering size measured across
the diagonal.  However, I've noticed that the sensitivity of a picker
is also highly dependent on how zoomed away one
is from the object one is picking. If one is zoomed in very close to
the cell, then a larger radius of pixels around
the a cell is sensitive to a pick, though if one is zoomed out away
from the cell, then a smaller radius is sensitive.
Anyone know if there is a way to remove this dependence on the zoom
level? Is there perhaps some formula for
setting the tolerance based on camera position, or perhaps some other
scheme for making pick sensitivity zoom
independent?

Thanks!
Gary