[vtkusers] filter points by normal attribute?

[Budd Hirons]

is there a way to glyph a vtkPolyDataSet and render only the glyphs that 
meet a certain criteria in the normal attribute?  I am using the pointdata 
scalar to color the glyphs already, and would like to filter the input 
dataset or edit the glyphs somehow and only visibly render the glyph points 
that have a given normal attribute.

thanks,
budd.