[Paraview] Using Threshold

[Michael Cohen]

Hi all,
I have been using the Threshold filter for some time and now I am 
discovering it doesn't do what I want it to do.
I have a surface made up of a set of points, each point has a location 
in 2-space, and a function value attached to it, -2, -1, 0 or 1.
I would like to filter out all points that don't have a particular 
value, say zero.
I plot the full shape, and place a Threshold filter on top of it, 
excluding everything above (say) 0.2,a nd everything below -0.2.
If I tick the box that says "All scalars", none of my points show up, 
even though I can see by the colour coding that there are some points 
with the zero value.  If I untick the box then those points do appear, 
but so do neighbouring points with other values.  What's going on here?

Thanks,
Michael