[Paraview] Using Threshold

[David Thompson]

>>> 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?
>> 
>> The threshold filter copies cells, not points, from the input dataset to the output dataset. So, since you have point data it must use a rule to decide whether or not a given input cell should be included in the output based on the points that make up the cells. When you click on "All scalars," it uses the rule that *all* the points of a given input cell must be inside the threshold range (i.e., between -0.2 and 0.2) in order for the cell to be copied. It sounds like none of your cells have all 0 values at their points, so the output is empty. When "All scalars" is unchecked, the rule is that *any* point of a given input cell matching the threshold criterion will result in the input cell being copied to the output. In this case, cells with *any single* point value between -0.2 and 0.2 will cause the cell (and *all* of its points, even those which don't meet the threshold) being copied to the output. So, what is it that you really want to do? You might consider clipping by the scalar field (that will get the shape right but break cells up into pieces) or converting the input dataset from cells that connect points to just a point cloud by using the glyph filter to create a VERTEX cell at each input point. You could then threshold the point cloud. In that case you'll get just the points that match as output, but they won't be connected to other points as they were in the original mesh.
> 
> I understand now.  I definitely want to break it up into a point cloud, probably using the glyph filter to create a vertex cell at each point. How do I go about this?

Hi Michael,

Load your data. Go to the Filters menu and choose "Glyph". When the filter options appear in the Object Inspector panel, set the "Glyph Type" to "2D Glyph". Then, a "2D Glyph" box will appear. Select "Vertex" in its drop-down menu and click the filter's Apply button.

Note that if you have more than 5000 points, you'll need to turn off the "Mask Points" option or it will randomly choose 5000 points and only output those (this was done so that rendering of large datasets would be possible, but since you plan to subset the output of the Glyph filter with a Threshold filter there is no need to mask points).

	David