[Paraview] Thresholding an image

[David Doria]

I am trying to look for the pixels in an image whose magnitude are
above a certain value. I loaded the image, then tried to use the
Threshold filter. The tooltip says "Requires an attribute array with 1
component."

Question 1) Would it make sense to allow this filter to operate on one
of the channels of the image instead of having to extract it manually?

To generate this attribute array, I used the Calculator filter to make
an array called AbsX with abs(MetaImage_X). Then I applied a Threshold
filter to this new array. The values in this array are in [0 512].
When I leave the Lower Threshold all the way down and the Upper
Threshold all the way up, I can view the image just fine. In the
Display tab, the Color By is set to AbsX, like I want. However, as
soon as I move the Lower Threshold slider, the image disappears. When
I look in the Display tab, the only option for Color By is now Solid
Color.

Question 2) Why would the Color By options change based on the
threshold that is set? (I'd also ask why the image is disappearing,
but I think it might have to do with the coloring).

If I then try to glyph the output of the threshold filter (to draw
spheres or something at the location of the pixels that remain), the
Scalars option of the glyphs shows my AbsX array, but it is not
enabled. The only choice is under Vectors, and it is my MetaImage
array.

Question 3) Why can I not glyph the AbsX array?

Sorry for the long winded question, but hopefully someone can shed
some light on these issues!

Thanks,

David