[vtkusers] dark glyphs

[David.Pont at ForestResearch.co.nz]

Hi Chris,
     I have had problems with glyphs disappearing with negative values.
Your lighting must be such that they are still just visible, lucky you, I
was completely unaware for a while, then quite baffled for even longer. A
scaling transform is applied to glyphs according to their scalar values,
negative values turn the glyphs inside out, specifically the normals point
inward, so they dont reflect light. You might be able to manipulate the
input data (take absolute value, or add a fixed offset).
My fix was to pass the output through vtkPolyDataNormals. This is 'costly'
but works. There is still a problem with zero and small (absolute) values
which effectively disappear, and very large (absolute) values that swallow
nearby small glyphs.
A real remedy would be a way of normalising the range of glyph sizes within
a user specified range so the smallest (signed) value maps to a specified
small size, and the largest (signed) value maps to a specified large size.
Could even define a non-linear mapping within the range. I suppose this
would have to be done inside vtkGlyph3D?

  Dave Pont



|--------+--------------------------------->
|        |          "Christopher M.        |
|        |          Navarro"               |
|        |          <cnavarro at ncsa.uiuc.edu|
|        |          >                      |
|        |          Sent by:               |
|        |          vtkusers-admin at public.k|
|        |          itware.com             |
|        |                                 |
|        |                                 |
|        |          06/06/2003 15:12       |
|        |                                 |
|--------+--------------------------------->
  >------------------------------------------------------------------------------------------------------------|
  |                                                                                                            |
  |      To:     vtkusers at public.kitware.com                                                                   |
  |      cc:                                                                                                   |
  |      Subject:     [vtkusers] dark glyphs                                                                   |
  >------------------------------------------------------------------------------------------------------------|





When I create a lookup table that has a range from negative to positive
values, say -1 to 1, all of the glyphs associated with negative scalars
are very dark.  Why is that?  Is there something that can be done to
correct this?  Thanks.

-chris

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