[vtk-developers] Rendering scalar data with NaNs

[David Thompson]

> I’ve recently run into an issue with rendering fields with NaN  
> values in it.... I propose making the changes in the attached  
> patches (created with git-format-patch).  First, a NanColor was  
> added to vtkLookupTable (as a 4-tuple RGBA) and added to  
> vtkColorTransferFunction (as a 3-tuple RGB) so that these respective  
> lookup tables would return a specific color for NaN.  Next,  
> vtkScalarsToColorsPainter was modified to use a 2D texture map that  
> uses the second dimensions to distinguish between real and NaN  
> values.  I also added a test for rendering these NaN values along  
> with the other non-finite –INF and INF values.
>
> Does anyone object to these changes, or do they seem reasonable to  
> put into VTK?
Hi Ken,

For cell data, the patch seems really useful. For point data I'm less  
sure. For example, the attached images show first your test and second  
what happens when 2 neighboring cells both have NaNs. Without the  
texture map, color interpolation looks bad (bars 1, 3). With the  
texture map, the valid values between NaNs appear as a single pixel  
(bars 2, 4). Perhaps a filter with a feature similar to vertex  
splitting in vtkPolyDataNormals is in order? It could modify polydata  
as required to insert extra vertices along edges with a single NaN, so  
that the rendered output would only display NaN in about half of each  
cell.

Also, I've long wanted a way to specify a multidimensional texture map  
with hue mapped to one axis and saturation to another. Will this  
change prevent it? If so, would you mind moving the NanColor handling  
in the mapper to a subclass?

	Thanks,
	David

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