[vtk-developers] quadratic cell types

David Thompson david.thompson at kitware.com
Fri Feb 1 00:31:44 EST 2013


Hi Mark,

> Hi, I filed bug report #0013827 regarding contouring and isovolumes for quadratic cells, but since then I’ve been doing some reading and came across this paper:
> Methods and Framework for Visualizing Higher-Order Finite Elements by Schroeder, et al.
>  
> And also this discussion from last June:
> http://www.vtk.org/pipermail/vtkusers/2012-June/124131.html
>  
> And looked at the README.html referenced in the above discussion. So if I understand it right, one has to write an adaptor to work with  VTK_QUADRATIC_* cells that will supply a quadratic parameterization?

You do not need to use the vtkGenericDataSet (the "adaptor" route) to use the quadratic cells; they may appear in a vtkUnstructuredGrid. However, isocontouring and other operations on these cells will break them up into a fixed linear approximation.

I haven't had a chance to look at the bug report you've filed, so I can't comment on the odd colors and contour shapes. Feel free to keep pinging me about it, but I am oversubscribed right now.

If you want smoother approximations to the contour, you can use the vtkTessellatorFilter to adaptively subdivide the elements based on scalar field and/or geometric curvature before contouring/clipping/etc.

  http://vtk.org/doc/nightly/html/classvtkTessellatorFilter.html

This will generally produce results that look better but not "right" as the edge subdivision decisions are made locally and can result in long, unsubdivided edges near short, subdivided edges.

	Hope this helps,
	David


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