Dear Gordon,

Thank you for the reply. Let me use two quotes from the MACET paper (Edge Transformations for Improving Mesh Quality of Marching Cubes), in the context of isosurface extraction.

"For each configuration, the set of active edges, i.e., edges where the function f has different signs at the endpoints, independently determines both the geometry and topology of the mesh. The topology is determined by the configuration of active edges for each cell. The geometry is determined by the location of the isovalue along each active edge. This independence leaves room to change the geometry of the mesh while keeping the topology intact. Our modification to MC explores this independence. Some edge transformations do not change the underlying topology: <<they only change the geometry of the isosurface.>>"

"In this work, we use a cubic spline interpolation to reconstruct the derivatives of the scalar field. <<We also use the corresponding cubic spline to reconstruct the scalar field itself>> at the sampling grid vertices. This means there might be more than one root in active edges. However, since the topology has already been determined by the lookup into the MC tables and the intersection calculation returns only one vertex, this presents no practical problems."

I want to leverage this independence of topology/geometry and get the geometry using cubic splines, in order to extract isolines from a volume dataset (a slice of it sor instace); preferably, using the teem library as cited in the paper. Do you have any pointers on how to proceed?

Best regards,
-Gustavo


On Tue, Dec 24, 2013 at 12:30 AM, Gordon L. Kindlmann <glk@uchicago.edu> wrote:
Hello,

Sorry for the slow reply.

If you're talking about connected isolines from 2D images, there is nothing in Teem geared towards this (and nothing has been removed either).

I'm not sure what the contours.h header was, but it was probably in the "seek" library (teem/src/seek).  Seek does support marching-cubes syle isosurface extraction, but like even fixed Marching Cubes it will not generate topologically accurate contours for any 3D filter (it only generates closed surfaces, but these don't match the topology of a true tri-linear interpolation, let alone BC-cubic splines).

If all you want is a dense sampling of a 3D isocontour, in a continuous volume reconstructed with BC-splines or other kernels, then you can use the "pull" library for particle-system based feature extraction.  Unfortunately there aren't currently any stand-alone demos of using pull for this purpose.  The demos here:
are for scale-space ridge line detection.  But again, this will only give a dense sampling, and there is no guarantee that it is "dense enough" to help produce a topologically correct mesh, if that's what you need.

Through a set of options, you can also tell pull to only work in 2D.

If you can provide some public datasets that could become part of teem/data and teem/Examples, that would help provide the incentive to create example programs.

Gordon

On Dec 15, 2013, at 6:38 AM, Gustavo CHAVEZ <gustavo.chavezchavez@kaust.edu.sa> wrote:

Dear Teem-users,

I'm trying to extract isolines from scalar fields (volume datasets in nrrd or raw format), based on BC-splines using the kernels of teem/nrrd. I am basically looking for a sophisticated extension of marching squares (that uses linear interpolation only).

I took a look at the source code of MACET, AFRONT, and the tutorial of nrrd (although this is for indirect volume rendering) with no luck. I also saw that back in 2006 teem distribution used to have a contours.h library which is no longer supported. Maybe you move this functionality somewhere else? or, is isocontour extraction out of the scope of the current capabilities of teem?

Best regards,

-Gustavo
Chávez

Graduate student of Computer Science



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