hexahedral mesh rendering

[Yuanqiao Wu]

Dear developer,

I am working on a huge FE model with hexahedral mesh. I've tried a lot of other kinds of toolboxes and software to load the mesh. None of them works. 

I got suggestions from my mentor and she highly recommended Iso2Mesh. But seems like the default element type is the tetrahedral mesh. Any idea on how to switch to the hexahedral kind? 

Best,

Qiao

[Qianqian Fang]

hi Yuanqiao,

by "working on", do you want to create a hex mesh? or want to load/display/process an existing hex mesh?

if it was the former, unfortunately iso2mesh does not create hex mesh. the meshing utilities we use, CGAL and tetgen, under the hood are mostly designed for tetrahedral mesh generators (maybe Laurent Rineau from CGAL team can comment).

in this case, please check out BioMesh3D from U. of Utah:

https://www.sci.utah.edu/publications/callahan09/scirun_meshing.pdf

https://www.nitrc.org/projects/biomesh3d/

if it was the 2nd case, iso2mesh can display hex mesh (or any polyhedral mesh) using plotmesh. you will have to find a format reader to read it in matlab, and I can show you a simple command to display/process.

Qianqian

[Yuanqiao Wu]

Hi Qianqian,

I mean the second case. We created the FE model from CT scan (converting each voxel into a FE element) and had trouble in display the model. 

Here is an example of the connectivity file (elem):

and node location file (node):

This is the command that I use: figure, plotmesh (node,elem)

This is part of the FE model from different views: 

An example will be helpful.

Thanks in advance. 

Best,

Yuanqiao 

[Qianqian Fang]

I am confused - didn't you say you had a hexahedral mesh? I am expecting 8-node solid elements, thus an elem array of N-by-8.

https://en.wikipedia.org/wiki/Types_of_mesh#/media/File:Three_Dim_Grid.PNG

did you manually split each hex into smaller tetrahedra?

in other words, what is the column number of your elem? and what do the numbers in each column mean?

if you plan to do an FEM analysis on this domain, and your FEM modeler does support tetrahedral mesh, my strongly recommend you to create a coarse tetrahedral mesh instead - despite your box-based mesh has voxel-level density, it is rather inaccurate and inefficient because the voxel boundaries you saw are largely due to discretization errors and does not reflect the shape of the actual boundary. Creating a coarse tetrahedral mesh not only can reduce your mesh density dramatically, but also help you restore that smooth surface that was lost after rasterization.

You can consider running v2m or v2s to convert your volume to a tetrahedral or triangular mesh. Run the examples in iso2mesh/samples to see how it works.

Qianqian

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[Yuanqiao Wu]

Hi Qianqian,

Sorry I might be not clear on stating the case.

Yes, you are right. We had hexahedral mesh already, which is 8-node solid elements. 

We have two file, one is node connectivity file and the other one is the node location file.

The node connectivity file is a N-by-8 array, which stores the node index. For example, the first row of this file: 1 2 5 4  179 180 185 184. It means the first element is formed by node 1,2,5,4,179,180,185,184. 

The node location file is a M-by-3 array, which stores the node location in space. For example, the first row of this file:  7.424 7.304 1. It means the first node (1 in the node connectivity file).

Hope I make it clear enough. If not, please let me know and I am happy to chat.

As for why not tetrahedral mesh, the answer is: yes we do have one model with tetrahedral mesh. The aim of this project is to compare the accuracy of FE model with different element types in capturing the experimental results.

Best,

Yuanqiao