FAQ - Frequently asked questions

Many points covered in this FAQ are illustrated via Python scripts. Remember that the Python API is almost 100% identical to the C++ one, with the rules listed here.

Questions

General questions

1. I am a complete newbie, where should I start?
2. What is a mesh, what is a field?
3. What is a the difference between MED file, MEDCoupling and the SALOME MED module?
4. How can I visualize a mesh and/or a field?
5. What does a P0- (or P1-) field mean?
6. What does surjective/old-2-new/new-2-old format mean?
7. What is the implicit numbering of a structured mesh?

MEDCoupling scripts in Python

1. "Can you show me a simple example to get me started"
2. "When trying to execute my Python script I have 'ImportError: No module named MEDCoupling'"
3. "How to build a mesh from scratch"
4. "How to build a field from scratch"
5. "How to write/read a mesh to/from a file"
6. "How to control the validity of my mesh"
7. "How can I read/write groups on a mesh"
8. "How can I transform a structured mesh into an unstructured one"

Projection, interpolation, remapping

1. How to project a field from one mesh to the other
2. Which formula are used in the field projection algorithms

C++ specific questions

1. Is there some coding guidelines that I should follow
2. My C++ program produces a SIGSEGV, what should I do

Answers

General questions

I am a complete newbie, where should I start?

Take a look at the getting started section and the Tutorial - MEDCoupling/MEDLoader in Python.

What is a mesh, what is a field?

Take a look at Terminology - Meshes, fields, interpolation

What is a the difference between MED file, MEDCoupling and the SALOME MED module?

Take a look at The MED constellation: MEDCoupling, MEDLoader, MED file, etc ...

How can I visualize a mesh and/or a field?

Use the PARAVIS module of SALOME to visualize your MED file. The following dedicated fitlers have been written specifically for MED files: Extract group, Extract cell types, ELNO Mesh, ELNO Points, ELNO Surface.

What does a P0- (or P1-) field mean?

Take a look at Terminology - Meshes, fields, interpolation

What does surjective/old-2-new/new-2-old format mean?

Take a look at Array indexing and numbering

What is the implicit numbering of a structured mesh?

When converting a structured mesh to unstructured one, or when storing a field onto a structured mesh, the numbering convention detailed in MEDCoupling::MEDCouplingStructuredMesh::buildUnstructured() is used.

MEDCoupling scripts in Python

"Can you show me a simple example to get me started"

TODO

"When trying to execute my Python script I have 'ImportError: No module named MEDCoupling'"

Check that the environment variables PYTHONPATH and LD_LIBRARY_PATH (PATH under Windows) are correctly set. If you have a full SALOME installation, use the 'shell' command that will automatically set up everything as it should be:

cd <salome_install>
salome shell

With a custom installation you may want to set the variable manually:

export PYTHONPATH=<install_root>/lib/python2.7/site-packages/salome
export LD_LIBRARY_PATH=<install_root>/lib/salome

"How to build a mesh from scratch"

Take a look at this example: Standard build of an unstructured mesh from scratch

"How to build a field from scratch"

Take a look at this example: Standard build of a tensor field on cells with no time attached

"How to write/read a mesh to/from a file"

For starter, take a look at the basic MEDLoader API.

"How to control the validity of my mesh"

Use the methods MEDCouplingUMesh::checkConsistencyLight() or MEDCouplingUMesh::checkConsistency()

"How can I read/write groups on a mesh"

Take a look at Reading a mesh. and Writing a mesh..

"How can I transform a structured mesh into an unstructured one"

Use the method MEDCouplingCMesh::buildUnstructured()

Projection, interpolation, remapping

How to project a field from one mesh to the other

This the job of the interpolation algorithms in the MED library. For starters, take a look at the general introduction on interpolation. Also this simple example gives a good first illustration. Finally, if you are intereseted in parallel projection (C++ only!), you should take a look at the DEC.

Which formula are used in the field projection algorithms

The documentation for non P0 field (i.e. non cell-based fields) is still an on-going work, but for the P0->P0 case, this page gives a good overview.

C++ specific questions

Is there some coding guidelines that I should follow

Yes. Please:

• document your code (this is true for Python too!)
• write some tests (this is true for Python too!)
• and finally, take a look at the page Note for C++ developpers

My C++ program produces a SIGSEGV, what should I do

Re-compile in debug mode (with CMAKE_BUILD_TYPE=Debug), and use either valgrind or gdb to spot the place where the segfault happens. The most common source of mistake is some memory mis-allocation and/or deallocation. With this respect using the auto pointer class MCAuto can be of great help.