$Id: FAQ,v 1.51 2005/02/04 19:42:26 geuzaine Exp $

This is the Gmsh FAQ

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Section 1: The basics

* 1.1 What is Gmsh?

Gmsh is an automatic three-dimensional finite element mesh generator,
primarily Delaunay, with built-in pre- and post-processing
facilities. Its primal design goal is to provide a simple meshing tool
for academic problems with parametric input and advanced visualization
capabilities.  One of the strengths of Gmsh is its ability to respect
a characteristic length field for the generation of adapted meshes on
lines, surfaces and volumes.

* 1.2 What are the terms and conditions of use?

Gmsh is distributed under the terms of the GNU General Public
License. See the file doc/LICENSE for more information, or go to the
GNU foundation's web site at http://www.gnu.org.

* 1.3 What does 'Gmsh' mean?

Nothing ;-)

(Note that in the US, people tend to pronounce 'Gmsh' as
'Gee-mesh'. Yeah.)

* 1.4 Where can I find more information?

<http://www.geuz.org/gmsh/> is the primary location to obtain
information about Gmsh. You will for example find a complete reference
manual as well as a searchable archive of the Gmsh mailing list
(<gmsh@geuz.org>) on this webpage.

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Section 2: Installation

* 2.1 Which OSes does Gmsh run on?

Gmsh is known to run on Windows 95/98/NT/2000/XP, Linux, Mac OS X,
Compaq Tru64 Unix (aka OSF1, aka Digital Unix), Sun OS, IBM AIX, SGI
IRIX, FreeBSD and HP-UX. It should compile on any Unix-like operating
system, provided that you have access to a recent C and C++ compiler.

* 2.2 Are there additional requirements to run Gmsh?

You should have the OpenGL libraries installed on your system, and in
the path of the library loader. A free replacement for OpenGL can be
found at <http://www.mesa3d.org>.

* 2.3 What do I need to compile Gmsh from the sources?

You need a C and a C++ compiler (e.g. the GNU compilers gcc and g++)
as well as the GSL (version 1.2 or higher; freely available from
http://sources.redhat.com/gsl/) and FLTK (version 1.1.x, configured
with OpenGL support; freely available from http://www.fltk.org).
You'll also need the jpeg library if you want to save jpeg images, and
the libpng and zlib libraries if you want to save png images.

Under Windows, you will need the Cygwin tools and compilers (freely
available from http://www.cygwin.com).

* 2.4 How to I compile Gmsh?

Just type 

./configure; make; make install

If you change some configuration options (type ./configure --help to
get the list of all available choices), don't forget to do 'make
clean' before rebuilding Gmsh.

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Section 3: General problems

* 3.1 Gmsh [from a binary distribution] complains about missing
libraries.

Try 'ldd gmsh' (or 'otool -L gmsh' on Mac OS X) to check if all the
required shared libraries are installed on your system. If not,
install them. If it still doesn't work, recompile Gmsh from the
sources.

* 3.2 Gmsh keeps re-displaying its graphics when other windows
partially hide the graphical window.

Disable opaque move in your window manager.

* 3.3 The graphics display very slowly.

Are you are executing Gmsh from a remote host (via the network)
without GLX?  You should turn double buffering off (with the -nodb
command line option).

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Section 4: Geometry module

* 4.1 Does Gmsh support NURBS curves/surfaces?

Not yet.

* 4.2 Gmsh is very slow when I use many transformations (Translate,
Rotate, Symmetry, Extrude, etc. ). What's wrong?

The default behavior of Gmsh is to check and suppress all duplicate
entities (points, lines and surfaces) each time a transformation
command is issued. This can slow down things a lot if many
transformations are performed. There are two solutions to this
problem:
- you may save the unrolled geometry in another file (e.g. with 
  gmsh file.geo -0 > flat.geo), and use this new file for subsequent
  computations;
- you may set the 'Geometry.AutoCoherence' option to 0. This will
  prevent any automatic duplicate check/replacement. If you still
  need to remove the duplicates entities, simply add 'Coherence;' at 
  strategic locations in your geo files (e.g. before the creation of
  line loops, etc.).

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Section 5: Mesh module

* 5.1 What should I do when the 2D unstructured algorithm fails?

Try one of the other 2D algorithms, e.g.:
- on the command line: gmsh -algo tri
- in the interface: Tools->Options->Mesh->2D->Isotropic algorithm (Triangle)
- in input files: Mesh.Algorithm = 3
The old 2D algorithm may disappear once all its features are
integrated in the new ones, so please don't send bug reports on the
old algorithm anymore.

* 5.2 The new 2D unstructured algorithms also fail! Then what?

Send us your geometry, and we will investigate. Please keep the
following in mind though: 2D (surface) meshes are generated by
projecting a 2D mesh in the mean plane of the surface. This gives nice
results only if the surface curvature is small enough. Otherwise you
_have_ to cut the surface in pieces. For example, using half circles
to define a cylinder will fail with the unstructured algorithm (you
should define arcs with angles smaller than Pi, and thus define the
cylinder with at least three patch surfaces).

* 5.3 What should I do when the 3D unstructured algorithm fails?

The 3D algorithm is still very experimental. Try to change some
characteristic lengths in your input file to generate meshes that
better suit the geometrical details of your structure.

* 5.4 I changed the characteristic lengths, but the 3D algorithm still
does not work. What should I do?

Buy a professional mesh generator ;-) You can also try to use Netgen
instead of the default algorithm for the 3D mesh. Note that all
surface meshes have to be oriented with exterior normals in this case.

* 5.5 The 3D algorithm is reaaaaally slow. Can you improve it?

We are working on it. But since we have a (very) limited amount of
time to spend on the development of Gmsh, this may take a while. For
very big meshes, see the answer to the previous question...

* 5.6 The quality of the elements generated by the 3D algorithm is
very bad.

Upgrade to Gmsh >= 1.54 and use "Optimize quality". If badly shaped
elements still exist due to the surface recovery step, you can try to
use Netgen instead of the default algorithm for the 3D mesh. Note that
all surface meshes have to be oriented with exterior normals in this
case.

* 5.7 Non-recombined 3D extruded meshes sometimes fail.

The swapping algorithm is not very clever at the moment. Try to change
the surface mesh a bit, or recombine your mesh to generate prisms or
hexahedra.

* 5.8 Tools->Visibility does not seem to work with extruded meshes.

This is partially fixed in Gmsh >= 1.54. Since numbers are explicitly
assigned to mesh entities in the extrude commands (which partially
destroys the geometry/mesh relationship), the Visibility tool will
only work as expected when displaying Elementary entities. To
visualize extruded Physical entities, the only solution is to save the
mesh, and to read it again.

* 5.9 Does Gmsh support curved elements?

Yes, Gmsh can generate both 1st order and 2nd order elements. To
generate second order elements, click on 'Second order elements' in
the mesh menu after the mesh is completed. To always generate 2nd
order elements, select 'Generate second order elements' in the mesh
option panel. From the command line, you can also use '-order 2'.

* 5.10 Can I import an existing surface mesh in Gmsh and use it to
build a 3D mesh?

Yes, either in the form of a STL triangulation, or by using the
"Discrete Surface" commands. Note that Gmsh cannot currently modify
the surface mesh you provide in this way, so the surface mesh has to
be conform (without gaps, hanging nodes, etc.) and must contain
surface elements having the (final) desired sizes.

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Section 6: Solver module

* 6.1 How do I integrate my own solver with Gmsh?

If you want to simply launch a program from within Gmsh, just edit the
options to define your solver commands (e.g. Solver.Name0,
Solver.Executable0, etc.), and set the ClientServer option to zero
(e.g. Solver.ClientServer0 = 0).

If you want your solver to interact with Gmsh (for error messages,
option definitions, post-processing, etc.), you will need to link your
solver with the 'GmshClient.c' file and add the appropriate function
calls inside your program. You will of course also need to define your
solver commands in an option file, but this time you should set the
ClientServer variable to 1 (e.g. Solver.ClientServer = 1). A complete
example on how to build a solver that interacts with Gmsh is available
at http://www.geuz.org/gmsh/doc/mysolver.tgz.

* 6.2 On Windows, Gmsh does not seem to find the solver
executable. What's wrong?

The solver executable (e.g. 'getdp.exe') has to be in your path. If
not, simply go to the solver options
(e.g. Solver->GetDP->Options->Executable) to specify its location.

On recent versions of Microsoft Windows (XP SP2), it seems that you
need to execute the solver (e.g. launch 'getdp.exe') at least once
independently of Gmsh in order to authorize future executions.

* 6.3 Can I launch Gmsh from my solver (instead of launching my solver
from Gmsh) in order to monitor a solution?

Sure. A simple C program showing how to do this is given in
'utils/misc/callgmsh.c'.

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Section 7: Post-processing module

* 7.1 How do I compute a section of a plot?

Use 'View->Plugins->Cut Plane'.

* 7.2 Can I save isosurfaces to a files?

Yes: first run the CutMap plugin to extract the isosurface, then use
'View->Save As' to save the new view.

* 7.3 Can Gmsh generate isovolumes?

Yes, with the CutMap plugin (set the extractVolume to -1 or 1 to
extract the positive or negative levelset). 

* 7.4 How do I animate my plots?

If the views contain multiple time steps, you can press the 'play'
button under the graphic window, or change the time step by hand in
the view option panel. You can also use the left and right arrow keys
to change the time step in all visible views in real time.

If you want to loop through different views instead of time steps, you
can use the 'Loop through views instead of time steps' option in the
view option panel, or use the up and down arrow keys.

* 7.5 How do I visualize a deformed mesh?

Load a vector view containing the displacement field, and select
'Vector type->Displacement' in the view options. If the displacement
is too small (or too large), you can scale it with the 'Displacement
factor' option. (Remember that you can drag the mouse in all numeric
input fields to slide the value!)

Another option is to use the "general raise expressions" (in
View->Options->Offset) on a scalar view, with the displacement map
selected as the raise data source.

* 7.6 Can I visualize a field on a deformed mesh?

Yes, there are several ways to do that. 

The easiest is to load two views: the first one containing a
displacement field (a vector view that will be used to deform the
mesh), and the second one containing the field you want to display
(this view has to contain the same number of elements as the
displacement view). You should then set 'Vector type' to
'Displacement' in the first view, as well as set 'Data source' to
point to the second view. (You might want to make the second view
invisible, too. If you want to amplify or decrease the amount of
deformation, just modify the 'Displacement factor' option.)

Another solution is to use the "general raise expressions" (in
View->Options->Offset) on the field you want to display, with the
displacement map selected as the raise data source.

And yet another solution is to use the DiplacementRaise plugin.

* 7.7 Can I color the arrows representing a vector field with data
from a scalar field?

Yes: load both the vector and the scalar fields (the two views must
have the same number of elements) and, in the vector field options,
select the scalar view in 'Data source'.

* 7.8 Can I color isovalue surfaces with data from another scalar
view?

Yes, using the CutMap plugin.

* 7.9 Is there a way to save animations?

Yes. For example, have a look at tutorial/t8.geo or
demos/anim.script.

* 7.10 Is there a way to visualize only certain components of
vector/tensor fields?

Yes: use 'View->Plugin->Extract'.

* 7.11 Can I do arithmetic operations on a view? Can I perform
operations involving different views?

Yes, with the Evaluate plugin.

* 7.12 Some plugins seem to create empty views. What's wrong?

There can be several reasons:

- the plugin might be written for specific element types only (for
  example, only for scalar triangles or tetrahedra). In that case, you
  should transform your view before running the plugin (you can use
  Plugin(DecomposeinSimplex) to transform all quads, hexas, prisms and
  pyramids into triangles and tetrahedra).

- the plugin might expect a mesh while all you provide is a point
  cloud. In 2D, you can use Plugin(Triangulate) to transform a point
  cloud into a triangulated surface. A 3D version of this plugin is
  not available yet but it is on our TODO list.

- the input parameters are out of range.

In any case, you can automatically remove all empty views with
'View->Remove->Empty Views' in the GUI, or with "Delete Empty Views;"
in a script.
