8. Post-processing module

Gmsh's post-processing module can handle multiple scalar, vector or tensor datasets along with the geometry and the mesh. The datasets can be given in several formats: in human-readable “parsed” format (these are just part of a standard input script, but are usually put in separate files with a ‘.pos’ extension), in native MSH files (ASCII or binary files with ‘.msh’ extensions: see File formats), or in standard third-party formats (like MED: http://www.code-aster.org/outils/med/).

Once loaded into Gmsh, scalar fields can be displayed as iso-value lines and surfaces or color maps, whereas vector fields can be represented either by three-dimensional arrows or by displacement maps. (Tensor fields are currently displayed as Von-Mises effective stresses. To display other (combinations of) components, use Plugin(Extract): see Post-processing plugins.)

In Gmsh's jargon, each dataset is called a “view”. Each view is given a name, and can be manipulated either individually (each view has its own button in the GUI and can be referred to by its index in a script) or globally (see the PostProcessing.Link option in Post-processing options list).

By default, Gmsh treats all post-processing views as three-dimensional plots, i.e., draws the scalar, vector and tensor primitives (points, lines, triangles, tetrahedra, etc.) in 3D space. But Gmsh can also represent each post-processing view containing scalar points as two-dimensional (“X-Y”) plots, either space- or time-oriented:

• in a `2D space' plot, the scalar points are taken in the same order as they are defined in the post-processing view: the abscissa of the 2D graph is the curvilinear abscissa of the curve defined by the point series, and only one curve is drawn using the values associated with the points. If several time steps are available, each time step generates a new curve;
• in a `2D time' plot, one curve is drawn for each scalar point in the view and the abscissa is the time step.

Although visualization is usually mostly an interactive task, Gmsh exposes all the post-processing commands and options to the user in its scripting language to permit a complete automation of the post-processing process (see e.g., ‘t8.geo’, and ‘t9.geo’).

The two following sections summarize all available post-processing commands and options. Most options apply to both 2D and 3D plots (colormaps, point/line sizes, interval types, time step selection, etc.), but some are peculiar to 3D (lightning, element selection, etc.) or 2D plots (abscissa labels, etc.). Note that 2D plots can be positioned explicitly inside the graphical window, or be automatically positioned in order to avoid overlaps.

Sample post-processing files in human-readable “parsed” format and in the native MSH file format are available in the ‘tutorial’ directory of Gmsh's distribution (‘.pos’ and ‘.msh’ files). The “parsed” format is defined in the next section (cf. the View command); the MSH format is defined in File formats.