Create basics.py

examples/basics.py

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+# ------------------------------------------------------------------------------
+#
+#  Gmsh Python tutorial 1
+#
+#  Geometry basics, elementary entities, physical groups
+#
+# ------------------------------------------------------------------------------
+
+# The Python API is entirely defined in the `gmsh.py' module (which contains the
+# full documentation of all the functions in the API):
+from __future__ import annotations
+
+import sys
+
+import gmsh
+
+# Before using any functions in the Python API, Gmsh must be initialized:
+gmsh.initialize()
+
+# Next we add a new model named "t1" (if gmsh.model.add() is not called a new
+# unnamed model will be created on the fly, if necessary):
+gmsh.model.add("t1")
+
+# The Python API provides direct access to each supported geometry (CAD)
+# kernel. The built-in kernel is used in this first tutorial: the corresponding
+# API functions have the `gmsh.model.geo' prefix.
+
+# The first type of `elementary entity' in Gmsh is a `Point'. To create a point
+# with the built-in CAD kernel, the Python API function is
+# gmsh.model.geo.addPoint():
+# - the first 3 arguments are the point coordinates (x, y, z)
+# - the next (optional) argument is the target mesh size close to the point
+# - the last (optional) argument is the point tag (a stricly positive integer
+#   that uniquely identifies the point)
+lc = 1e-2
+gmsh.model.geo.addPoint(0, 0, 0, lc, 1)
+
+# Note that in addition to the default ``camelCase'' function name `addPoint',
+# the Python API also defines a ``snake case'' alias, i.e. `add_point'. You can
+# use either interchangeably; all the tutorials are written using the camelCase
+# convention for consistency.
+
+# The distribution of the mesh element sizes will be obtained by interpolation
+# of these mesh sizes throughout the geometry. Another method to specify mesh
+# sizes is to use general mesh size Fields (see `t10.py'). A particular case is
+# the use of a background mesh (see `t7.py').
+#
+# If no target mesh size of provided, a default uniform coarse size will be used
+# for the model, based on the overall model size.
+#
+# We can then define some additional points. All points should have different
+# tags:
+gmsh.model.geo.addPoint(0.1, 0, 0, lc, 2)
+gmsh.model.geo.addPoint(0.1, 0.3, 0, lc, 3)
+
+# If the tag is not provided explicitly, a new tag is automatically created, and
+# returned by the function:
+p4 = gmsh.model.geo.addPoint(0, 0.3, 0, lc)
+
+# Curves are Gmsh's second type of elementery entities, and, amongst curves,
+# straight lines are the simplest. The API to create straight line segments with
+# the built-in kernel follows the same conventions: the first 2 arguments are
+# point tags (the start and end points of the line), and the last (optional one)
+# is the line tag.
+#
+# In the commands below, for example, the line 1 starts at point 1 and ends at
+# point 2.
+#
+# Note that curve tags are separate from point tags - hence we can reuse tag `1'
+# for our first curve. And as a general rule, elementary entity tags in Gmsh
+# have to be unique per geometrical dimension.
+gmsh.model.geo.addLine(1, 2, 1)
+gmsh.model.geo.addLine(3, 2, 2)
+gmsh.model.geo.addLine(3, p4, 3)
+gmsh.model.geo.addLine(4, 1, p4)
+
+# The third elementary entity is the surface. In order to define a simple
+# rectangular surface from the four curves defined above, a curve loop has first
+# to be defined. A curve loop is defined by an ordered list of connected curves,
+# a sign being associated with each curve (depending on the orientation of the
+# curve to form a loop). The API function to create curve loops takes a list
+# of integers as first argument, and the curve loop tag (which must be unique
+# amongst curve loops) as the second (optional) argument:
+gmsh.model.geo.addCurveLoop([4, 1, -2, 3], 1)
+
+# We can then define the surface as a list of curve loops (only one here,
+# representing the external contour, since there are no holes--see `t4.py' for
+# an example of a surface with a hole):
+gmsh.model.geo.addPlaneSurface([1], 1)
+
+# Before they can be meshed (and, more generally, before they can be used by API
+# functions outside of the built-in CAD kernel functions), the CAD entities must
+# be synchronized with the Gmsh model, which will create the relevant Gmsh data
+# structures. This is achieved by the gmsh.model.geo.synchronize() API call for
+# the built-in CAD kernel. Synchronizations can be called at any time, but they
+# involve a non trivial amount of processing; so while you could synchronize the
+# internal CAD data after every CAD command, it is usually better to minimize
+# the number of synchronization points.
+gmsh.model.geo.synchronize()
+
+# At this level, Gmsh knows everything to display the rectangular surface 1 and
+# to mesh it. An optional step is needed if we want to group elementary
+# geometrical entities into more meaningful groups, e.g. to define some
+# mathematical ("domain", "boundary"), functional ("left wing", "fuselage") or
+# material ("steel", "carbon") properties.
+#
+# Such groups are called "Physical Groups" in Gmsh. By default, if physical
+# groups are defined, Gmsh will export in output files only mesh elements that
+# belong to at least one physical group. (To force Gmsh to save all elements,
+# whether they belong to physical groups or not, set the `Mesh.SaveAll' option
+# to 1.) Physical groups are also identified by tags, i.e. stricly positive
+# integers, that should be unique per dimension (0D, 1D, 2D or 3D). Physical
+# groups can also be given names.
+#
+# Here we define a physical curve that groups the left, bottom and right curves
+# in a single group (with prescribed tag 5); and a physical surface with name
+# "My surface" (with an automatic tag) containing the geometrical surface 1:
+gmsh.model.addPhysicalGroup(1, [1, 2, 4], 5)
+gmsh.model.addPhysicalGroup(2, [1], name="My surface")
+
+# We can then generate a 2D mesh...
+gmsh.model.mesh.generate(2)
+
+# ... and save it to disk
+gmsh.write("t1.msh")
+
+# Remember that by default, if physical groups are defined, Gmsh will export in
+# the output mesh file only those elements that belong to at least one physical
+# group. To force Gmsh to save all elements, you can use
+#
+# gmsh.option.setNumber("Mesh.SaveAll", 1)
+
+# By default, Gmsh saves meshes in the latest version of the Gmsh mesh file
+# format (the `MSH' format). You can save meshes in other mesh formats by
+# specifying a filename with a different extension. For example
+#
+#   gmsh.write("t1.unv")
+#
+# will save the mesh in the UNV format. You can also save the mesh in older
+# versions of the MSH format: simply set
+#
+#   gmsh.option.setNumber("Mesh.MshFileVersion", x)
+#
+# for any version number `x'. As an alternative, you can also not specify the
+# format explicitly, and just choose a filename with the `.msh2' or `.msh4'
+# extension.
+
+# To visualize the model we can run the graphical user interface with
+# `gmsh.fltk.run()'. Here we run it only if "-nopopup" is not provided in the
+# command line arguments:
+if "-nopopup" not in sys.argv:
+    gmsh.fltk.run()
+
+# Note that starting with Gmsh 3.0, models can be built using other geometry
+# kernels than the default "built-in" kernel. To use the OpenCASCADE CAD kernel
+# instead of the built-in kernel, you should use the functions with the
+# `gmsh.model.occ' prefix.
+#
+# Different CAD kernels have different features. With OpenCASCADE, instead of
+# defining the surface by successively defining 4 points, 4 curves and 1 curve
+# loop, one can define the rectangular surface directly with
+#
+# gmsh.model.occ.addRectangle(.2, 0, 0, .1, .3)
+#
+# After synchronization with the Gmsh model with
+#
+# gmsh.model.occ.synchronize()
+#
+# the underlying curves and points could be accessed with
+# gmsh.model.getBoundary().
+#
+# See e.g. `t16.py', `t18.py', `t19.py' or `t20.py' for complete examples based
+# on OpenCASCADE, and `examples/api' for more.
+
+# This should be called when you are done using the Gmsh Python API:
+gmsh.finalize()