3 \page constructing_meshes_page Constructing meshes
5 To create a mesh on geometry, it is necessary to create a mesh object by choosing
6 - a geometrical shape produced in the Geometry module (<em>main shape</em>);
7 - <em>meshing parameters</em>, including
8 - \ref basic_meshing_algos_page "meshing algorithms" and
9 - \ref about_hypo_page "hypotheses" specifying constraints to be
10 taken into account by the chosen meshing algorithms.
12 Then you can launch mesh generation by invoking \ref compute_anchor "Compute" command.
13 The generated mesh will be automatically shown in the Viewer. You can
14 switch off automatic visualization or limit mesh size until which it is
15 automatically shown in \ref mesh_preferences_page (<em>Automatic update</em> entry).
17 \note Sometimes \a hypotheses term is used to refer to both algorithms
20 Mesh generation on the geometry is performed in the bottom-up
21 flow: nodes on vertices are created first, then edges are divided into
22 segments using nodes on vertices; the node of segments are then
23 used to mesh faces; then the nodes of faces are used to mesh
24 solids. This automatically assures the conformity of the mesh.
26 It is required to choose a meshing algorithm for every dimension of
27 sub-shapes up to the highest dimension to be generated. Note
28 that some algorithms generate elements of several dimensions, and
29 others of only one. It is not necessary to define meshing
30 parameters for all dimensions at once; you can start from 1D
31 meshing parameters only, compute the 1D mesh, then define 2D meshing
32 parameters and compute the 2D mesh (note that 1D mesh will not be
35 An algorithm of a certain dimension chosen at mesh creation is applied
36 to discretize every sub-shape of this dimension. It is possible to
37 specify a different algorithm or hypothesis to be applied to one or
38 a group of sub-shapes by creating a \ref constructing_submeshes_page
39 "sub-mesh". You can specify no algorithms at all at mesh object
40 creation and specify the meshing parameters on sub-meshes only; then
41 only the sub-shapes, for which an algorithm and a hypothesis (if any)
42 have been defined will be discretized.
44 \n Construction of a mesh on a geometry includes at least two
45 (\ref create_mesh_anchor "mesh creation" and
46 \ref compute_anchor "computing") of the following steps:
48 <li> \ref create_mesh_anchor "Creation of a mesh object", where you
49 can specify meshing parameters to apply to all sub-shapes of the
51 <li> \ref constructing_submeshes_page "Creation of sub-meshes",
52 (optional) where you can specify meshing parameters to apply to the
53 selected sub-shapes.</li>
54 <li> \ref evaluate_anchor "Evaluating mesh size" (optional) can be
55 used to know an approximate number of elements before their actual generation.</li>
56 <li> \ref preview_anchor "Previewing the mesh" (optional) can be
57 used to generate mesh of only lower dimension(s) in order to
58 visually estimate it before full mesh generation, which can be much
60 <li> \ref submesh_order_anchor "Changing sub-mesh priority"
61 (optional) can be useful if there are concurrent sub-meshes
63 <li> \ref compute_anchor "Computing the mesh" uses defined meshing
64 parameters to generate mesh elements.</li>
65 <li> \ref edit_anchor "Editing the mesh" (optional) can be used to
66 \ref modifying_meshes_page "modify" the mesh of a lower dimension before
67 \ref compute_anchor "computing" elements of an upper dimension.</li>
70 \anchor create_mesh_anchor
71 <h2>Creation of a mesh object</h2>
72 <em>To construct a mesh:</em>
74 <li>Select a geometrical object for meshing.</li>
75 <li>In the \b Mesh menu select <b>Create Mesh</b> or click <em>"Create
76 Mesh"</em> button in the toolbar.
79 \image html image32.png
80 <em>"Create Mesh" button</em>
83 The following dialog box will appear:
85 \image html createmesh-inv.png
88 <li> To filter off irrelevant meshing algorithms, you can
89 select <b>Mesh Type</b> in the corresponding list from <b>Any,
90 Hexahedral, Tetrahedral, Triangular </b> and \b Quadrilateral (there
91 can be less items for the geometry of lower dimensions).
93 Selection of a mesh type hides all meshing algorithms that cannot
94 generate elements of this type.</li>
96 <li>Apply \subpage basic_meshing_algos_page "meshing algorithms" and
97 \subpage about_hypo_page "hypotheses" which will be used to compute
100 "Create mesh" dialog box contains several tab pages titled \b 3D,
101 \b 2D, \b 1D and \b 0D. The title of each page reflects the
102 dimension of the sub-shapes the algorithms listed on
103 this page affect and the maximal dimension of elements the algorithms
104 generate. For example, \b 3D page lists the algorithms that affect
105 3D sub-shapes (solids) and generate 3D mesh elements
106 (tetrahedra, hexahedra etc.)
108 As soon as you have selected an algorithm, you can create a
109 hypothesis (or select an already created one). A set of accessible
110 hypotheses includes only the hypotheses that can be used by the
114 - Some page(s) can be disabled if the geometrical
115 object does not include shapes (sub-shapes) of the corresponding
116 dimension(s). For example, if the input object is a geometrical face,
117 \b 3D page is disabled.
118 - Some algorithms affect the geometry of several dimensions,
119 i.e. 1D+2D or 1D+2D+3D. If such an algorithm is selected, the
120 dialog pages related to the corresponding lower dimensions are
122 - \b 0D page refers to 0D geometry (vertices) rather than
123 to 0D elements. Mesh module does not provide algorithms that
124 produce 0D elements. Currently \b 0D page provides only one
125 algorithm "Segments around vertex" that allows specifying the required
126 size of mesh edges about the selected vertex (or vertices).
128 For example, you need to mesh a 3D object.
130 First, you can change a default name of your mesh in the \b Name
131 box. Then check that the selected geometrical object indicated in
132 \b Geometry field, is what you wish to mesh; if not, select
133 the correct object in the Object Browser. Click "Select" button
134 near \b Geometry field if the name of the object has not yet
135 appeared in \b Geometry field.
137 \image html image120.png
138 <em>"Select" button</em>
141 Now you can define 3D Algorithm and 3D Hypotheses, which will be
142 applied to discretize the solids of your geometrical object using
143 3D elements. Click the <em>"Add Hypothesis"</em> button to create
144 and add a hypothesis.
146 \image html image121.png
147 <em>"Add Hypothesis" button</em>
149 Click the <em>"Plus"</em> button to enable adding more additional hypotheses.
151 Click the <em>"Edit Hypothesis"</em> button to change the values for the
154 \image html image122.png
155 <em>"Edit Hypothesis" button</em>
158 Most 2D and 3D algorithms can work without hypotheses using
159 default meshing parameters. Some algorithms do not require any
160 hypotheses. After selection of an algorithm "Hypothesis" field of
161 the dialog can contain:
163 <li> <em>\<Default\></em> if the algorithm can work using default
165 <li> <em>\<None\></em> if the algorithm requires a hypothesis defining
167 <li> If the algorithm does not use hypotheses, this field is grayed.</li>
169 After selection of an algorithm <b>Add. Hypothesis</b> field can contain:
171 <li> <em>\<None\></em> if the algorithm can be tuned
172 using an additional hypothesis.</li>
173 <li> If the algorithm does not use additional hypotheses, this field is grayed.</li>
176 Proceed in the same way with 2D and 1D Algorithms and Hypotheses that
177 will be used to mesh faces and edges of your geometry. (Note
178 that any object has edges, even if their existence is not
179 apparent, for example, a sphere has 4 edges). Note that the
180 choice of hypotheses and lower dimension algorithms depends on
181 the higher dimension algorithm.
183 If you wish you can select other algorithms and/or hypotheses
184 for meshing some sub-shapes of your CAD model by \ref constructing_submeshes_page.
186 Some algorithms generate mesh of several dimensions, while others
187 produce mesh of only one dimension. In the latter case there must
188 be one Algorithm and zero or several
189 Hypotheses for each dimension of your object, otherwise you will
190 not get any mesh at all. Of course, if you wish to mesh a face,
191 which is a 2D object, you do not need to define a 3D Algorithm and
194 In the <b>Object Browser</b> the structure of the new mesh is
195 displayed as follows:
196 \image html image88.jpg
199 <li>a mesh name (<em>Mesh_mechanic</em>);
200 <li>a reference to the geometrical object on the basis of
201 which the mesh has been constructed (\a mechanic);</li>
202 <li><b>Applied hypotheses</b> folder containing the references
203 to the hypotheses chosen at the construction of the mesh;</li>
204 <li><b>Applied algorithms</b> folder containing the references
205 to the algorithms chosen at the construction of the mesh.</li>
206 <li><b>SubMeshes on Face</b> folder containing the sub-meshes
207 defined on geometrical faces. There also can be folders for
208 sub-meshes on vertices, edges, wires, shells, solids and
210 <li><b>Groups of Faces</b> folder containing the groups of mesh
211 faces. There also can be folders for groups of nodes, edges,
212 volumes 0D elements and balls.</li>
215 There is an alternative way to assign Algorithms and Hypotheses by
216 clicking <b>Assign a set of hypotheses</b> button and selecting among
217 pre-defined sets of algorithms and hypotheses. In addition to the built-in
218 sets of hypotheses, it is possible to create custom sets by editing
219 CustomMeshers.xml file located in the home directory. CustomMeshers.xml
220 file must describe sets of hypotheses in the
221 same way as ${SMESH_ROOT_DIR}/share/salome/resources/smesh/StdMeshers.xml
222 file does (sets of hypotheses are enclosed between \<hypotheses-set-group\>
225 <?xml version='1.0' encoding='us-ascii'?>
226 <!DOCTYPE meshers PUBLIC "" "desktop.dtd">
228 <hypotheses-set-group>
229 <hypotheses-set name="My favorite hypotheses"
230 hypos="AutomaticLength"
231 algos="CompositeSegment_1D, Quadrangle_2D, GHS3D_3D"/>
232 </hypotheses-set-group>
235 If the file contents are incorrect, there can be an error at
236 activation of Mesh module: <em>"fatal parsing error: error
237 triggered by consumer in line ..."</em>
240 \image html hypo_sets.png
241 List of sets of hypotheses. Tag <em>[custom]</em> is
242 automatically added to the sets defined by the user.
245 - \a "Automatic" in the names of predefined sets of hypotheses
246 does not actually mean that they are suitable for meshing any
248 - The list of sets of hypotheses can be shorter than in the
249 above image depending on the geometry dimension.
253 Consider trying a sample script for construction of a mesh from our
254 \ref tui_creating_meshes_page "TUI Scripts" section.
256 \anchor evaluate_anchor
257 <h2>Evaluating mesh size</h2>
259 After the mesh object is created and all hypotheses are assigned and
260 before \ref compute_anchor "Compute" operation, it is possible to
261 calculate the eventual mesh size. For this, select the mesh in
262 the <b>Object Browser</b> and from the \b Mesh menu select \b
263 Evaluate. The result of evaluation will be displayed in the following
266 \image html mesh_evaluation_succeed.png
268 \anchor preview_anchor
269 <h2>Previewing the mesh</h2>
271 Before \ref compute_anchor "the mesh computation", it is also possible
272 to see the mesh preview. This operation allows to incrementally
273 compute the mesh, dimension by dimension, and to discard an
276 For this, select the mesh in the Object Browser. From the \b Mesh menu
277 select \b Preview or click "Preview" button in the toolbar or activate
278 "Preview" item from the pop-up menu.
281 \image html mesh_precompute.png
282 <em>"Preview" button</em>
285 Select <b>1D mesh</b> or <b>2D mesh</b> preview mode in the Preview dialog.
287 \image html preview_mesh_1D.png "1D mesh preview shows nodes computed on geometry edges"
289 \image html preview_mesh_2D.png "2D mesh preview shows edge mesh elements, computed on geometry faces"
291 <b>Compute</b> button computes the whole mesh.
293 When the Preview dialog is closed, the question about the storage of temporarily
294 created mesh elements appears:
296 \image html preview_tmp_data.png
298 These elements can be kept in the mesh.
301 \anchor submesh_order_anchor
302 <h2>Changing sub-mesh priority</h2>
304 If the mesh contains concurrent \ref constructing_submeshes_page "sub-meshes",
305 it is possible to change the priority of their computation, i.e. to
306 change the priority of applying algorithms to the shared sub-shapes of
309 <em>To change sub-mesh priority:</em>
311 Choose "Change sub-mesh priority" from the Mesh menu or a pop-up
312 menu. The opened dialog shows a list of sub-meshes in the order of
315 There is an example of sub-mesh order modifications taking a Mesh created on a Box
316 shape. The main Mesh object:
318 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=20</li>
319 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis<b>Max Element Area</b>
322 The first sub-mesh <b>Submesh_1</b> created on <b>Face_1</b> is:
324 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=4</li>
325 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
327 The second sub-mesh <b>Submesh_2</b> created on <b>Face_2</b> is:
329 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=8</li>
330 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
333 And the last sub-mesh <b>Submesh_3</b> created on <b>Face_3</b> is:
335 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=12</li>
336 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
339 The sub-meshes become concurrent if they share sub-shapes that can be
340 meshed with different algorithms (or different hypotheses). In the
341 example, we have three sub-meshes with concurrent algorithms, because
342 they have different hypotheses.
344 The first mesh computation is made with:
346 \image html mesh_order_123.png
347 <em>"Mesh order SubMesh_1, SubMesh_2, SubMesh_3"</em></center>
349 \image html mesh_order_123_res.png
350 <em>"Result mesh with order SubMesh_1, SubMesh_2, SubMesh_3 "</em></center>
352 The next mesh computation is made with:
354 \image html mesh_order_213.png
355 <em>"Mesh order SubMesh_2, SubMesh_1, SubMesh_3"</em></center>
357 \image html mesh_order_213_res.png
358 <em>"Result mesh with order SubMesh_2, SubMesh_1, SubMesh_3 "</em></center>
360 And the last mesh computation is made with:
362 \image html mesh_order_321.png
363 <em>"Mesh order SubMesh_3, SubMesh_2, SubMesh_1"</em></center>
364 <center>\image html mesh_order_321_res.png
365 <em>"Result mesh with order SubMesh_3, SubMesh_2, SubMesh_1 "</em></center>
367 As we can see, each mesh computation has a different number of result
368 elements and a different mesh discretization on the shared edges (the edges
369 that are shared between <b>Face_1</b>, <b>Face_2</b> and <b>Face_3</b>)
371 Additionally, sub-mesh priority (the order of applied algorithms) can
372 be modified not only in a separate dialog box, but also in
373 the <b>Preview</b>. This helps to preview different mesh results,
374 modifying the order of sub-meshes.
376 \image html mesh_order_preview.png
377 <em>"Preview with sub-mesh priority list box"</em></center>
379 If there are no concurrent sub-meshes under the Mesh object, the user
380 will see the following information.
382 \image html mesh_order_no_concurrent.png
383 <em>"No concurrent submeshes detected"</em></center>
386 \anchor compute_anchor
387 <h2>Computing the mesh</h2>
389 It is equally possible to skip \ref evaluate_anchor "the Evaluation"
390 and \ref preview_anchor "the Preview" and to \b Compute the mesh after
391 the hypotheses are assigned. For this, select your mesh in
392 the <b>Object Browser</b>. From the \b Mesh menu or the context menu
393 select \b Compute or click \a "Compute" button of the toolbar.
396 \image html image28.png
397 <em>"Compute" button</em>
400 After the mesh computation finishes, the Mesh Computation information
401 box appears. If you close this box and click "Compute" button again,
402 without previously changing meshing parameters, the mesh will NOT be
403 re-computed and the Mesh Computation information box will be shown
404 with the same contents. (To fully re-compute the mesh, invoke
405 \ref clear_mesh_anchor "Clear Mesh Data" command before).
407 If the mesh computation has been a success, the box shows information
408 on the number of entities of different types in the mesh.
410 \image html meshcomputationsucceed.png
412 \anchor meshing_failed_anchor
413 If the mesh computation has failed, the information about the cause of the
414 failure is provided in \b Errors table.
416 \image html meshcomputationfail.png
418 After you select an error in \b Errors table, <b>Show Sub-shape</b> button allows
419 visualizing in magenta the geometrical entity meshing of which failed
420 (Name of this entity or its ID and type is shown in \a Sub-shape column).
423 \image html failed_computation.png
424 <em>3D algorithm failed to compute mesh on a box shown using <b>Show
425 Sub-shape</b> button</em>
428 <b>Publish Sub-shape</b> button publishes the sub-shape, whose meshing
429 has failed, in the Geometry component as a child of the main shape, which
430 allows analyzing the problematic geometry and creating a sub-mesh on it in
431 order to locally tune the hypotheses.
433 If the failure is caused by an invalid input mesh and the algorithm has
434 found which mesh entities are bad, <b>Show bad Mesh</b>
435 button appears in the dialog. Clicked, it shows the bad mesh entities in
436 the Viewer in magenta. Sometimes the shown mesh entities are too small
437 or/and hidden by other mesh elements. They can be seen after
438 switching the mesh to Wireframe visualization mode or switching off
439 the visualization of faces and volumes (if any).
441 <b>Bad Mesh to Group</b> button creates groups of bad mesh entities
442 to facilitate their analysis.
445 \image html show_bad_mesh.png
446 <em>Edges bounding a hole in the surface are shown in magenta using <b>Show
447 bad Mesh</b> button</em>
450 \note Mesh Computation Information box does not appear if you set
451 "Mesh computation/Show a computation result notification" preference
452 to the "Never" value. This option gives the possibility to control mesh
453 computation reporting. There are the following possibilities: always
454 show the information box, show only if an error occurs or never.
455 By default, the information box is always shown after mesh computation operation.
459 <h2>Editing the mesh</h2>
461 It is possible to \ref modifying_meshes_page "edit the mesh" of a
462 lower dimension before generation of the mesh of a higher dimension.
464 For example you can generate a 2D mesh, modify it using e.g.
465 \ref pattern_mapping_page, and then generate a 3D mesh basing on the
466 modified 2D mesh. The workflow is as follows:
467 - Define 1D and 2D meshing algorithms.
468 - Compute the mesh. 2D mesh is generated.
469 - Apply \ref pattern_mapping_page.
470 - Define 3D meshing algorithms without modifying 1D and 2D algorithms
472 - Compute the mesh. 3D mesh is generated.
474 \note Nodes and elements added \ref adding_nodes_and_elements_page
475 "manually" cannot be used in this workflow because the manually created
476 entities are not attached to any geometry and thus (usually) cannot be
477 found by the mesher paving a geometry.
479 <b>See Also</b> a sample TUI Script demonstrates the possibility of
480 \ref tui_editing_while_meshing "Intermediate edition while meshing"