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.
14 \note Sometimes \a hypotheses term is used to refer to both algorithms
17 Mesh generation on the geometry is performed in the bottom-up
18 flow: nodes on vertices are created first, then edges are divided into
19 segments using nodes on vertices; the segments of edges are then
20 used to mesh faces; then the mesh of faces is used to mesh
21 solids. This automatically assures the conformity of the mesh.
23 It is required to choose a meshing algorithm for every dimension of
24 sub-shapes up to the highest dimension to be generated. Note
25 that some algorithms generate elements of several dimensions, and
26 others of only one. It is not necessary to define meshing
27 parameters for all dimensions at once; you can start from 1D
28 meshing parameters only, compute the 1D mesh, then define 2D meshing
29 parameters and compute the 2D mesh (note that 1D mesh will not be
32 An algorithm of a certain dimension chosen at mesh creation is applied
33 to discretize every sub-shape of this dimension. It is possible to
34 specify a different algorithm or hypothesis to be applied to one or
35 a group of sub-shapes by creating a \ref constructing_submeshes_page
36 "sub-mesh". You can specify no algorithms at all at mesh object
37 creation and specify the meshing parameters on sub-meshes only; then
38 only the sub-shapes, for which an algorithm and a hypothesis (if any)
39 have been defined will be discretized.
41 \n Construction of a mesh on a geometry includes at least two (mesh
42 creation and computing) of the following steps:
44 <li> \ref create_mesh_anchor "Creation of a mesh object", where you
45 can specify meshing parameters to apply to all sub-shapes of the
47 <li> \ref constructing_submeshes_page "Creation of sub-meshes",
48 (optional) where you can specify meshing parameters to apply to the
49 selected sub-shapes.</li>
50 <li> \ref evaluate_anchor "Evaluating mesh size" (optional) can be
51 used to know an approximate number of elements before their actual generation.</li>
52 <li> \ref preview_anchor "Previewing the mesh" (optional) can be
53 used to generate mesh of only lower dimension(s) in order to
54 visually estimate it before full mesh generation, which can be much
56 <li> \ref submesh_order_anchor "Changing sub-mesh priority"
57 (optional) can be useful if there are concurrent sub-meshes
59 <li> \ref compute_anchor "Computing the mesh" uses defined meshing
60 parameters to generate mesh elements.</li>
61 <li> \ref edit_anchor "Editing the mesh" (optional) can be used to
62 \ref modifying_meshes_page "modify" the mesh of a lower dimension before
63 \ref compute_anchor "computing" elements of an upper dimension.</li>
66 \anchor create_mesh_anchor
67 <h2>Creation of a mesh object</h2>
68 <em>To construct a mesh:</em>
70 <li>Select a geometrical object for meshing.</li>
71 <li>In the \b Mesh menu select <b>Create Mesh</b> or click <em>"Create
72 Mesh"</em> button in the toolbar.
75 \image html image32.png
76 <em>"Create Mesh" button</em>
79 The following dialog box will appear:
81 \image html createmesh-inv.png
84 <li> To filter off irrelevant meshing algorithms, you can
85 select <b>Mesh Type</b> in the corresponding list from <b>Any,
86 Hexahedral, Tetrahedral, Triangular </b> and \b Quadrilateral (there
87 can be less items for the geometry of lower dimensions).
89 Selection of a mesh type hides all meshing algorithms that cannot
90 generate elements of this type.</li>
92 <li>Apply \subpage basic_meshing_algos_page "meshing algorithms" and
93 \subpage about_hypo_page "hypotheses" which will be used to compute
96 "Create mesh" dialog box contains several tab pages titled \b 3D,
97 \b 2D, \b 1D and \b 0D. The title of each page reflects the
98 dimension of the sub-shapes the algorithms listed on
99 this page affect and the maximal dimension of elements the algorithms
100 generate. For example, \b 3D page lists the algorithms that affect
101 3D sub-shapes (solids) and generate 3D mesh elements
102 (tetrahedra, hexahedra etc.)
104 As soon as you have selected an algorithm, you can create a
105 hypothesis (or select an already created one). A set of accessible
106 hypotheses includes only the hypotheses that can be used by the
110 - Some page(s) can be disabled if the geometrical
111 object does not include shapes (sub-shapes) of the corresponding
112 dimension(s). For example, if the input object is a geometrical face,
113 \b 3D page is disabled.
114 - Some algorithms affect the geometry of several dimensions,
115 i.e. 1D+2D or 1D+2D+3D. If such an algorithm is selected, the
116 dialog pages related to the corresponding lower dimensions are
118 - \b 0D page refers to 0D geometry (vertices) rather than
119 to 0D elements. Mesh module does not provide algorithms that
120 produce 0D elements. Currently \b 0D page provides only one
121 algorithm "Segments around vertex" that allows specifying the required
122 size of mesh edges about the selected vertex (or vertices).
124 For example, you need to mesh a 3D object.
126 First, you can change a default name of your mesh in the \b Name
127 box. Then check that the selected geometrical object indicated in
128 \b Geometry field, is what you wish to mesh; if not, select
129 the correct object in the Object Browser. Click "Select" button
130 near \b Geometry field if the name of the object has not yet
131 appeared in \b Geometry field.
133 \image html image120.png
134 <em>"Select" button</em>
137 Now you can define 3D Algorithm and 3D Hypotheses, which will be
138 applied to discretize the solids of your geometrical object using
139 3D elements. Click the <em>"Add Hypothesis"</em> button to create
140 and add a hypothesis.
142 \image html image121.png
143 <em>"Add Hypothesis" button</em>
145 Click the <em>"Plus"</em> button to enable adding more additional hypotheses.
147 Click the <em>"Edit Hypothesis"</em> button to change the values for the
150 \image html image122.png
151 <em>"Edit Hypothesis" button</em>
154 Most 2D and 3D algorithms can work without hypotheses using
155 default meshing parameters. Some algorithms do not require any
156 hypotheses. After selection of an algorithm "Hypothesis" field of
157 the dialog can contain:
159 <li> <em>\<Default\></em> if the algorithm can work using default
161 <li> <em>\<None\></em> if the algorithm requires a hypothesis defining
163 <li> If the algorithm does not use hypotheses, this field is grayed.</li>
165 After selection of an algorithm <b>Add. Hypothesis</b> field can contain:
167 <li> <em>\<None\></em> if the algorithm can be tuned
168 using an additional hypothesis.</li>
169 <li> If the algorithm does not use additional hypotheses, this field is grayed.</li>
172 Proceed in the same way with 2D and 1D Algorithms and Hypotheses that
173 will be used to mesh faces and edges of your geometry. (Note
174 that any object has edges, even if their existence is not
175 apparent, for example, a sphere has 4 edges). Note that the
176 choice of hypotheses and lower dimension algorithms depends on
177 the higher dimension algorithm.
179 If you wish you can select other algorithms and/or hypotheses
180 for meshing some sub-shapes of your CAD model by \ref constructing_submeshes_page.
182 Some algorithms generate mesh of several dimensions, while others
183 produce mesh of only one dimension. In the latter case there must
184 be one Algorithm and zero or several
185 Hypotheses for each dimension of your object, otherwise you will
186 not get any mesh at all. Of course, if you wish to mesh a face,
187 which is a 2D object, you do not need to define a 3D Algorithm and
190 In the <b>Object Browser</b> the structure of the new mesh will be
191 displayed as follows:
194 \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>
208 There is an alternative way to assign Algorithms and Hypotheses by
209 clicking <b>Assign a set of hypotheses</b> button and selecting among
210 pre-defined sets of algorithms and hypotheses. In addition to the built-in
211 sets of hypotheses, it is possible to create custom sets by editing
212 CustomMeshers.xml file located in the home directory. CustomMeshers.xml
213 file must describe sets of hypotheses in the
214 same way as ${SMESH_ROOT_DIR}/share/salome/resources/smesh/StdMeshers.xml
215 file does (sets of hypotheses are enclosed between <hypotheses-set-group>
219 \image html hypo_sets.png
220 List of sets of hypotheses. Tag <em>[custom]</em> is
221 automatically added to the sets defined by the user.
225 - \a "Automatic" in the names of predefined sets of hypotheses
226 does not actually mean that they are suitable for meshing any
228 - The list of sets of hypotheses can be shorter than in the
229 above image depending on the geometry dimension.
233 Consider trying a sample script for construction of a mesh from our
234 \ref tui_creating_meshes_page "TUI Scripts" section.
236 \anchor evaluate_anchor
237 <h2>Evaluating mesh size</h2>
239 After the mesh object is created and all hypotheses are assigned and
240 before \ref compute_anchor "Compute" operation, it is possible to
241 calculate the eventual mesh size. For this, select the mesh in
242 the <b>Object Browser</b> and from the \b Mesh menu select \b
243 Evaluate. The result of evaluation will be displayed in the following
246 \image html mesh_evaluation_succeed.png
248 \anchor preview_anchor
249 <h2>Previewing the mesh</h2>
251 Before \ref compute_anchor "the mesh computation", it is also possible
252 to see the mesh preview.
254 For this, select the mesh in the Object Browser. From the \b Mesh menu
255 select \b Preview or click "Preview" button in the toolbar or activate
256 "Preview" item from the pop-up menu.
259 \image html mesh_precompute.png
260 <em>"Preview" button</em>
263 Select <b>1D mesh</b> or <b>2D mesh</b> preview mode in the Preview dialog.
265 \image html preview_mesh_1D.png "1D mesh preview shows nodes computed on geometry edges"
267 \image html preview_mesh_2D.png "2D mesh preview shows edge mesh elements, computed on geometry faces"
269 <b>Compute</b> button computes the whole mesh.
271 When the Preview dialog is closed, the question about the storage of temporarily
272 created mesh elements appears:
274 \image html preview_tmp_data.png
276 These elements can be kept in the mesh.
279 \anchor submesh_order_anchor
280 <h2>Changing sub-mesh priority</h2>
282 If the mesh contains concurrent \ref constructing_submeshes_page "sub-meshes",
283 it is possible to change the priority of their computation, i.e. to
284 change the priority of applying algorithms to the shared sub-shapes of
287 <em>To change sub-mesh priority:</em>
289 Choose "Change sub-mesh priority" from the Mesh menu or a pop-up
290 menu. The opened dialog shows a list of sub-meshes in the order of
293 There is an example of sub-mesh order modifications taking a Mesh created on a Box
294 shape. The main Mesh object:
296 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=20</li>
297 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis<b>Max Element Area</b>
300 The first sub-mesh <b>Submesh_1</b> created on <b>Face_1</b> is:
302 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=4</li>
303 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
305 The second sub-mesh <b>Submesh_2</b> created on <b>Face_2</b> is:
307 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=8</li>
308 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
311 And the last sub-mesh <b>Submesh_3</b> created on <b>Face_3</b> is:
313 <li><i>1D</i> <b>Wire discretisation</b> with <b>Number of Segments</b>=12</li>
314 <li><i>2D</i> <b>Triangle (Mefisto)</b> with Hypothesis <b>MaxElementArea</b>=1200</li>
317 The sub-meshes become concurrent if they share sub-shapes that can be
318 meshed with different algorithms (or different hypotheses). In the
319 example, we have three sub-meshes with concurrent algorithms, because
320 they have different hypotheses.
322 The first mesh computation is made with:
324 \image html mesh_order_123.png
325 <em>"Mesh order SubMesh_1, SubMesh_2, SubMesh_3"</em></center>
327 \image html mesh_order_123_res.png
328 <em>"Result mesh with order SubMesh_1, SubMesh_2, SubMesh_3 "</em></center>
330 The next mesh computation is made with:
332 \image html mesh_order_213.png
333 <em>"Mesh order SubMesh_2, SubMesh_1, SubMesh_3"</em></center>
335 \image html mesh_order_213_res.png
336 <em>"Result mesh with order SubMesh_2, SubMesh_1, SubMesh_3 "</em></center>
338 And the last mesh computation is made with:
340 \image html mesh_order_321.png
341 <em>"Mesh order SubMesh_3, SubMesh_2, SubMesh_1"</em></center>
342 <center>\image html mesh_order_321_res.png
343 <em>"Result mesh with order SubMesh_3, SubMesh_2, SubMesh_1 "</em></center>
345 As we can see, each mesh computation has a different number of result
346 elements and a different mesh discretization on the shared edges (the edges
347 that are shared between <b>Face_1</b>, <b>Face_2</b> and <b>Face_3</b>)
349 Additionally, sub-mesh priority (the order of applied algorithms) can
350 be modified not only in a separate dialog box, but also in
351 the <b>Preview</b>. This helps to preview different mesh results,
352 modifying the order of sub-meshes.
354 \image html mesh_order_preview.png
355 <em>"Preview with sub-mesh priority list box"</em></center>
357 If there are no concurrent sub-meshes under the Mesh object, the user
358 will see the following information.
360 \image html mesh_order_no_concurrent.png
361 <em>"No concurrent submeshes detected"</em></center>
364 \anchor compute_anchor
365 <h2>Computing the mesh</h2>
367 It is equally possible to skip \ref evaluate_anchor "the Evaluation"
368 and \ref preview_anchor "the Preview" and to \b Compute the mesh after
369 the hypotheses are assigned. For this, select your mesh in
370 the <b>Object Browser</b>. From the \b Mesh menu select \b Compute or
371 click "Compute" button of the toolbar.
374 \image html image28.png
375 <em>"Compute" button</em>
378 After the mesh computation finishes, the Mesh Computation information
379 box appears. If you close this box and click "Compute" button again,
380 without previously changing meshing parameters, the mesh will NOT be
381 re-computed and the Mesh Computation information box will be shown
382 with the same contents. (To fully re-compute the mesh, invoke
383 \ref clear_mesh_anchor "Clear Mesh Data" command before).
385 If the mesh computation has been a success, the box shows information
386 on the number of entities of different types in the mesh.
388 \image html meshcomputationsucceed.png
390 \anchor meshing_failed_anchor
391 If the mesh computation has failed, the information about the cause of the
392 failure is provided in \b Errors table.
394 \image html meshcomputationfail.png
396 After you select an error, <b>Show Sub-shape</b> button allows
397 visualizing in magenta the geometrical entity that causes the error.
400 \image html failed_computation.png
401 <em>3D algorithm failed to compute mesh on a box shown using <b>Show
402 Sub-shape</b> button</em>
405 <b>Publish Sub-shape</b> button publishes the sub-shape, whose meshing
406 has failed, in the Geometry component as a child of the main shape, which
407 allows analyzing the problematic geometry and creating a sub-mesh on it in
408 order to locally tune the hypotheses.
410 If the failure is caused by an invalid input mesh and the algorithm has
411 found which mesh entities are bad, <b>Show bad Mesh</b>
412 button appears in the dialog. Clicked, it shows the bad mesh entities in
413 the Viewer in magenta. Sometimes the shown mesh entities are too small
414 or/and hidden by other mesh elements. They can be seen after
415 switching the mesh to Wireframe visualization mode or switching off
416 the visualization of faces and volumes (if any).
418 <b>Bad Mesh to Group</b> button creates groups of bad mesh entities
419 to facilitate their analysis.
422 \image html show_bad_mesh.png
423 <em>Edges bounding a hole in the surface are shown in magenta using <b>Show
424 bad Mesh</b> button</em>
427 \note Mesh Computation Information box does not appear if you set
428 "Mesh computation/Show a computation result notification" preference
429 to the "Never" value. This option gives the possibility to control mesh
430 computation reporting. There are the following possibilities: always
431 show the information box, show only if an error occurs or never.
432 By default, the information box is always shown after mesh computation operation.
436 <h2>Editing the mesh</h2>
438 It is possible to \ref modifying_meshes_page "edit the mesh" of a
439 lower dimension before generation of the mesh of a higher dimension.
441 For example you can generate a 2D mesh, modify it using e.g.
442 \ref pattern_mapping_page, and then generate a 3D mesh basing on the
443 modified 2D mesh. The workflow is as follows:
444 - Define 1D and 2D meshing algorithms.
445 - Compute the mesh. 2D mesh is generated.
446 - Apply \ref pattern_mapping_page.
447 - Define 3D meshing algorithms without modifying 1D and 2D algorithms
449 - Compute the mesh. 3D mesh is generated.
451 \note Nodes and elements added \ref adding_nodes_and_elements_page
452 "manually" cannot be used in this workflow because the manually created
453 entities are not attached to any geometry and thus (usually) cannot be
454 found by the mesher paving a geometry.
456 <b>See Also</b> a sample TUI Script demonstrates the possibility of
457 \ref tui_editing_while_meshing "Intermediate edition while meshing"