1 .. _constructing_meshes_page:
7 To create a mesh on geometry, it is necessary to create a mesh object by choosing
9 * a geometrical shape produced in the Geometry module (*main shape*);
10 * *meshing parameters*, including
11 * :ref:`basic_meshing_algos_page` and
12 * :ref:`about_hypo_page` specifying constraints to be taken into account by the chosen meshing algorithms.
14 Then you can launch mesh generation by invoking :ref:`compute_anchor` command.
15 The generated mesh will be automatically shown in the Viewer. You can
16 switch off automatic visualization or limit mesh size until which it is
17 automatically shown in :ref:`mesh_preferences_page` (**Automatic update** entry).
20 Sometimes *hypotheses* term is used to refer to both algorithms and hypotheses.
22 Mesh generation on the geometry is performed in the bottom-up
23 flow: nodes on vertices are created first, then edges are divided into
24 segments using nodes on vertices; the nodes of segments are then
25 used to mesh faces; then the nodes of faces are used to mesh
26 solids. This automatically assures the conformity of the mesh.
28 It is required to choose a meshing algorithm for every dimension of
29 sub-shapes up to the highest dimension to be generated. Note
30 that some algorithms generate elements of several dimensions, and
31 others of only one. It is not necessary to define meshing
32 parameters for all dimensions at once; you can start from 1D
33 meshing parameters only, compute the 1D mesh, then define 2D meshing
34 parameters and compute the 2D mesh (note that 1D mesh will not be
37 An algorithm of a certain dimension chosen at mesh creation is applied
38 to discretize every sub-shape of this dimension. It is possible to
39 specify a different algorithm or hypothesis to be applied to one or
40 a group of sub-shapes by creating a :ref:`constructing_submeshes_page`.
41 You can specify no algorithms at all at mesh object
42 creation and specify the meshing parameters on sub-meshes only; then
43 only the sub-shapes, for which an algorithm and a hypothesis (if any)
44 have been defined will be discretized.
46 .. note:: Construction of a mesh on a geometry includes at least two (:ref:`create_mesh_anchor` and :ref:`compute_anchor`) of the following steps:
48 * :ref:`create_mesh_anchor`, where you can specify meshing parameters to apply to all sub-shapes of the main shape.
49 * :ref:`constructing_submeshes_page`, (optional) where you can specify meshing parameters to apply to the selected sub-shapes.
50 * :ref:`evaluate_anchor` (optional) can be used to know an approximate number of elements before their actual generation.
51 * :ref:`preview_anchor` (optional) can be used to generate mesh of only lower dimension(s) in order to visually estimate it before full mesh generation, which can be much longer.
52 * :ref:`submesh_order_anchor` (optional) can be useful if there are concurrent sub-meshes defined.
53 * :ref:`compute_anchor` uses defined meshing parameters to generate mesh elements.
54 * :ref:`edit_anchor` (optional) can be used to :ref:`modifying_meshes_page` the mesh of a lower dimension before :ref:`compute_anchor` elements of an upper dimension.
57 .. _create_mesh_anchor:
59 Creation of a mesh object
60 #########################
62 **To construct a mesh:**
64 #. Select a geometrical object for meshing.
65 #. In the **Mesh** menu select **Create Mesh** or click **"Create Mesh"** button in the toolbar.
67 .. image:: ../images/image32.png
71 **"Create Mesh" button**
73 The following dialog box will appear:
75 .. image:: ../images/createmesh-inv.png
78 #. To filter off irrelevant meshing algorithms, you can select **Mesh Type** in the corresponding list from **Any, Hexahedral, Tetrahedral, Triangular** and **Quadrilateral** (there can be less items for the geometry of lower dimensions). Selection of a mesh type hides all meshing algorithms that cannot generate elements of this type.
80 #. Apply :ref:`basic_meshing_algos_page` and :ref:`about_hypo_page` which will be used to compute this mesh.
82 "Create mesh" dialog box contains several tab pages titled **3D**, **2D**, **1D** and **0D**. The title of each page reflects the dimension of the sub-shapes the algorithms listed on this page affect and the maximal dimension of elements the algorithms generate. For example, **3D** page lists the algorithms that affect 3D sub-shapes (solids) and generate 3D mesh elements (tetrahedra, hexahedra etc.)
84 As soon as you have selected an algorithm, you can create a hypothesis (or select an already created one). A set of accessible hypotheses includes only the hypotheses that can be used by the selected algorithm.
87 * Some page(s) can be disabled if the geometrical object does not include shapes (sub-shapes) of the corresponding dimension(s). For example, if the input object is a geometrical face, **3D** page is disabled.
88 * Some algorithms affect the geometry of several dimensions, i.e. 1D+2D or 1D+2D+3D. If such an algorithm is selected, the dialog pages related to the corresponding lower dimensions are disabled.
89 * **0D** page refers to 0D geometry (vertices) rather than to 0D elements. Mesh module does not provide algorithms that produce 0D elements. Currently **0D** page provides only one algorithm "Segments around vertex" that allows specifying the required size of mesh edges about the selected vertex (or vertices).
91 For example, you need to mesh a 3D object.
93 First, you can change a default name of your mesh in the **Name** box. Then check that the selected geometrical object indicated in **Geometry** field, is what you wish to mesh; if not, select the correct object in the Object Browser. Click "Select" button near **Geometry** field if the name of the object has not yet appeared in **Geometry** field.
94 .. image:: ../images/image120.png
100 Now you can define 3D Algorithm and 3D Hypotheses, which will be applied to discretize the solids of your geometrical object using 3D elements. Click the **"Add Hypothesis"** button to create and add a hypothesis.
101 .. image:: ../images/image121.png
105 **"Add Hypothesis" button**
107 Click the **"Plus"** button to enable adding more additional hypotheses.
109 Click the **"Edit Hypothesis"** button to change the values for the current hypothesis.
110 .. image:: ../images/image122.png
114 **"Edit Hypothesis" button**
116 Most 2D and 3D algorithms can work without hypotheses using default meshing parameters. Some algorithms do not require any hypotheses. After selection of an algorithm "Hypothesis" field of the dialog can contain:
118 * **\<Default\>** if the algorithm can work using default parameters.
119 * **\<None\>** if the algorithm requires a hypothesis defining its parameters.
120 * If the algorithm does not use hypotheses, this field is grayed.
122 After selection of an algorithm **Add. Hypothesis** field can contain:
124 * **\<None\>** if the algorithm can be tuned using an additional hypothesis.
125 * If the algorithm does not use additional hypotheses, this field is grayed.
128 Proceed in the same way with 2D and 1D Algorithms and Hypotheses that will be used to mesh faces and edges of your geometry. (Note that any object has edges, even if their existence is not apparent, for example, a sphere has 4 edges). Note that the choice of hypotheses and lower dimension algorithms depends on the higher dimension algorithm.
130 If you wish you can select other algorithms and/or hypotheses for meshing some sub-shapes of your CAD model by :ref:`constructing_submeshes_page`.
132 Some algorithms generate mesh of several dimensions, while others produce mesh of only one dimension. In the latter case there must be one Algorithm and zero or several Hypotheses for each dimension of your object, otherwise you will not get any mesh at all. Of course, if you wish to mesh a face, which is a 2D object, you do not need to define a 3D Algorithm and Hypotheses.
134 In the **Object Browser** the structure of the new mesh is displayed as follows:
136 .. image:: ../images/image88.jpg
141 * a mesh name (**Mesh_mechanic**);
142 * a reference to the geometrical object on the basis of which the mesh has been constructed (*mechanic*);
143 * **Applied hypotheses** folder containing the references to the hypotheses chosen at the construction of the mesh;
144 * **Applied algorithms** folder containing the references to the algorithms chosen at the construction of the mesh.
145 * **SubMeshes on Face** folder containing the sub-meshes defined on geometrical faces. There also can be folders for sub-meshes on vertices, edges, wires, shells, solids and compounds.
146 * **Groups of Faces** folder containing the groups of mesh faces. There also can be folders for groups of nodes, edges, volumes 0D elements and balls.
149 There is an alternative way to assign Algorithms and Hypotheses by clicking **Assign a set of hypotheses** button and selecting among pre-defined sets of algorithms and hypotheses. In addition to the built-in sets of hypotheses, it is possible to create custom sets by editing CustomMeshers.xml file located in the home directory. CustomMeshers.xml file must describe sets of hypotheses in the same way as ${SMESH_ROOT_DIR}/share/salome/resources/smesh/StdMeshers.xml file does (sets of hypotheses are enclosed between \<hypotheses-set-group\> tags). For example:
152 <?xml version='1.0' encoding='us-ascii'?>
153 <!DOCTYPE meshers PUBLIC "" "desktop.dtd">
155 <hypotheses-set-group>
156 <hypotheses-set name="My favorite hypotheses"
157 hypos="AutomaticLength"
158 algos="CompositeSegment_1D, Quadrangle_2D, GHS3D_3D"/>
159 </hypotheses-set-group>
162 If the file contents are incorrect, there can be an error at activation of Mesh module: **"fatal parsing error: error triggered by consumer in line ..."**
164 .. image:: ../images/hypo_sets.png
167 List of sets of hypotheses. Tag **[custom]** is automatically added to the sets defined by the user.
170 * *"Automatic"* in the names of predefined sets of hypotheses does not actually mean that they are suitable for meshing any geometry.
171 * The list of sets of hypotheses can be shorter than in the above image depending on the geometry dimension.
174 Consider trying a sample script for construction of a mesh from our :ref:`tui_creating_meshes_page` section.
181 After the mesh object is created and all hypotheses are assigned and before :ref:`compute_anchor` operation, it is possible to calculate the eventual mesh size. For this, select the mesh in the **Object Browser** and from the **Mesh** menu select **Evaluate**.
182 The result of evaluation will be displayed in the following information box:
184 .. image:: ../images/mesh_evaluation_succeed.png
192 Before :ref:`compute_anchor` , it is also possible to see the mesh preview. This operation allows to incrementally compute the mesh, dimension by dimension, and to discard an unsatisfactory mesh.
194 For this, select the mesh in the Object Browser. From the **Mesh** menu select **Preview** or click "Preview" button in the toolbar or activate "Preview" item from the pop-up menu.
196 .. image:: ../images/mesh_precompute.png
202 Select **1D mesh** or **2D mesh** preview mode in the Preview dialog.
204 .. image:: ../images/preview_mesh_1D.png
208 "1D mesh preview shows nodes computed on geometry edges"
211 .. image:: ../images/preview_mesh_2D.png
215 "2D mesh preview shows edge mesh elements, computed on geometry faces"
218 **Compute** button computes the whole mesh.
220 When the Preview dialog is closed, the question about the storage of temporarily created mesh elements appears:
222 .. image:: ../images/preview_tmp_data.png
225 These elements can be kept in the mesh.
228 .. _submesh_order_anchor:
230 Changing sub-mesh priority
231 ##########################
233 If the mesh contains concurrent :ref:`constructing_submeshes_page`, it is possible to change the priority of their computation, i.e. to change the priority of applying algorithms to the shared sub-shapes of the Mesh shape.
235 **To change sub-mesh priority:**
237 Choose "Change sub-mesh priority" from the Mesh menu or a pop-up menu. The opened dialog shows a list of sub-meshes in the order of their priority.
239 There is an example of sub-mesh order modifications taking a Mesh created on a Box shape. The main Mesh object:
241 * *1D* **Wire discretisation** with **Number of Segments** =20
242 * *2D* **Triangle: Mefisto** with Hypothesis **Max Element Area**
245 The first sub-mesh **Submesh_1** created on **Face_1** is:
247 * *1D* **Wire discretisation** with **Number of Segments** =4
248 * *2D* **Triangle: Mefisto** with Hypothesis **MaxElementArea** =1200
250 The second sub-mesh **Submesh_2** created on **Face_2** is:
252 * *1D* **Wire discretisation** with **Number of Segments** =8
253 * *2D* **Triangle: Mefisto** with Hypothesis **MaxElementArea** =1200
256 And the last sub-mesh **Submesh_3** created on **Face_3** is:
258 * *1D* **Wire discretisation** with **Number of Segments** =12
259 * *2D* **Triangle: Mefisto** with Hypothesis **MaxElementArea** =1200
262 The sub-meshes become concurrent if they share sub-shapes that can be meshed with different algorithms (or different hypotheses). In the example, we have three sub-meshes with concurrent algorithms, because they have different hypotheses.
264 The first mesh computation is made with:
266 .. image:: ../images/mesh_order_123.png
270 **"Mesh order SubMesh_1, SubMesh_2, SubMesh_3"**
273 .. image:: ../images/mesh_order_123_res.png
277 **"Result mesh with order SubMesh_1, SubMesh_2, SubMesh_3 "**
279 The next mesh computation is made with:
281 .. image:: ../images/mesh_order_213.png
285 **"Mesh order SubMesh_2, SubMesh_1, SubMesh_3"**
287 .. image:: ../images/mesh_order_213_res.png
291 **"Result mesh with order SubMesh_2, SubMesh_1, SubMesh_3 "**
293 And the last mesh computation is made with:
295 .. image:: ../images/mesh_order_321.png
299 **"Mesh order SubMesh_3, SubMesh_2, SubMesh_1"**
302 .. image:: ../images/mesh_order_321_res.png
306 **"Result mesh with order SubMesh_3, SubMesh_2, SubMesh_1 "**
308 As we can see, each mesh computation has a different number of result
309 elements and a different mesh discretization on the shared edges (the edges
310 that are shared between **Face_1**, **Face_2** and **Face_3**)
312 Additionally, sub-mesh priority (the order of applied algorithms) can
313 be modified not only in a separate dialog box, but also in
314 the **Preview**. This helps to preview different mesh results,
315 modifying the order of sub-meshes.
317 .. image:: ../images/mesh_order_preview.png
321 **"Preview with sub-mesh priority list box"**
323 If there are no concurrent sub-meshes under the Mesh object, the user
324 will see the following information.
326 .. image:: ../images/mesh_order_no_concurrent.png
330 **"No concurrent submeshes detected"**
338 It is equally possible to skip :ref:`evaluate_anchor`
339 and :ref:`preview_anchor` and to **Compute** the mesh after
340 the hypotheses are assigned. For this, select your mesh in
341 the **Object Browser**. From the **Mesh** menu or the context menu
342 select **Compute** or click *"Compute"* button of the toolbar.
344 .. image:: ../images/image28.png
350 After the mesh computation finishes, the Mesh Computation information
351 box appears. If you close this box and click "Compute" button again,
352 without previously changing meshing parameters, the mesh will NOT be
353 re-computed and the Mesh Computation information box will be shown
354 with the same contents. (To fully re-compute the mesh, invoke
355 :ref:`clear_mesh_anchor` command before).
357 .. _meshing_result_anchor:
362 If the mesh computation has been a success, the box shows information on the number of entities of different types in the mesh.
364 .. image:: ../images/meshcomputationsucceed.png
367 .. _meshing_failed_anchor:
372 If the mesh computation has failed, the information about the cause of the failure is provided in **Errors** table.
374 .. image:: ../images/meshcomputationfail.png
377 After you select an error in **Errors** table, **Show Sub-shape** button allows
378 visualizing in magenta the geometrical entity meshing of which failed
379 (Name of this entity or its ID and type is shown in *Sub-shape* column).
381 .. image:: ../images/failed_computation.png
385 3D algorithm failed to compute mesh on a box shown using **Show Sub-shape** button
387 **Publish Sub-shape** button publishes the sub-shape, whose meshing
388 has failed, in the Geometry component as a child of the main shape, which
389 allows analyzing the problematic geometry and creating a sub-mesh on it in
390 order to locally tune the hypotheses.
392 If the failure is caused by an invalid input mesh and the algorithm has
393 found which mesh entities are bad, **Show bad Mesh**
394 button appears in the dialog. Clicked, it shows the bad mesh entities in
395 the Viewer in magenta. Sometimes the shown mesh entities are too small
396 or/and hidden by other mesh elements. They can be seen after
397 switching the mesh to Wireframe visualization mode or switching off
398 the visualization of faces and volumes (if any).
400 **Bad Mesh to Group** button creates groups of bad mesh entities to facilitate their analysis.
402 .. image:: ../images/show_bad_mesh.png
405 Edges bounding a hole in the surface are shown in magenta using **Show bad Mesh** button
408 Mesh Computation Information box does not appear if you set :ref:`show_comp_result_pref` preference to the "Never" value. This option gives the possibility to control mesh computation reporting. There are the following possibilities: always show the information box, show only if an error occurs or never. By default, the information box is always shown after mesh computation operation.
415 It is possible to :ref:`modifying_meshes_page` of a
416 lower dimension before generation of the mesh of a higher dimension.
418 For example you can generate a 2D mesh, modify it using e.g. :ref:`pattern_mapping_page`, and then generate a 3D mesh basing on the modified 2D mesh. The workflow is as follows:
420 * Define 1D and 2D meshing algorithms.
421 * Compute the mesh. 2D mesh is generated.
422 * Apply :ref:`pattern_mapping_page`.
423 * Define 3D meshing algorithms without modifying 1D and 2D algorithms and hypotheses.
424 * Compute the mesh. 3D mesh is generated.
427 Nodes and elements added :ref:`adding_nodes_and_elements_page` cannot be used in this workflow because the manually created entities are not attached to any geometry and thus (usually) cannot be found by the mesher paving a geometry.
429 **See Also** a sample TUI Script demonstrates the possibility of :ref:`tui_editing_while_meshing`.
434 basic_meshing_algos.rst
