I want to access node coordinate in cmesh

[shusaku625]

I'm studying how to use t8code in finite element method.
When I create matrix, I have to acces node coordinate in each element.
Please tell me how to access each node coordinate in cmesh.

[jmark]

Hello @syusaku625,

I recommend to check out this tutorial https://github.com/DLR-AMR/t8code/wiki/Step-6-Computing-stencils and associated source code https://github.com/DLR-AMR/t8code/blob/main/tutorials/general/t8_step6_stencil.cxx .

This shows how to access information like volume, centroid, etc per element. For node coordinates there are the following functions:

/** Compute the coordinates of a given vertex of an element if a geometry
 * for this tree is registered in the forest's cmesh.
 * \param [in]      forest     The forest.
 * \param [in]      ltree_id   The forest local id of the tree in which the element is.
 * \param [in]      element    The element.
 * \param [in]      corner_number The corner number, in Z-order, of the vertex which should be computed.
 * \param [out]     coordinates On input an allocated array to store 3 doubles, on output
 *                             the x, y and z coordinates of the vertex.
 */
void
t8_forest_element_coordinate (t8_forest_t forest, t8_locidx_t ltree_id, const t8_element_t *element, int corner_number,
                              double *coordinates);

/** Compute the coordinates of a point inside an element inside a tree.
 *  The point is given in reference coordinates inside the element and gets
 *  converted to reference coordinates inside the tree. After that, the point
 *  is converted to global coordinates inside the domain. If needed, the element
 *  is stretched by the given stretch factors (the resulting mesh is then 
 *  no longer non-overlapping).
 * 
 * \param [in]      forest            The forest.
 * \param [in]      ltreeid           The forest local id of the tree in which the element is.
 * \param [in]      element           The element.
 * \param [in]      ref_coord         The reference coordinates of the point inside the element.
 * \param [in]      num_coords        The number of coordinate sets in ref_coord (dimension x double).
 * \param [out]     coords_out        On input an allocated array to store 3 doubles, on output
 *                                    the x, y and z coordinates of the point inside the domain.
 * \param [in]      stretch_factors   An element-wise array with d (dimension) doubles per element, 
 *                                    which are used to stretch the element beyond its original boundary.
 *                                    The array must be of length forest->local_num_elements * d.
 *                                    If NULL, no stretching is applied.
 */
void
t8_forest_element_from_ref_coords (t8_forest_t forest, t8_locidx_t ltreeid, const t8_element_t *element,
                                   const double *ref_coord, const size_t num_coords, double *coords_out,
                                   sc_array_t *stretch_factors);

If you are only interested in node coordinates on cmesh level then I suggest to use the following function:

void
t8_geometry_evaluate (t8_cmesh_t cmesh, t8_gloidx_t gtreeid, const double *ref_coords, const size_t num_coords,
                      double *out_coords);

Does this answer your question?