Feature: Add subelement scheme for quad and triangle hanging node resolution by lenaploetzke · Pull Request #2395 · DLR-AMR/t8code

lenaploetzke · 2026-08-13T13:58:48Z

Describe your changes here:
This PR adds a general subelement scheme with specializations for quad and triangle hanging node resolution. It also provides two examples and one test.
I implemented only the functions necessary for the visualization. Next steps are all functions necessary for simulations. This means for example neighbors.

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codecov · 2026-08-13T14:19:33Z

Codecov Report

❌ Patch coverage is 84.27481% with 103 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.35%. Comparing base (ea082ff) to head (ba8cf74).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
.../t8_schemes/t8_subelement/t8_subelement_scheme.hxx	76.40%	80 Missing ⚠️
.../t8_subelement/specializations/t8_scheme_quads.hxx	91.75%	8 Missing ⚠️
...es/t8_subelement/specializations/t8_scheme_tri.hxx	90.69%	8 Missing ⚠️
src/t8_forest/t8_forest_subelement.cxx	93.87%	3 Missing ⚠️
src/t8_schemes/t8_scheme.hxx	85.71%	2 Missing ⚠️
src/t8_forest/t8_forest_adapt.cxx	91.66%	1 Missing ⚠️
src/t8_schemes/t8_subelement/t8_subelement.cxx	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2395      +/-   ##
==========================================
+ Coverage   82.28%   82.35%   +0.07%     
==========================================
  Files         125      131       +6     
  Lines       20703    21352     +649     
==========================================
+ Hits        17035    17584     +549     
- Misses       3668     3768     +100

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spenke91

Thanks a lot @lenaploetzke for this great feature! I'm glad to see the subelement concept being molded into an actual implementation 👍
I like the overall code design and think it is great that we barely have to change the interface of the normal schemes. As you will see in one of the comments, I feel like we might be missing one layer of inheritance between "a subelement scheme for triangles" and "a subelement schemes for triangles that resolves haning nodes".

Aside from that, most of my comments are minor suggestion and remarks, as usual :-)

spenke91 · 2026-09-01T11:18:46Z

+  if ((t8_forest_get_tree_element_offset (forest_from, which_tree) + lelement_id) % 2 == 0) {
+    return 1;
+  }
+  return 0;


I think this is actually the local element ID, not the global one. It relies on the tree's member variable
elements_offset, which is process-local, see here:

spenke91 · 2026-09-01T11:19:04Z

 }
+
+int
+t8_test_adapt_even_global_id ([[maybe_unused]] t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree,


Suggested change

t8_test_adapt_even_global_id ([[maybe_unused]] t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree,

t8_test_adapt_even_local_id ([[maybe_unused]] t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree,

spenke91 · 2026-09-01T11:19:29Z

+ * \param [in] elements     Pointers to a family or, if \a is_family is zero, pointer to one element.
+ */
+int
+t8_test_adapt_even_global_id (t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree, t8_eclass_t eclass,


Suggested change

t8_test_adapt_even_global_id (t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree, t8_eclass_t eclass,

t8_test_adapt_even_local_id (t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree, t8_eclass_t eclass,

I'd suggest to use the callback refine_every_nth_element_callback I recently added 🙂
(which is adjusted to global elements in #2415 )

spenke91 · 2026-09-01T11:42:14Z

@@ -0,0 +1,25 @@
+# t8_subelement
+
+This folder provides **subelement schemes**. Subelements are inserted *after* the standard recursive refinement and enable one additional refinement level that uses a different scheme.


Suggested change

This folder provides **subelement schemes**. Subelements are inserted *after* the standard recursive refinement and enable one additional refinement level that uses a different scheme.

This folder provides **subelement schemes**. Subelements are inserted *after* the standard recursive refinement and enable one additional refinement step that uses a different scheme.

I think we should not call it level to avoid confusion 🤔

spenke91 · 2026-09-01T12:08:25Z

+  void *scheme_context; /**< Anonymous implementation context. */
+
+ public:
+  // #################################____Constructor & Destructor...____###############################################


Is there a specific reason the constructor above is not sorted into this category?

spenke91 · 2026-09-03T06:53:36Z

+ * \param[in] forest Forest to be adapted.
+ */
+t8_forest_t
+t8_adapt_forest_2and (t8_forest_t forest)


Suggested change

t8_adapt_forest_2and (t8_forest_t forest)

t8_adapt_forest_2nd (t8_forest_t forest)

?

spenke91 · 2026-09-03T06:53:49Z

+                         prefix_removed_sub);
+
+  /* --- Second cycle: adapt around the larger circle. --- */
+  forest = t8_adapt_forest_2and (forest);


Suggested change

forest = t8_adapt_forest_2and (forest);

forest = t8_adapt_forest_2nd (forest);

spenke91 · 2026-09-03T06:55:25Z

+  struct t8_adapt_data adapt_data = {
+    { 0, 1, 0 }, /* Center of the circle. */
+    0.6,         /* Radius */
+    0.1,         /* Delta (transition band width) */
+    2,           /* Minlevel */
+    6            /* Maxlevel */
+  };
+


Wouldn't it be easier to define the whole adapt_data somewhere accessible from main and just adjust the radius then via adapt_data.radius?
Then we could also get rid of these two adapt wrappers. However, both is fine.

spenke91 · 2026-09-03T07:00:22Z

+  /* --- Adapt the forest: refine near the first circle, creating hanging nodes. --- */
+  forest = t8_adapt_forest (forest);
+  std::cout << "Subelements before removing: " << t8_forest_has_global_subelements (forest) << std::endl;
+  prefix = "t8_adapted1";


Might be convenient to adjust the naming because Paraview otherwise shows the two t8_adapted ptvu files as series in the open-file menu. Not a big problem, but maybe it would be nice to avoid it, e.g.,

Suggested change

prefix = "t8_adapted1";

prefix = "t8_adapted_1st";

spenke91 · 2026-09-03T07:43:50Z

+  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+*/
+
+/** \file t8_scheme_tri.hxx


I would suggest to use more descriptive names for these two files. I know the context is clear from the path, but the path is often not shown, e.g., in vs code. So maybe something like t8_subelement_scheme_tri.hxx or t8_scheme_hanging_nodes_tri.hxx?

lenaploetzke added 28 commits May 11, 2026 07:25

add example

0b677dd

Merge branch 'main' into subelement_quads

259aff0

begin to construct subelement scheme

2ce7d91

Got SEGV

b5f187a

Approach starting from standalone implementation

c4982fc

Implement several functions

8ca7cbf

remaining functions

1866bbe

working version of adapting normally

bdaec0b

Add to adapt

8de6100

Fix some problems

dc81571

working version!!

4318e55

begin cleanup

9be317d

documentation

9df30ff

Abstract subelement class

1193d93

begin to add tri scheme

80e0460

tri

e8c2a93

Working hybrid mesh

d580035

add non periodic example

7c4a26a

add shock example

33dbcbf

example now with radius

b5dcf75

inital level 0

69bb843

Merge branch 'main' into subelement_quads

cdb8ece

corrections

b85c3cb

Documentation and improvements

94ff6d7

clean up remaining files

5171f82

add global has subelements function

7b6d1eb

Added test.

3354b22

indent

8cad411

fix some errors

101ab57

lenaploetzke added 2 commits August 14, 2026 10:32

fix release mode

1713648

add vtk output to test

bf6552d

ole-alb assigned ole-alb and unassigned ole-alb Aug 17, 2026

sandro-elsweijer reviewed Aug 17, 2026

View reviewed changes

Comment thread src/t8_schemes/t8_scheme.cxx Outdated

lenaploetzke added 2 commits August 17, 2026 12:49

Merge branch 'main' into subelement_quads

0196d25

review and fix one hard coded number

2c6387b

spenke91 self-requested a review August 17, 2026 13:03

spenke91 self-assigned this Aug 17, 2026

lenaploetzke added 4 commits August 18, 2026 10:35

first two functions

cd28df8

also remove "is_subelement" from general scheme

751ed2c

remove unnecessary headers

d7aed6f

Small changes

ba8cf74

spenke91 requested changes Sep 3, 2026

View reviewed changes

spenke91 assigned lenaploetzke and unassigned spenke91 Sep 3, 2026

	t8_test_adapt_even_global_id ([[maybe_unused]] t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree,
	t8_test_adapt_even_local_id ([[maybe_unused]] t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree,

	t8_test_adapt_even_global_id (t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree, t8_eclass_t eclass,
	t8_test_adapt_even_local_id (t8_forest_t forest, t8_forest_t forest_from, t8_locidx_t which_tree, t8_eclass_t eclass,

		@@ -0,0 +1,25 @@
		# t8_subelement

		This folder provides subelement schemes. Subelements are inserted after the standard recursive refinement and enable one additional refinement level that uses a different scheme.

	t8_adapt_forest_2and (t8_forest_t forest)
	t8_adapt_forest_2nd (t8_forest_t forest)

	forest = t8_adapt_forest_2and (forest);
	forest = t8_adapt_forest_2nd (forest);

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lenaploetzke commented Aug 13, 2026 •

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