Problems adding vertex groups to mesh in modifier

I am trying to make a modifier that messes with vertex groups. To do what I am thinking about I have to add new v groups. I tried a view thinks but if it is compiling I always get one of two results.
1.) Everything seams fine but the modifier doesn’t seam to have created the v group I thought it should.
2.) If I try to apply the modifier it just states “Modifier returned error, skipping apply” but I am unable to get more information about it.

So now my a view questions to that topic:

  1. How is it Intendet to add, remove, edit vgroups on meshes in C and is there documentation on that topic other then searching through rnd/dna types.

  2. How to get more information about that what kind of error that Modifier returned and why?

  3. Is there documentation for modifier related types and so on other than source code comments.?

  4. Is something wrong with MOD_vertex_regroup.c? I mean yeah obviously but is that you can see at one glimpse that I missed?

here is MOD_vertex_regroup.c

#include “BKE_modifier.h”
#include “DNA_mesh_types.h”
#include “BKE_object_deform.h”

static Mesh *vertex_regroup_applyModifier( struct ModifierData *md,
const struct ModifierEvalContext *ctx,
struct Mesh *mesh
Mesh *result = mesh;
result = BKE_object_defgroup_add_name(mesh, “myName”);
//WM_main_add_notifier(NC_OBJECT | ND_DRAW, mesh);
return result;

ModifierTypeInfo modifierType_Vertex_regroup = {
/* name / “Vertex regroup”,
structName / “VertexRegroupModifierData”,
structSize / sizeof(VertexRegroupModifierData),
type / eModifierTypeType_None,
flags */ eModifierTypeFlag_AcceptsMesh,

/* copyData */ NULL,

/* deformVerts */ NULL,
/* deformMatrices */ NULL,
/* deformVertsEM */ NULL,
/* deformMatrices EM */ NULL,
/* applyModifier */ vertex_regroup_applyModifier,

/* initData */ NULL,
/* requiredDataMask */ NULL,
/* freeData */ NULL,
/* isDisable */ NULL,
/* updateDepsgraph */ NULL,
/* dependsOnTime */ NULL,
/* dependsOnNormals */ NULL,
/* foreachObjectLink */ NULL,
/* foreachIDLink */ NULL,
/* foreachTextLink */ NULL,
/* freeRuntimeData */ NULL,


ok seams like that just does not work since a modifier is not allowed to modify that kind of data :sob:

I would look at the Vertex Weight Edit/Mix modifiers for reference. They are able to edit existing vgroups so it should be possible.

I am don’t think you can to add or remove vgroups with a modfiier.

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