Creating a mesh geom progammatically

[Pierre-Terdiman]

Intro

Hello, I am interested in physics engines and starting to learn MuJoCo.

My setup

MuJoCo 3.2.2, C API, Windows

My question

I need help creating a mesh geom from code (i.e. without loading the data from a file). Everything else works (box, sphere, capsule, cylinder) but meshes do not. I am getting errors like these, so it seems it is just the names (rather than the mesh data) that are wrong. It makes sense, as the other geoms did not need names:

Error: empty name in mesh
Element name '', id 0

Code is basically like this:

	mjsGeom* geom = mjs_addGeom(params->mBody, null);
	mjs_defaultGeom(geom);
	geom->type = mjGEOM_MESH;
			
	mjsMesh* mesh = mjs_addMesh(mSpec, null);

	std::string* str = new std::string("convex");	// ### just a test, leak here

	geom->meshname = str;
	mesh->name = str;

	std::vector<float>* verts = new std::vector<float>;	// ### just a test, leak here
	// fill random vertices for test
	int maxhullvert = 16;
	mesh->maxhullvert = maxhullvert;
	for(int v=0; v<maxhullvert; v++)
	{
		verts->push_back((UnitRandomFloat() - 0.5f)*4.0f);
		verts->push_back((UnitRandomFloat() - 0.5f)*4.0f);
		verts->push_back((UnitRandomFloat() - 0.5f)*4.0f);
	}
	mesh->uservert = verts;

Am I missing a step or doing something wrong? Note that I managed to use names before to map actuators to joints, but for some reason it does not work for mapping meshes to geoms.

Minimal model and/or code that explain my question

No response

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[quagla]

Hi, strings can be assigned with mjs_setString, since the memory it's managed internally you can't assign a user-managed pointer to the name directly.

[Pierre-Terdiman]

Thank you for your answer.

Yes, I used mjs_setString before (for joints & actuators) but in this case it makes the code crash:

	mjs_setString(mesh->name, convexName);
	mjs_setString(geom->meshname, convexName);

The first mjs_setString call works (for mesh->name) but the second one crashes. And it seems to make sense, since geom->meshname is a null pointer.

// set string
void mjs_setString(mjString* dest, const char* text) {
  std::string* str = static_cast<std::string*>(dest);        <====== dest is null
  *str = std::string(text);
}

[Pierre-Terdiman]

Ah, wait, found it. It's because of that call to mjs_defaultGeom() in my code:

	mjsGeom* geom = mjs_addGeom(params->mBody, null);
	if(geom)
	{
		mjs_defaultGeom(geom);

For some reason it nullifies "meshname". This did not create issues with other geoms and it looked like a good idea to initialize everything to default values, but I guess this was a mistake.

Without that call I am now getting new error messages about the actual mesh data, so this first problem is fixed. Thank you.

[Pierre-Terdiman]

Alright, it all works now. Excellent.