Gerome

Do you want to do it as a standalone cmd? Expression or will it be an OP?
You can have a look at the traverse.C from the HDK samples, to get the nodes, then you can run through every geo, get the gdp and then use the function
gdp->primitives().entries() to get the number of polys of each geo.

I have nt seen how looks the DOP gravity. But if I understood your question,
you want to connect different geo to your SOP. Let say A and B

You have to add the number of inputs you want, when you declare your OP
void
newSopOperator(OP_OperatorTable *table)
{
table->addOperator(new OP_Operator(“sparticle”,
“Simple Particle”,
SOP_SParticle::myConstructor,
SOP_SParticle::myTemplateList,
1, // Min required sources
2, // Maximum sources
0));
}

then you loop through and you collect the GDP of each input


src = ( GU_Detail *)inputGeo(current_obj, context ))

If it answers your question I go into details.

The Y (green bit) is the up vector. But are you using the up Vector in POP to adjust the up vector?