使用prim中心点移动prims，做消散等等

vector cenpos = point(1, "P", i@myprim);
vector prim_cen = prim(0, "P", i@myprim);

vector4 pt_orient = point(1, "orient", i@myprim);


vector local_offset =cenpos - prim_cen;
vector rota_offset = qrotate(pt_orient, local_offset);


//rotate
vector axis = rand(@myprim);
float angle = chf("angle");
vector4 rot  = quaternion(angle, axis);
@P -=  prim_cen;
@P = qrotate(rot, @P);
@P +=  prim_cen;


@P +=local_offset ;