A Gentle Introduction to iRel

iRel is an interface for relating pairs of entities from other ITAPS interfaces, e.g. relating itaps.iMesh mesh to itaps.iGeom geometry. An iRel instance contains a list of pairs of ITAPS interfaces. Each pair contains relations between entities from each of the interfaces.

Starting Out

Before we can get started working with iRel, we’ll need to create a mesh and a geometry with a few entities each:

>>> from itaps import iMesh, iGeom, iRel
>>> mesh = iMesh.Mesh()
>>> geom = iGeom.Geom()
>>> mesh_ents = mesh.createVtx([[0,0,0], [1,1,1], [2,2,2]])
>>> geom_ents = [ geom.createBrick(1,1,1),
                  geom.createBrick(1,1,1),
                  geom.createBrick(1,1,1) ]

From here, we can create the iRel instance and a pair relating our newly-created mesh and geometry:

>>> rel = iRel.Rel()
>>> pair = rel.createPair(mesh, iRel.Type.entity, geom, iRel.Type.entity)

These snippets contain the prelude for most PyTAPS programs using iRel: we import the iBase, iMesh, iGeom, and iRel modules, create (or load) our mesh and geometry, then create an iRel instance and a relation pair. The relation pair object works like a bidirectional map, where keys on one side point to values on the other, and vice-versa. In the examples below, we’ll assume that the above snippets have already been executed.

Setting Relations

There are a few ways to set a relation between a pair of entities. All of the following are equivalent:

>>> pair.relate(mesh_ents[0], geom_ents[0])
>>> pair.left[ mesh_ents[0] ] = geom_ents[0]
>>> pair.right[ geom_ents[0] ] = mesh_ents[0]

Getting Relations

It’s equally straightforward to get a relation if you have one of the related entities:

>>> pair.left[ mesh_ents[0] ]
<itaps.iBase.Entity 0x2b80dc0>
>>> pair.left[ mesh_ents[0] ] == geom_ents[0]
True
>>> pair.right[ geom_ents[0] ]
<itaps.iBase.Entity 0x1>
>>> pair.right[ geom_ents[0] ] = mesh_ents[0]

Removing Relations

Naturally, one can delete a relation, too:

>>> del pair.left[ mesh_ents[0] ]

If we then attempt to retrieve this relation (from either side), we’ll get an exception:

>>> pair.left[ mesh_ents[0] ]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
iBase.TagNotFoundError: iMesh_getArrData: tag not foundfor tag "__MESH_ASSOCIATION0".  (MOAB Error Code: MB_TAG_NOT_FOUND)
>>> pair.right[ geom_ents[0] ]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
iBase.TagNotFoundError: Tag does not exist