I was wondering if it is possible to create four-sided surfaces?
Imagine a cage made of splines, and Houdini creates surfaces between them. Is this possible?

draetsch
I was wondering if it is possible to create four-sided surfaces?

How about Platonic Solid::Tetrahedron?

Imagine a cage made of splines, and Houdini creates surfaces between them. Is this possible?

I think so, if I understand what you mean. Might be a bit more work than you would hope, as Houdini won't automatically deal with triangles AFAIK.

To see what is possible, put down
Grid SOP:
Type Polygon
Connectivity Rows
Rows 3
Columns 5

Then connect to it:
Convert SOP:
Convert To: NURBS curve

Put down a fresh:
Grid SOP:
Type Polygon
Connectivity COls
Rows 5
Columns 3

And connect to it a convert sop
Convert To: NURBS curve

Now, connect the first convert into the LHS of a Skin SOP and the second convert into the RHS of a skin SOP.

You will then have transformed those curves into a four sided surface. If you turn on breakpoint display, you will see that the curves have breakpoints that match up nicely - this is the best way to construct the cage of nurbs splines so that they will convert to a smooth nurbs surface. The Skin sop, for better or worse, will convert curves which don't have matching breakpoints, but this will often not result in what you wanted.

- Brask