Topic - constrain to vertex position and orientation?

I have a deforming mesh and need to attach another object to a specific part of the deforming mesh.

An example would be adding glasses to an mdd deforming head.

I can position constrain a locator to a specific vertex, and add a normal constraint to that same vertex, but the mesh still swivels around that normal during the animation. Is it possible to further constrain the locator so the rotation stays locked to the mesh? In the above example, I'd position and normal constrain a locator to a vertex at the bridge of the nose, and add some kind of up vector constraint to a vertex at the center of the forehead to create stable orientation.

Anybody know of a way of doing this?
I saw this solution in the rigging master course (check training videos) --->
You have to use more than just normal constraint and vertex...
but the idea in the example from dvd is to build "two" up-vector, crossing the face probably to determine the center of your mesh, so you can constraint to the center...
Sorry, is a bit complex for me to explain everything... just a tip.

Hey John,

The approach I would take (although it's not very elegant), is to constraint a locator to the vertex position and vertex normal of the mesh. Then constrain a second locator to the vertex position an adjoining vertex.
Then position constrain your item to the first locator and direction constrain it to the normal position of the first locator. Using a direction constraint allows you to define an up vector, so use the second locator (the second vertex position) as the up vector for the direction constraint.

I've attached a video which shows how to set this up.

-Matt
Matt,

Thank you very much! That was exactly what I was looking for. I was trying to figure how to add an up vector, but couldn't see an obvious way. That'll work.

Would be nice if the vertex normal constraint could take an up vector directly.... 701??
OK, just tried it. Almost works. It appears that vertex normal constraints don't behave correctly with MDD deformed meshes. If I switch to a polygon normal constraint instead, it works.

Anyway, I have a working solution now, thanks!

-John

BTW, just went to report this as a bug, and found it already listed in the bugs database as Bug: 24773
Added a confirmation....
Hi Matt,

Awesome video. How did you look up the individual vertex ID numbers? Thanks for your time!

Model tab, select the mesh, select the vertex, right side halfway down,
click lists tab, further down select the INFO button, at the top of the window
below that should be the vertex number.