Last year while I was working at Red Rover, I heard the term “weak reference” in reference to a technique for referencing objects in 3DS Max. The Max TD used them for a variety of things. I didn’t quite understand what he was talking about at the time, since the last version of Max I used was 2.5 and I never did rigging or coding for it then.
More recently I’ve come to the same technique on my own both in Maya and in Cinema 4D, and was reminded of the name for it by that same TD over beers a few weeks back.
Essentially, weak references are variables on objects that contain a pointer to an object, and not a reference to it’s name. In Maya, for example, you may see rigging scripts written by TDs referencing specific objects by name or saving names of objects as they’re created and using those names to connect attributes or add constraints. In a clean scene this works fine, as long as the rigger is meticulous in their naming scheme and runs different stages of the rigging script in proper order.
But what happens when Maya namespaces become involved? As soon as you reference in an asset, every object that makes up that asset gets a namespace prefixed onto it’s name. If you’ve written scripts that require specific names, they break. If your layout files aren’t perfect and the namespace is different between two or more shots (as Maya is wont to append a number, regardless of what you specify), useful tools like character GUIs and the like break and you’re left doing namespace surgery in a text editor.
Weak references sidestep all this by giving you a permanent connection to an object regardless of name changes or namespace prefixes.
A good example is how I’m currently handling cameras in scenes. A decision was made early on, on the current project at work, to name cameras in layout files by the name of the shot / sequence. Normally this isnt a problem, but we’re using a renderer that’s not linked into Maya directly and therefore needs a command line batch exporter written. If all the cameras are named differently, and the camera’s animation has to be baked and exported as well, how do you go about picking the right object?
Using weak references, the problem becomes trivial. You create them as follows:
addAttr -ln "cameraObj" -at "message";
You’ve probably seen attribute connections of type message into IK handles and other things. The message attribute carries no data– that is, it never changes and causes no DAG recalculations. (This is doubly useful because you can connect the message attributes of two objects to message-typed user attributes in a cycle without causing a cycle error– more on that later.) However, the attribute can be used to get the name of the connected object like so:
listConnections object.messageAttribute;
It will return an array of strings. If you rename an object, you can get the object’s current name through the above command.
So where do you store these attributes? For the moment I’m using a trick I saw on the Siggraph ‘08 talk by Blue Sky on procedural rigging: I create non-executing script nodes and store connections on them. In the camera example above, every scene has a master script node. On that node are a few attributes, including its “type” and a .message connection to the render camera. It’s them trivial to find the camera’s name:
string $sel[] = `ls -type "script"`;
for ($s in $sel) {
if (`attributeQuery -node $s -ex "snCamera"`) {
// this should be the one you need
// normally I search for the type, but this is an example
string $conn[] = `listConnections ($s + ".snCamera")`;
// if it's only one connection incoming, then you're done.
print("Camera is named " + ($conn[0]) + "\n");
}
}
This technique can be extended to include all kinds of objects. It can also be very helpful for scripts like character GUIs that need to know what characters are present in a scene, and be able to change the positions of all those controls.
One final note on this for now: In Cinema 4D, every object and tag in a scene can be named the same. Searches for objects or tags by name are often fruitless because of this; if two objects or tags have the same name there’s really no easy way to tell which is which in a COFFEE script. What you can do, however, is create a user data variable that is of the Link type. This allows you to drag and drop an object into that variable’s edit field, and provides a permanent pointer to that object regardless of name. This is very useful in rigging; for example, you can always tell which joints in a leg are control joints, and which are bind joints, by creating links. You can also expose the links in XPresso and use the pointers as if you’d dragged an object onto the XPresso node window.
