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Monday, November 13, 2017

The Mighty Message Attribute

I recently had a discussion about storing relationships in Maya, and hadn’t realized the role of the message attribute wasn’t this universally cherished thing. In a previous post entitled ‘Don’t use string paths‘, I outlined why storing string paths is the devil’s work, but in that post I talked more about the API (MDagPath()), and didn’t really talk about the easiest way to free yourself from referencing maya nodes by strings: The message attribute.

For quite some time I have advocated storing relationships with message attrs. At the Maya SIGGRAPH User Event, when they asked me to speak about our modular rigging system, I kind of detailed how we leveraged those at Crytek in CryPed.

msg

I am not quite sure when I started using message attrs to convey relationships, I’m no brainiac, it could have been after seeing this 2003 post from Jason Schleifer on CGTalk:

image

Or maybe I read it in the Maya docs (unlikely):

“Message attributes only exist to formally declare relationships between nodes. By connecting two nodes via message attributes, a relationship between those nodes is expressed.”

So why does Maya use this, and why should I?

As you read in the docs above, when Maya wants to declare a relationship between a camera and image plane, they do so with a message attribute that connects them. This is important because this bond won’t be broken if the plane or it’s parent is renamed. As soon as you store the string path to a node in the DAG, that data is already stale.  It’s no longer valid.  When you query a message attribute, Maya returns the item, it’s DAG path will be valid, regardless of hierarchy or name changes.

Jason’s example above is maybe the most simple, in my image (a decade later) you can see the messages declaring many relationships abstracting the character at three main levels of interface, Character, ChatacterPart and RigPart. I talked about the basic ideas here in a 2013 post about object oriented python in Maya.

Though Rob vigorously disagreed in the comments there, I am still doing this today.  Here’s an example from the facial code we released in EPIC’s ARTv1 rigging tools some time ago. The face is abstracted on two levels, the ‘face’ and the ‘mask’, here I am only displaying the message connecting them:

wiring

By using properties as described in that previous blog post, below I am accessing the system, creating a face instance, walking down the message connection to the mask node, and then asking it for the attach locations. It’s giving me these transforms, by querying the DAG, live:

msg

So, that property looks like this:

    @property
    def attachLocations(self):
        return cmds.listConnections(self.node + '.attachLocations')
    @attachLocations.setter
    def attachLocations(self, locs):
        for loc in locs:
            utils.msgConnect(self.node + '.attachLocations', loc + '.maskNode')

Setting the attach locations through python would look like this, and it would rebuild the message attrs:

face.mask.attachLoactions = ['myLoc1', 'myLoc2']

Working like this, you have to think hard about what a rigger would want to access at what level and expose what’s needed. But in the end, as you see, through python, you have access to everything you need, and none of the data is stale.

How and when to use strings

There are times when the only way you can store a relationship is by using a string in some fashion. Here are some situations and how I have handled them in the past, feel free to leave a comment with your experiences.

  • Maya can’t store a relationship to something that doesn’t exist (has been deleted). It can’t store a relationship when it’s not open. In these situations, instead of storing the name in an attr, I stamp the two nodes with a string attr to store the relationship, then you query the world for another node with a matching stamped attr.
  • Many times you need to feed your class an initial interface node to build/wrap. Instead of feeding it a string name, you can query the world for node type, in the Ryse example above, the rigging and animation tools could query cmds.ls(type=’CryCharacter’), this would return all characters in the scene. This means all rigging and animation tools needed a common ‘working character’ combobox at the top to define the character the tool is operating on. If you don’t have a node type, you can use a special string attr to query for.
  • Sometimes you’re like saving joint names to serialize skinning data or something. You can use message attrs to play it safe here as well. Some pseudocode: For character in characters, if character identifier matches file on disk, for mesh in character.meshes if mesh in file skin it. For joint in character.joints if in file, add them to the skincluster, etc. Here you’re validating all your serialized string data against your class which is traversing the DAG live.
  • Message attrs can get SLOW if you’re tracking thousands of items, you should only be tracking important things you would want later. In CryPed, when we wanted to track all nodes that were created when a module was built, we would stamp them all with a string attr that was the function name that built the module. To track this kind of data HarryZ at Crytek had the pragmatic idea of just doing a global ls of the world when a buildout started and then one at the end and boolean them out, this caught all the intermediate and utility nodes and everything generated by the rigging code.
posted by Chris at 6:10 AM  

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