Breakdowns and autokeying


Ah, the life of an amateur programmer, so very full of backtracking because you did it wrong the first time (actually, as I understand it that might be all programmers). Now that I’ve had time to do the necessary refactoring, the ephemeral rig system supports being used to make breakdowns.

Doing breakdowns was a big part of the reason I needed an ephemeral rig system in the first place. The system we used on the Monkey test and New Pioneers, which we called Phantom Tools, had some limited ephemeral behavior built into manipulators. That made it possible to pose with a “broken” rig without having to move each part individually, though it was nowhere near as flexible as full ephemeral rigging, but it’s biggest limitation was that, being built into manipulators, it was completely separate from our breakdown tool. So breakdowns between poses that were significantly different from each other would tend to collapse, as there was no rig behavior in place while the breakdown was created.

The ephemeral rig graph, however, can now be run in either “control” or “breakdown” mode. In control mode the graph is activated by a callback placed on the node the control the user is currently manipulating, the same as I’ve been showing in the last few months of posts. In “breakdown” mode, on the other hand, the callback is instead placed on a slider object that can be used to slide the character between adjacent poses.

A note here about UI: so far, everything I’ve done for the UI on the ephemeral rig system (in-context menu aside) has been done with actual meshes in the scene. This is a really stupid way to do UI, but I ended up being backed into doing it that way to avoid even bigger headaches.

The selection map, for instance, could in theory be easily replaced by any one of many commercially available or free picker tools, and it was certainly my original plan to use mgPicker or a similar tool rather than continuing to roll my own. The problem with this is that I needed to do some pretty nonstandard stuff. For example, I wanted to be able to add stretch meters that would change color as the character was manipulated to give the user insight into how far off-model they were pulling a pose. It was necessary for this to update as the scene evaluated to be a useful indicator, and the easiest way to do that turned out to be just making it a part of the scene. No doubt I could have rolled my own QT-based interface that would have accomplished that goal, but that’s more work then I wanted to put into the picker.

The breakdown slider was originally going to be an exception to this: it’s easy enough to make a slider in a custom window that runs a command when dragged, and it’s much cleaner then having something that’s actually hanging around in the scene. The problem with this turned out to be related to how I have been running the graph and doing auto-keying.

As you may or may not recall from my earlier experiments, I’m handling auto-keying rather differently from the conventional Maya auto-keying. I want to treat poses as if they were drawings in a 2D animation package like Toon Boom, meaning that instead of keys on specific frames you have poses with duration. So if I go modify the pose on a frame that doesn’t happen to be it’s first frame (where the keyframe actually is), that doesn’t matter. I’m still modifying that pose.

Since I’m not relying on Maya’s auto-keying, I’ve needed to implement my own, and that means knowing when the user has just done something that requires setting another key. To do this, I have a global option called “doAutoKey”, and every time the ephemeral callback fires it sets this variable to True.


A brief note on coding style here--setting values by using get() and set() methods isn't considered to be good coding practice in Python and I really shouldn't be using them. The only reason they're present here is that I have a lot of global options I need to set and just setting them directly introduces problems with scope that I didn't know how to fix when I wrote this bit. So I ended up falling back on that particular practice because I was already familiar with it from pyMEL (which actually has kind of a reasonable excuse for that behavior that doesn't apply here) and I was in a hurry. It's not ideal and at some point I'm going to go back and purge it from my code.

Then when the user stops, I have another callback firing on each idle cycle. If the autokey option is set, it performs the autokey, and then turns it off so that subsequent idle cycles will not autokey again until the user activates it by interacting with the rig. In the old system I implemented this with a scriptJob, which was ugly. The new system just uses an idle callback, which is much cleaner.


In addition to performing autokey if necessary, the idle callback also builds the ephemeral graph and callback if it doesn't already exist. A callback on time change kills the graph:


...and then, when the user has alighted on a frame, the next idle cycle triggers the idle callback, setting the current frame to be the stored frame that the eph callback function will use to determine if it should run, and then building the graph again for this new frame. (Building the graph is one of the things that the setManipModes() function does, in addition to setting the manipulation modes for the character's nodes based on the current selection.) Basically it's the same concept as my old system, except it doesn't have to check the scene for some attribute that sets the current pose, and instead just handles it all in code (and all scriptJobs have been replaced by callbacks). It's vastly simpler and less brittle.

This worked great for the ephemeral rig interaction, but when I tried to drive it with a GUI slider it fell apart completely. Turns out that interacting with a GUI slider does not prevent Maya from firing idle callbacks, causing the system to interrupt the users interaction to set keys! This actually makes sense because unlike a manipulator, which interacts directly with Maya’s graph, the slider just executes a command whenever the user changes its value, and there’s no particular reason to assume it would execute those commands fast enough that Maya wouldn’t have time to do idle cycles in-between (and indeed, if you use the Shape Editor or any other built-in slider-based interface in Maya, you will very much see idle cycles happening all the time during manipulation).

With some additional work it would have been possible to get around this. For instance, I could have created the slider in QT and turned the idle callback on and off based on mouse events. But since I already had a whole bunch of machinery set up to have things work correctly when being driven by a transform node for the ephemeral interaction, I decided it was just easier (for this prototype, at any rate) to use the system I already had set up and make the slider an object in the scene.

Building the graph in breakdown mode has a few differences from building it in control mode. Unlike building a control graph, which has a specific point where you can start tracing the graph from (the node the user is manipulating), the breakdown graph does not necessarily have a clear point of entry, or may have multiple points (as it’s possible to breakdown specific nodes while the rest of the body reacts normally). In fact, it's possible to have multiple "islands" in the breakdown graph that do not even relate to each other.

The ephemeral graph building logic turned out to be surprisingly robust here--it’s possible to throw pretty much any set of nodes, connected or otherwise, at the graph, and they’ll organize themselves appropriately and establish which ones should be driving which. The main thing I had to add was a new type of constraint, a “NoConstraint,” so that nodes that do not have any drivers can still operate in the graph. This is never needed when building the control graph, because only the critical path is ever built, ensuring that the only node that does not have an input is the control node being manipulated by the user.


Creating the actual breakdown behavior depended on adding the ability to each node to know what it’s past and future matrices are, in addition to it’s current matrix. Because all the nodes are in world space (or at least in the same space) this wasn’t too difficult, as I can get their “world space” values right off their keyframes. Maya doesn’t usually evaluate the graph on frames other then the current frame, but you can ask it for the whole timeline’s worth of keyframes at any time. This is another big advantage of keeping all the control rig behavior outside the Maya graph--you can treat the actual world space location of each control throughout the shot as known information you can look at at any time, not something that must be evaluated before it can be known.

Here's how I look at the keys associated with a given node and figures out which ones represent the current, past, and future poses from the current time:


Note that this is a bit more complex then just using the findKeyframe command to find next or previous keys, because, since I'm treating them as poses rather then keyframes, the "current pose" may or may not actually be on the current frame.

Finally, a word to the wise: if you ask om2 what a transform node’s rotate values are, it will quite naturally and correctly give you radians, as God intended. But the keyframes? They are probably in degrees, also called the Devil’s Unit. Mark this warning well, lest you be deceived into feeding this unholy unit to a function designed to accept only pure and immaculate radians. It’s extremely confusing.