Skinning in Maya has changed many times over the years, but never more so than version 2011 and higher. Version 2011 introduced the concept of interactive skinning. With this concept the idea of skinning was changed so that any vertex could have many skinning inputs, all of which could do mathematical wonders together to move the geometry. The trick is to get everything “normalized,” or back into a 0–1 range. If you are using anything prior to 2010, there is a different way to skin. I won’t focus on those versions in this book—sorry.
First, let’s talk about the three different types of weight assigning.
1. Rigid bind: this allows a vertex to only have one joint influence it. It is an old method of skinning, still good for mechanical objects, and calculates extremely fast.
2. Smooth bind: this allows a vertex to have multiple joints influence it. You can paint smooth weights to fi ne tune the vertex/joint assignments. This “painting weights” is usually referred to when someone says they are “skinning.
3. Interactive skin bind: this is the newest method. It is, as it suggests, interactive. The workflow for this is to do an interactive skin bind first (as we have done in previous chapters). This method also allows you to paint weights for finer tuning after completing first rough adjustments with the interactive tool.
Many people use smooth binding. In group projects we have had some bug issues with the interactive skin binding method (though it could have been user error), but it is new and we may have been experiencing other issues as I mentioned before. Save often, and test. Many students who have hated skinning in the past, love the new interactive skin bind tool.
The next topic is that of normalization. This is a new option in post-2011 versions that shows up in a few places: in the smooth bind option window, in the Paint Skin Weights window, and also in the skin node itself. What does it mean? The Normalize Weights options are as follows:
None: This allows you to have weights that are over 1 or less than 0, but does not normalize them. It can cause issues with the rig according to the help documentation. I don’t use it. If you use it, email me and tell me why: firstname.lastname@example.org.
Interactive : When interactive is turned on skinning behaves as it did in earlier versions. All skinning weights are adjusted so that they only equal 1. (Don’t confuse the normalization method of interactive with interactive skin bind mode! Oh, the semantics issues.)
Post: Th is is the new mode (and is on by default) in which the actual normalization (where the numbers really are 0–1) happens when you deform the mesh, not when you paint the weights. So, any tricks you used to have about setting one weight value to 1, thus automatically setting all other weights to 0 will no longer work. You have to paint weights to 1 for one joint and then physically paint 0 as well for all of the other joints. Interactive binding needs this type of math in order to work.
I’m not usually one to go through all of the options, and usually show it at work instead. However, with skinning, it is easier to talk about the options so you are well equipped when you tackle your particular skinning problems.
We have seen what type of binding methods there are, and how the numbers are normalized. The last thing to keep in mind is the bind pose. The bind pose is the “neutral” state of the geometry vs. the skeleton system. Think of it as the “zero, zero, zero” point which all things should come back to. In the latest versions of Maya you are allowed to have multiple neutral or bind poses. This is super-useful for when you have multiple pieces of geometry being bound to the same skeleton. There is an option titled Allow Multiple Bind Poses in the skinning options box. If turned OFF, all geometry must be bound at the same time. If you want to bind the separate pieces at separate times, turn the option ON.
To help with the chaos of skinning, you will find that the option titled Maintain Max Influxes will help limit the amount of joints that can influence a vertex. If you set it to 1, that would equal rigid binding. If you set it to 2, that’s a little more smooth. If set to 3, there’s more influence and that can help you simulate skin better. If set to 5 or higher, you have made yourself a skinning nightmare: only do that if you really have a reason. The gotcha built in is that if you set the max influence to a low number, say 2, and want to paint in a third joint, the system will keep the influence at 2 by removing influence from an existing joint and giving it to the third joint. In this way Maya maintains a total of two joints influencing the vertex. This can be surprising when skinning at 3 am in the morning and not understanding why your skinning assignment keeps subtracting itself from joints you had working. In the Paint Skin Weights window that we’ll see in a little bit, you can click on the button “hold weights” to keep the weights on a joint.
Extra effort: I once went on an “I hate skinning” rampage and created a skinless low-/high-res leg. It was a low-res leg cut up and parent-constrained to a skeleton. The cut up a proxy leg, wrap deformed a low-res leg, which had a poly smooth on it to a high res. No skinning. See file LowRes_HighRes_wToe_v3.mb for that handy work.
Excerpt from Rig it Right! By Tina O’Hailey © 2014 Taylor & Francis Group All Rights Reserved.