Mirror skin weights problem

Is mirroring skin weights not working correctly?  Are bones that are influencing the skinCluster getting skipped when mirrored?  Are the bones around the skipped bone(s) getting arbitrary weights?  Have you tried every script/export/hack you can think of to fix this?  Hopefully you got here first..  Here’s the fix! [using Maya 2011, but it might work in previous versions]

1. Select your skin
2. Open the Mirror Skin Weights option box
3. Make sure your mirror axis and direction are correct
4. Set Influence Association 1 to: One to One
5. Mirror the weights

Why this worked for me:
In my hierarchy, I had two joint chains that started in the same position, but weren’t parented to each other (they were constrained).  With the default Influence Associate setting at Closest Point, Maya didn’t know what to do with that situation.  So it found the first joint and put weights on it, skipped the second joint, and then distributed the skipped weights on the surrounding joints.  In retrospect, this makes total sense, but at the time I wasn’t aware of any other options (even using a script that saved weights per vertex still had issues).  In the Maya help file, it says that the One to One option “Associates the joints in situations when the skinned objects have identical skeleton hierarchies.”  This was the case with my rig; it had an identical hierarchy on either side of the body.  When I switched to that option, Maya knew exactly what to do and where to mirror the weights.  If you’re working with identical hierarchies, you might as well just use this option from here on out.

Now if only I hadn’t spent 4 hours trying various hacks and scouring the internet for solutions…