View Full Version : Forearm twist

08-14-2011, 09:27 AM
I've been looking at forearm twist rigging lately. How complex does this need to be? How many joints do you need to have? At the moment I'm particularly working with figuring out the best way to do the skinning; the rigging control can come later, I just want to get a good method down for good deformation.

The most successful method I've seen is one that uses 4 twist joints along the length of the forearm; the weights are just about perfect by default and the twist is nice and smooth. But I notice that a lot of game characters only seem to have one twist joint. The HumanIK skeleton generator in Maya2012 also generates only one twist joint. So is that actually sufficient?

thanks for any tips!

08-15-2011, 12:03 PM
One twist joint is usually enough for game characters especially if it's 3rd person. If it were first person and you see a lot of the arms (e.g. Mirror's Edge or something like that) you might want to go up to two or three in rare instances.

The way the engine handles the skinning also has a big influence on what is most appropriate. For example, some game engines now use "dual quaternion" style skinning which preserves volume very well while joints twist counter to each other. In that case you can most likely get away with only one twist joint even if it's going to be seen closely.

Another factor to take into account is the data/processing cost. For most engines the rotation data from the joints needs to be baked (so you get one key per frame) which means having 4 additional joints per arm can ultimately mean a significant increase in the amount of memory the animations end up taking up.

Rick Stirling
08-15-2011, 12:10 PM
1 or 2 is plenty.

Mark Dygert
08-15-2011, 12:14 PM
yep 1-2 and it really depends on the required motions. 3 is almost over kill, 4 is just throwing more joints at a problem that probably exists some place else, either in the edge flow or the skinning.

08-15-2011, 01:03 PM
If you use dual quaternion skinning you don't even need one.

Rick Stirling
08-15-2011, 01:45 PM
DQ is not a magic bullet - it will help candy wrapper issues when twisting bones, but creates ballooning in other areas. A linear/dq blend is great however, but that's an extra rendering cost. Correctional twist deformers are one of the cheapest solutions.

08-18-2011, 09:13 AM
If you use dual quaternion skinning you don't even need one.

Even with DQ, you still need twist bones.