They can be categorized as parametric and non-parametric. Meaning that joints will keep relative distance between each other (non-parametric) when changing the driving surface or not (parametric).
I recently compared them. Mainly for performance reasons, but keeping functionality in mind. The screenshots are from simplified setup versions.
- + Fastest non-parametric setup
- - Limited twist control: Only start and end. No center twist control
- - Undesirable behavior when modifying curve-cvs (overshooting joints)
- - Gets exponentially slower when increasing joint count
- + Easy to add dynamics/simulation on curve (because it is only one)
Joints attached to nurbs surface
- + Full local control
- Average speed
Curve / Upcurve
Two curves. One has the skinning joints attached to it. The skinning joints either aim at the next skinning joint (spline-IK like behavior) or are rotated at the average of aiming to next/previous (ribbon like behavior). On the second curve are the up-objects for the aimconstraints. Both curves are deformed in the same way, so that rotation along the joint-curve (twist) will only deform the Upcurve, so there will be a smooth twist driven by the up-objects.
- + Can be non-parametric (motionpath) or parametric (pointOnCurveInfo)
- + Fastest setup when using pointOnCurveInfo (only when using one aimconstrain upwards)
- - Slowest setup when using motion path
- + Full local control between start and end, depending on setup
- Note: motion path rotation should be used carefully (almost never?). Following curve results in more extreme rotation (bigger difference between neighbors) than average of aiming up+down. So deformation will look bad in most cases.
- Note: Motion path should be created manually (createNode motionPath). Because the MEL command (motionPath) creates unnecessary doubleLinear nodes that create cycle warnings and the command is not undoable.
Muscle > Bonus Rigging > Create Muscle Spline...
- (+) Has a lot of functionality by default
- - Joints get oriented to curve (same problem as motion path rotation... The average of aiming on upper and lower joint is probably always better)
- + Nice tangent/curve control attributes from start/mid/end ctrls
- + Seems to be a bit faster than Spline-IK (but it is not non-parametric like the Spline-IK. So they are not fully comparable)
- + Jiggle options (for hair, props, tail, antenna, .. ?)
- Local twist, but limited control over it (flip at 180), because it is calculated in blackbox node.
- Note: For easy to animate curves set small "Tangent Length" value (0.01) on start/end ctrl and increase on the center ctrl to have automated bend when moving center ctrl. Also edit "uValue" attribute on first (from 0.0 to 0.01) and last joint (1.0 to 0.99), so they get oriented to the curve.
- Wiredeformer is parametric
- Curve added as influence to skinCluster (component mode on) is non-parametric
Curve / Upcurve setups are the most flexible and offer the most control, which probably results in the best deformation for most cases. But can be slower depending on the setup.
To get a better idea what "fastest" / "slowest" for these setups mean. Here are the fps numbers when adding eight chains (two for each limb, six joints each) of either type to a base rig that runs at 34 fps
34.0 base rig
28.6 (-5.4) pointoncurve
27.7 (-6.3) splineik
27.2 (-6.8) ribbon
26.7 (-7.3) motionpath
26.3 (-7.7) pointoncurve avrg