For a human spine, if the upper torso (rib cage) rotates on Y, the vertebrae on the lumbar region barely rotate, or not at all in the case of the first one or two right on top of the sacrum. I just did a test with a “torso” piece of geometry and it seems like I was wrong: “no normals” for the follow curve solver is not the best option, because as I said earlier, vertebrae have to rotate progressively. I'm sure we'll know more as we encounter various situations in real projects, as these tests, although do provide some insights, are rather dry when compared to what you actually need when you start rigging some real organic or mech, so we, or at least me, won't know for sure what's the best approach beforehand. In this situation, from what I've managed to test thus far, curve normals are to be avoided.
#HOUDINI FX CLICK IK SOLVER EXPORT MANUAL#
So, trying to extract the essence from here, I'd say that in general when you're rigging this curve in order to be used as a spine, you don't necessarily need to have manual control over bones' orientation (vertebrae twist progressively only a bit in relation to their upper and lower neighbor), but you need to have them stable as in keep their initial rotation.
Another, is this curve rig works well in some cases, but for realistic humanoids or any creature that is not upright (spine on horizontal) there's a needed for a bit more flexibility, therefore a spine has to have two ctrl nulls (a la XSI) which is better for this case as it allows independent twisting at chest and hip level.
That's a very nice setup! Two things: I've managed to brake it by moving on YZ the locked on X null (swing if you can), just a little, not too extreme.
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