StrikePose FABRIK chain - handling bone twists properly!

Added: 2026-05-27

[00:00:01]Hello. I have another devlog for you. Uh so, I've been working on the IK, making sure the base uh fabric chains are working properly. Uh this is uh our character's chain. We can see there's also like new bone visuals that actually look like bones.

[00:00:16]Um and there's an important reason for why we wanted these visuals too is because it now supports orientations.

[00:00:22]So, I'm just going to get a free bone too. Uh I haven't uh added an effector yet, but once I add an effector, um I can start like moving it around, and what we'll see this affects the bone orientations. You can see the cyan arrow, that's the forward direction, that's mostly given by the chain. And then there's the uh yellow arrow, which represents the bone up direction. Uh this represents the internal rotations, which are then translated into the actual rotation of the bone.

[00:00:50]Um and this is sort of computed automatically. Some of them, like you see like just by quirk of the rotation, like it ends up like the up direction ends up like being to the side, and this one like ends up being this way. Uh but generally shouldn't matter. Um it's just kind of like whatever the system chose for internal representation. And for testing, this actually kind of helps.

[00:01:09]You can also see the numbers, that's the dot product between these two, that should always be zero because these are supposed to be perpendicular to each other. Um so, now you can see like I can move this around, like um it does compute the orientations, which is um you know, the forward, but what it also does uh is it now computes the uh up axis. So, if I start twisting this one, it's a little bit hard to do, you can see that the whole chain is essentially going to follow that twist.

[00:01:39]Um the way it kind of propagates through the chain is like uh it essentially goes from like zero here, and then like this one gets least influence more and more and more until it's like the full twist.

[00:01:49]If there's multiple effectors, it's also going to combine those as well.

[00:01:53]Um but here for this test, I have just like this one.

[00:01:57]Uh but you can see like it it like when when it twists like this um it's propagating correctly through the chain. It's like a little bit of a figure out make sure like it's kind of translated properly.

[00:02:09]Uh though it kind of works is like um relatively simple on paper. It was like a few issues that kind of had to be dealt with so like that rotations are handled properly. But essentially what it does, um let me move to the effector and let's say I did again. Just going to like reinitialize.

[00:02:26]Um what it essentially does in the background, um it takes the up direction. Then from the effector, it'll figure out um what's the rotation around this axis and then like it will transplant that like twist. Uh initially like it has the original vector and the like new twisted up vector. It transplants it to the previous bone, computes like the twist for this bone, transplants it back transplants it back and just kind of propagates through the chain so it can handle like you know over the chain for example like this.

[00:03:00]Uh I'm going to like try to curl it a bit.

[00:03:03]If it's like this, it's still um still properly translating the rotation it's propagating through the chain.

[00:03:12]Um Also like I can take this thing so I have like you know we have the new functionality to sort of auto rig it to the few tweaks to this as well. So I can take this bone chain. I have the mesh for this thing. So I'll just grab this and repair secondary.

[00:03:28]Um they essentially auto rigs this thing. And now if I do this, you see the mesh is um the mesh is being transformed by the bones.

[00:03:41]And if I twist it, uh you see you see if I twist the mesh, this is affecting the rotation as well.

[00:03:48]Maybe a little bit hard to see. Um Let me see if I can actually change.

[00:03:53]Give it a better material. I don't know what would be very well visible. Let's try this one.

[00:03:59]This might be too glowy.

[00:04:01]Uh Oops.

[00:04:05]Let's try this thing, maybe.

[00:04:08]I don't think that's too It kind of shows you the texture a bit. So, you can see like how it sort of propagates. I don't want to also expose like more controls for for this, so you can um so you can adjust how much you know, it kind of like goes. So, like you can, for example, have it like have the effect that if you twist it, it's going to twist the whole thing, or maybe like it's going to fall off very quickly. It's going to twist like this end bit a little bit more control. Now, it's like essentially completely even.

[00:04:40]Uh but, I want to expose more controls.

[00:04:41]But, you can kind of see like how when I twist this, it's properly propagating.

[00:04:47]Uh the base algorithm seems to be working, so I can kind of move on to the next phases.

[00:04:52]I might actually end up like I need to do a few more tests, but I might actually end up releasing this very soon as a basic like fabric chain.

[00:05:03]Uh let's see. Where's the chain?

[00:05:05]Uh there's the rig. So, you'll get like, you know, this component, uh and you'll be able to play with it. Uh that way you can also get some feedback.

[00:05:13]Make sure like it kind of works for lots of like edge cases. You can play, you know, with the with this tool. I need to like polish up a few things. And again, kind of continue working on the more parts of it, like adding constraints.

[00:05:23]And once this is kind of like, you know, I'm confident this is working pretty well, because this is a building block, I'm going to build a system for, you know, full body like avatars like this, which requires a lot of extra behaviors.

[00:05:36]Um Just kind of like make sure they handle like properly. Like, you know, limbs bend the way limbs do. And it's handling like, you know, different things like, right?

[00:05:45]But, it's going to use this kind of system in the background, you know, to sort of like do stuff. Like for example, like you know, this thing, this chain, this could be your spine and oh, it twisted it all around.

[00:05:58]Um And you know, like like this could be your head and like as you twist around, you know, you want your spine, your body like you know, you want like a head bone to twist, you want neck to twist and maybe like the spine twist a bit less.

[00:06:10]Um So, there's going to be Um there's going to be like lots of like, you know, kind of use cases.

[00:06:18]And also you can just kind of use it for, you know, for building whatever or just messing around. It can be like useful for robots, you know, chains, like you know, people want to make like chains, wires, like stuff that needs to connect to things.

[00:06:29]Um so, hopefully like you'll have like a lots of fun with it.

[00:06:33]Anyway, thank you for watching and I hope maybe you'll be able to play with this very soon. I just need to clean up a few things and make sure it's okay and in okay state and um I can release the initial version. Hopefully, we'll see.

[00:06:47]No promises.

[00:06:48]But we'll see. Woah.