Smart Bones

1What a smart bone solves

Rig a plain elbow and bend it hard, and the joint pinches into an ugly crease, the classic rig problem. A smart bone fixes that without you re-posing the elbow every single time it bends. The idea is simple: you record what the joint should look like at its extreme, and tie that correction to how far the bone has rotated. From then on the fix fades in automatically as the joint bends and fades out as it straightens. It is the difference between a rig that only looks right at rest and one that holds up in motion.

2The Actions panel

Everything smart-bone happens in the Actions panel. Open it and you will see tabs across the top: All, Regular, and Smart Bones. There is also a permanent Mainline row, which represents your ordinary main timeline and reads either "recording here" or "main timeline" depending on where you are working. An action is a small self-contained pocket of keyframes that sits outside the main timeline until you want it. A smart bone is a special kind of action wired to a control bone's angle.

3Work in Rig mode first

Smart bones are part of the rest skeleton's behaviour, so build them in Rig mode, the mode that edits the rest skeleton with nothing on the timeline. Before you make a smart bone, make sure the joint you want to fix already has its bones in place and bound to the artwork. You cannot fix a bend that has no bones. Get the elbow, eyelid, or hand rigged and bending (badly is fine, that is what we are here to fix) before you open the Actions panel to start the smart bone.

4Name the action

Start by giving your fix a clear name. Click into the Action name field in the Actions panel and type something you will recognise later, like "Elbow fix L" or "Blink" or "Fist R". Good names matter more than they seem to, because a busy rig can have many smart bones and you will be picking them out of a list. Be specific about left and right, and about the joint. "Fix" alone will haunt you in a month when you have four of them.

5Click Add smart bone

With the name typed, click Add smart bone. This creates a new smart-bone action and drops you into recording it, which is why the Mainline row switches to "recording here", you are now editing inside the action's own little timeline, not the main scene. Note the sibling button Add action makes a plain reusable action instead (a saved wave, say); for a corrective fix you specifically want Add smart bone. A new row appears with a Smart bone kind badge and a key count.

6Choose the control bone

A smart bone needs to know what drives it. On the smart-bone row, open the Control bone select and choose the bone whose rotation should trigger the fix. For an elbow correction, the control bone is the forearm bone: as it rotates, the fix comes in. For a blink you might make a dedicated little control bone that does nothing but drive the eyelid action. If you ever see "Missing bone" here, the control bone was deleted or renamed, and you will need to point it at a real bone again.

7Set the dial range

Now tell the smart bone how much rotation maps to the whole fix, using the Dial range, which reads [min] to [max] deg and defaults to 0 to 90. Those two numbers are the control bone's angle at the start of the action and at the end of it. So 0 to 90 means: at 0 degrees the joint is at rest (no correction), and by 90 degrees the joint is fully bent (full correction applied). Set the range to match the real travel of your joint, for a wide-swinging elbow you might use 0 to 120.

8How the range maps to frames

This is the mental model that makes smart bones click. The action has its own tiny timeline, and the dial range stretches across it: the min angle lines up with the action's first frame, and the max angle lines up with its last frame. The first frame already holds your rest pose (the joint as it sits). So all you have to record is the last frame: the corrected pose at full bend. Everything in between the studio blends smoothly, which is why rotating the control bone later scrubs cleanly through the fix.

9Record the corrected end pose

Move to the last frame of the action, then pose the joint the way it should look at full bend. Use the bone tools to rotate, and if needed nudge points or reshape so the elbow keeps its volume instead of pinching. For a blink, close the eyelid cleanly at the end frame. For a fist, curl the fingers into a tidy fist. Because you are recording inside the action (Mainline still says "recording here"), these keys are stored in the smart bone, not on your scene. This corrected end pose is the entire payload of the fix.

10Test it: rotate the control bone

Time to see it work. Leave the action and, back on the rig, rotate the control bone through its range. As it swings from the min angle toward the max, watch the joint smoothly adopt your correction, the elbow that used to pinch now keeps its shape all the way through the bend. Rotating that control bone effectively scrubs through the action cleanly. If the fix comes in too early or too late, your dial range does not match the joint's real travel; adjust the min and max degrees and test again.

11Classic use: fix a folding elbow or knee

The elbow (and its cousin the knee) is the textbook smart-bone job. A plain rigged elbow pinches into a crease when it folds past a right angle. Make a smart bone whose control bone is the forearm, set the dial range to cover the elbow's full bend (say 0 to 120 degrees), and at the end frame reshape the joint so it keeps a rounded, fleshy volume instead of collapsing. Do the same for knees. Suddenly your character can bend and crouch without their limbs turning to origami.

12Classic use: a blink and a fist

Two more bread-and-butter smart bones. For a blink, make a small dedicated control bone that does nothing else, set a dial range like 0 to 90, and at the end frame close the eyelid shape cleanly over the eye. Now turning that one bone blinks the eye, and you can key it in seconds. A fist works the same way: one control bone, and at the end frame curl all the fingers into a tidy closed hand. Both give you a complex shape change from a single, easy-to-key dial.

13Apply, rename, and manage your actions

Each action row has a few controls to know. The kind badge reads Action or Smart bone so you can tell them apart, and the row shows a live key count. Per-row Apply ("Stamp these keyframes onto the timeline") bakes an action's keys onto your main timeline at the playhead, which is mostly for plain reusable actions rather than smart bones. Double-click a row to rename it. Use the Smart Bones tab to filter down to just your corrections when the list gets long.

14When to reach for smart bones instead of scripting

If you come from a desktop tool, you might expect to write a bit of Lua scripting to automate rig behaviour. FoundryInk has no scripting surface, and it does not need one for this. Smart bones, plain actions, and control bones (bones that drive other bones via Angle from, Scale from, or Position from with a Ratio) cover the automation you would otherwise script. A corrective bend, an automatic follow-through, a one-dial complex shape, these are all handled by the tools on this page.

15What good looks like

A well-made smart bone is invisible: the joint just bends nicely, at every angle, with no pop or crease anywhere in the dial range. Test it by dragging the control bone slowly from min to max and back; the correction should fade in and out with no sudden jumps. Check the extremes especially, a fix that looks great at full bend but pops at the halfway point usually means your end pose is too aggressive. When your elbows, knees, blinks, and fists all behave without a second thought, your rig is ready to animate.

Smart bone questions