Smart Bones for Vectors
A plain bind gets you most of the way, but the last stretch of a bend, the very fold of an elbow or knee, often creases or thins no matter how you Flexi Bind it. A smart bone fixes that for good by recording the correct shape and playing it back automatically as the joint moves. This tutorial walks the whole smart-bone flow on vector art: the Actions panel, Add smart bone, a Control bone, a Dial range, and recording the corrected pose.
1What a smart bone actually does
A smart bone ties a whole corrected pose to the rotation of one bone. You rotate a chosen Control bone, and as it turns through a range of degrees, the rig blends smoothly into a pose you recorded earlier. It works like a dial: at zero degrees the joint is relaxed, at ninety it is fully folded and clean, and every angle in between is interpolated for you. It is the difference between an elbow that crimps and one that folds like a real arm.
2The problem it solves
Flexi Bind gets a vector elbow to bend as one shape, but push the fold to its extreme and the crease can still pinch, thin out, or bulge oddly. You could fix it by hand every single time you pose that arm, but that is a chore you would repeat in every shot. A smart bone lets you fix it once, record that correction, and have it apply forever whenever the elbow closes. Build the correction one time, reap it in every scene.
3Start in Rig mode
Smart bones are built on the rest rig, so work on the vector surface in Rig mode. Click Rig at the top; the banner confirms Rig mode, where edits shape the skeleton and nothing lands on the main timeline. This keeps your smart-bone setup separate from any animation you make later. You will do the recording inside a dedicated action, not on the main timeline, so nothing here disturbs a shot.
4Open the Actions panel
Smart bones are managed in the Actions panel. Open it and you will see tabs for All, Regular, and Smart Bones, plus a permanent Mainline row that represents your main timeline and shows whether you are recording there or into an action. Each action row carries a kind badge (Action or Smart bone) and a key count. This panel manages every recorded motion, so get comfortable here.
5Add a smart bone
To create one, type a clear label into the Action name field, something like Elbow Fix or Left Knee, then click Add smart bone. A new row appears with the Smart bone badge, and recording now points into that action rather than the Mainline. Naming it well matters: a character often ends up with several smart bones, and Left Elbow Fix beats Action 7 every time you go looking.
6Set the Control bone
A smart bone needs to know which bone drives it. In the smart-bone row, set the Control bone select to the joint whose rotation should trigger the correction, for an elbow fix, that is the forearm bone that bends. From now on, rotating that control bone is what scrubs through your recorded pose. If the select ever shows Missing bone, the chosen bone was renamed or deleted, so pick the current one again.
7Set the Dial range
Now define how far the control bone travels to play the whole correction, using the Dial range, which reads as a min value to a max value in deg and defaults to 0 to 90. For an elbow that goes from straight to fully folded, 0 to 90 is a natural fit: at 0 degrees the arm is relaxed, at 90 it is fully bent with your recorded correction in full effect. Match the range to the real travel of that joint.
8Record the corrected pose
With the control bone and dial range set, rotate the control bone to the extreme of the range (say the fully folded 90 degrees) and then sculpt the joint until it looks right: nudge the points near the crease, adjust the curves, fix any pinch or bulge. Because the recorder is aimed at your smart-bone action, that corrected shape is captured as the pose for that angle. Rotate back toward zero and the rig blends out of the correction on its own.
9Fix a clean elbow bend
The classic use is the elbow. With the arm Flexi Bound, add a smart bone named for it, set the Control bone to the forearm, the Dial range to 0 to 90, rotate the forearm to full fold, and reshape the inner crease so it stays full and rounded instead of pinching. Record it. Now every time that arm closes, the elbow folds like a real elbow, no hand-fixing, no crimp. This single fix instantly lifts the quality of every arm move.
10Fix a clean knee bend
Knees get the same treatment. Add a smart bone, set the Control bone to the lower-leg bone, set a Dial range that matches how far the knee bends (0 to 90 is a good start, widen it for a deep crouch), rotate to the folded extreme, and reshape the back of the knee and the front of the thigh so they meet cleanly. Record. A character can now squat, kneel, and run without its knees folding into origami.
11Blinks with a smart bone
Smart bones are not only for limbs. Give the eyelid its own control bone, add a smart bone with that as the Control bone, and set a Dial range from open to closed. Rotate the control bone to the closed end and shape the eyelid points so they sweep down over the eye in a natural arc, then record. Now a single dial turns a full blink, and you can trigger it any time the character needs to blink without re-drawing eyelids.
12Mouth shapes with a smart bone
You can drive a mouth's opening with a smart bone too: a control bone that rotates from a closed mouth to a wide open one, with the lip points reshaped at the open extreme and recorded. That gives you a smooth open-and-close on a single dial. For full lip-sync, where you swap distinct phoneme shapes rather than blend one open-close, FoundryInk also has a dedicated Switch / Lip-sync panel that shows one drawn mouth sub-layer at a time; use a smart bone for jaw openness and the Switch panel for the actual phoneme shapes.
13Test the dial
Check your smart bone before you rely on it. Switch the vector surface to Test mode and rotate the control bone slowly across its whole Dial range, watching the joint sweep from relaxed to corrected. Nothing in Test is saved, so scrub it back and forth freely. If the correction pops in too suddenly, your dial range is too narrow; if the fold still pinches at the extreme, go back to Rig and reshape the recorded pose a little more.
14Stack several smart bones
A finished character carries a small orchestra of smart bones: both elbows, both knees, a blink, maybe a mouth and a wrist. Keep them tidy in the Actions panel under the Smart Bones tab, each clearly named and driven by its own Control bone. They do not interfere with each other, because each responds only to its own control bone's rotation. Build them up over a session and your rig becomes much easier to pose.
Build a smart bone
Action, control bone, dial range, a recorded fix. Links into the app. Saved on this device.
Smart bone questions
It is a dial that plays a recorded pose as one bone rotates. You pick a Control bone and a Dial range in degrees, then record a corrected shape at the extreme. Turning the control bone through its range blends the rig into that pose automatically, so a joint folds cleanly without hand-fixing every time.
When the extreme of a vector bend misbehaves, an elbow or knee that pinches or bulges at full fold, even after Flexi Bind. If your bind is smooth all the way through its range, you may not need a smart bone. Reach for one to correct the fold you would otherwise fix by hand in every shot.
In Rig mode, open the Actions panel, type a name into the Action name field, and click Add smart bone. A new row appears with the Smart bone badge, and recording now points into that action instead of the Mainline. Then set its Control bone and Dial range.
It maps the control bone's rotation to your recorded pose. The default 0 to 90 degrees means the correction plays fully as the bone swings from zero to ninety. Match it to the joint's real travel, wider for a deep knee bend, so the fix lands at the right angle instead of early or late.
Rotate the control bone to the extreme of the dial range, then reshape the joint, nudging the crease points and curves until it looks right. With the recorder aimed at your smart-bone action, that shape is captured for that angle. The rig interpolates every angle between relaxed and corrected, so you only sculpt the extreme.
Yes. Give the eyelid or jaw its own control bone, add a smart bone, set a dial range from open to closed, reshape at the closed extreme, and record. For distinct phoneme shapes in full lip-sync, pair a smart bone for jaw openness with the dedicated Switch / Lip-sync panel that swaps drawn mouth shapes.
The Dial range is probably too narrow, so the whole pose crams into a few degrees. Widen the range to match the joint's real swing, then scrub the control bone across it in Test mode to confirm the fix eases in smoothly rather than snapping on.
The bone you assigned was renamed or deleted after you set it. Open the smart-bone row and pick the current bone in the Control bone select again. Naming bones clearly before you build smart bones helps you avoid this in the first place.
Your joints fold flawlessly now
Elbows and knees that fold like the real thing, blinks and mouths on a single dial, every fix recorded once and reused forever. Your vector rig just got a whole lot more professional. Next, we pull everything together and rig a complete character from backbone to fingertips, the full skeleton with limits, IK, and dynamics.