Pose constraints

What it is#

A constraint stack is an IPoseProcessor: it runs after the layers have mixed and before skinning, and it corrects the pose towards a set of goals. There are four kinds of goal and there is not a fifth:

| Kind | Asks for | Measured in | |---|---|---| | Position | a joint, or a point on one, is at a place | metres | | Orientation | a joint's rotation aligns with one | radians | | Aim | an axis on a joint points at something | radians | | Distance | two joints are a certain distance apart | metres |

Each has a region form — anywhere on this shelf, within five degrees — and an additive form, an offset from wherever the animation put you. Absolute goals average; additive goals sum on top. Averaging two recoils would make them weaker than one, which is the opposite of what additive means.

What it is for#

Anything where the pose has to agree with the world: a hand on a rail, a foot on a slope, a chin clear of a shoulder, two hands on a rifle, a gun pointed where the player is aiming. The claim doc 34 makes is narrower and more useful than "IK": one authored contact, played on bodies of visibly different proportions, lands on the same spot on all of them.

You do not want it for the shape of a motion. A constraint corrects a pose; it does not author one.

Using it#

A stack belongs to a character and holds its goals. Build one against the rig, hand it the proxy shapes if any goal names a surface, and add it to the animator's processors — it runs after the layers have mixed and before skinning, which is the only place a correction can see the pose it is correcting:

A goal names the joint it is about, the chain it may bend to get there, and where *there* is. Adding one returns a handle, and the handle is how it goes away again:

Disposing eases the goal out rather than dropping it, for the same reason a frame that fails to resolve does: a hand that stops being asked for is a hand that lets go, not one that teleports.

Most goals are not written in code at all. They are marked up on the clip — begin, end, easeIn, easeOut, a priority by name — and the stack plays them from the clip's own tags, which is what the rest of this page is about.

Where a goal is#

A goal is expressed in an IConstraintFrame, and resolving is allowed to fail — a prop that has not spawned, a socket nothing is bound to. Failing eases the goal out rather than snapping it.

| Frame | Where | |---|---| | WorldFrame | a fixed place | | JointFrame | one of the character's own joints | | EntityFrame | whatever is bound to a named slot | | SocketFrame | a named attachment point on whatever is in a slot | | ProvidedFrame | whatever the game wrote this frame — a raycast, a navmesh query | | SurfaceFrame | a place on the surface of one of the character's own proxy shapes | | AttachmentFrame | one of the character's own sockets, after it has been adapted | | TrajectoryFrame | any of the above, moving over the clip | | ScreenFrame | where a camera would have to be for a subject to land there in the picture |

Proxy shapes, and why a contact is portable#

A SurfaceFrame names a shape and a normalised coordinate on it — a fraction round and along, not a distance. That is what makes one authored contact fit any body: the coordinate resolves against whatever size that character's belly actually is.

The shapes themselves are a .vxproxyshapes beside the model: seven primitives, each parented to a joint, loaded with ProxyShapeCache.Get and baked once per rig. The editor for one has a viewport of its own — the body and its shapes drawn, and the selected shape moved, turned and resized with the same gizmo an entity uses.

⚠ rig: is an authoring-time reference and the bake ignores it. A set is worn by whichever body loads it — that is the whole reason a shape names its joint instead of indexing it — but the *editor* cannot pose one without a skeleton, and a file that does not say which model it was drawn against leaves every panel showing it guessing. Filling it in is what makes the viewport, the coarse generator and the audit work; the row is at the top of the panel, so a new set's first message is which reference to fill in.

A .vxshapevocab declares the names a project's sets may use, which turns "somebody called it palm-l on this body and left-palm on that one" from a bug nobody can see into an import error. It has an editor of its own: names, tags and body plans in one list, with a class member that names an undeclared shape marked as you type it rather than at the next build.

Proposing contacts#

Propose Contacts in the clip editor plays the clip against the scene it was marked up in — the authoringContext: a .vxanim names — and lists the contacts it noticed, most confident first. Nothing is applied: each row has its own Add, and there is no accept-all, because the failure mode of proximity heuristics is confident nonsense and accept-all is the button everybody presses.

What it watches is the body's own proxy shapes. Nothing else in a project says which joints reach for things, and a shape is a named point somebody cared enough to write down — a palm, a fingertip, a heel — so adding one to the set adds it to the pass. Two consequences worth knowing:

• A shape on the root is not watched. Once the scene has been folded in, the root carries every
• prop the scene put there; a mug looking for contacts with the character is the pass run backwards.
• The chain root is two joints above the effector. Wrist, elbow, shoulder is the two-bone chain
• the solver is written for. The immediate parent would propose constraints that can only bend one
• joint; the skeleton root would propose ones that move the whole body to reach a mug.

So the button needs three files to be in place: the clip names a sequence, the sequence names a subject whose asset is the model, and some .vxproxyshapes names that same model in its rig:. Miss any one and it says which — it will not guess a body.

Priority is a name#

priority: contact, not priority: 400. A raw integer has no meaning across a project, and two authors will pick 70 and 700 for the same intent. A .vxpriorities declares the ladder:

contact+1 is a legal sub-step, clamped to ±99 — which is why the importer warns when two rungs are less than a hundred apart.

How conflicts resolve#

Goals are grouped by the chain they move, because two goals that move the same joints have to agree with each other and two that do not never meet. Within a group the shipped arbiter averages the absolute goals by weight, sums the additive ones on top, and satisfies position → orientation → aim.

Priority is a share taken off the top rather than a hard ordering, so a priority that fades in causes no jump. A satisfied region goal takes no share at all — "satisfied anywhere inside" has to mean *silent* anywhere inside, or a bound is a pin.

Three things the default does not do, stated plainly because you will hit all three: it does not guarantee a high-priority goal is satisfied exactly when a low-priority one conflicts, it does not redistribute error towards the root, and it does not resolve two hands each targeting the other. Those need a staged solve, which is what IConstraintArbiter is for.

Joint limits#

A rig may say how far each joint turns from where it was modelled — a swing cone and a twist range, about one axis:

The arbiter clamps every joint on a chain back inside its limit after the solve, and the residual goes up to say what that cost.

⚠ Limited is a flag rather than "a zero limit means free". Zero swing and zero twist is a joint welded to its bind pose — a legitimate thing to author, and also exactly what every rig exported before the fields existed deserialises to. Without the flag, adding them would have frozen every character in every project.

⚠ The clamp cuts and does not redistribute. A solver that knew about limits could bend more at the elbow because the shoulder ran out; this takes the correction the solver produced and removes the parts a joint may not do. What comes out is a pose that is *legal* and further from the goal. That is the same limitation the arbiter states above, and a staged arbiter is what fixes it.

⚠ A rig with no limits pays one boolean. Skeleton.HasLimits is checked before anything else, so the swing–twist decomposition never runs on the rigs — almost all of them — that declare none.

Seeing it fail#

ConstraintGizmos draws what the last solve did: the effector, the resolved frame, the chain the solver was allowed to move, the proxy shape a surface goal is anchored to, and — the one that matters — a line from where the effector ended up to where it was wanted, graded green to red.

AddAnimation registers the evaluation and skinning passes in the same one line physics has had all along; EngineLoop cannot include them in its default set, because the dependency only runs one way and the engine has no name for an animator.

It reads ConstraintStack.LastSolved rather than re-resolving, so what is drawn is what happened. A goal that never resolved is drawn grey and labelled unresolved, because a residual of zero says both "landed" and "never ran" and those mean opposite things.

Cost#

The stage is off until something uses it: with no goals, Solve returns before it computes anything. Measured on a hundred characters, a build with the stage installed and no goals is indistinguishable from one without it; two solved goals each costs about 380 µs more.

When that is too much, ConstraintGovernor has three knobs — rate, detail and scope — and reports what it dropped. It reserves the floor for every character still waiting rather than spending the budget on the most important ones in order, which is the difference between a hundred characters at one-in-four and thirty-seven characters at full rate.

Seams#

| Seam | The default | What else fits | |---|---|---| | IConstraintFrame | nine shipped | a physics query, a navmesh, a spline, another system's spatial data | | IBindingSource | a transform and its sockets | a table of participants resolved per interaction | | IConstraintArbiter | one weighted pass | a staged solve, error redistribution, exact satisfaction of a layer | | IConstraintScheduler | plans nothing | dependency discovery between characters, simultaneous multi-character solves | | IChainSolver | two-bone analytic | an iterative solver for a long chain, a data-driven one | | IProxyShapePoser | a joint's transform | a shape sized by a morph weight or a simulation | | IClipMetadataExtension | timed notes | a project's own tag kinds, checked at import and carried untouched |

Every one has a second implementation, and SeamTests fails the build if one does not.

Examples#

The portable contact, in code. A hand a quarter of the way round the belly and six tenths along it — not 8 cm to the left of a joint, which is what makes the same goal land correctly on a body a third narrower:

The same contact is what a .vxanim's markup produces, and the constraints: block near the top of this page is that goal written where an author can see it. Code is for the ones a game decides at runtime — the rail it is actually holding, the target it is actually aiming at.

A goal that may fail to resolve. EntityFrame and SocketFrame answer only while something is bound to the slot, and that is the ordinary case rather than the error case:

Holster the rifle and the frame stops resolving: the goal eases out and the hand goes back to whatever the animation said, rather than reaching for a socket that is no longer anywhere.

Whether it works on every body is not a question this page can answer by inspection, which is what the variation harness is for.

See also#

• Move sets — the other half of doc 34, and independent of this one
• The variation harness — whether a marked-up clip actually works on every body