# `BB.Error.Kinematics.FABRIK.UnsupportedJoint`
[🔗](https://github.com/beam-bots/bb_ik_fabrik/blob/main/lib/bb/error/kinematics/fabrik/unsupported_joint.ex#L5)

The requested chain contains a joint FABRIK cannot solve for.

FABRIK is not Jacobian-based. It is a heuristic that iteratively repositions
joints along a chain, treating each as a point connected by a fixed-length link,
and a `:planar` or `:floating` joint has no meaningful interpretation in that
scheme — there is no single point to reposition, and no fixed length between one
and the next.

Refusing is better than approximating: a wrong answer from a solver that
silently ignored three or six degrees of freedom looks exactly like a right one.

## This is a property of the chain, not the robot

A legged robot whose body floats is a perfectly good FABRIK problem when each
leg is solved from the body to the foot, because those chains contain nothing but
revolute joints. Scope the solve past the multi-DoF joint with `source_link` and
FABRIK handles it:

    # The floating base is not part of this problem.
    BB.IK.FABRIK.solve(robot, state, :body, :front_left_foot, target, [])

For a chain that genuinely must include the multi-DoF joint, use a
Jacobian-based solver — `bb_ik_dls` is dimension-agnostic and needs no special
case.

# `t`

```elixir
@type t() :: %BB.Error.Kinematics.FABRIK.UnsupportedJoint{
  __exception__: term(),
  bread_crumbs: term(),
  class: term(),
  joint: atom(),
  joint_type: :planar | :floating,
  path: term(),
  source_link: atom() | nil,
  splode: term(),
  stacktrace: term(),
  target_link: atom() | nil,
  vars: term()
}
```

# `exception`

```elixir
@spec exception(opts :: Keyword.t()) :: %BB.Error.Kinematics.FABRIK.UnsupportedJoint{
  __exception__: term(),
  bread_crumbs: term(),
  class: term(),
  joint: term(),
  joint_type: term(),
  path: term(),
  source_link: term(),
  splode: term(),
  stacktrace: term(),
  target_link: term(),
  vars: term()
}
```

Create an `Elixir.BB.Error.Kinematics.FABRIK.UnsupportedJoint` without raising it.

## Keys

- :joint
- :joint_type
- :source_link
- :target_link

---

*Consult [api-reference.md](api-reference.md) for complete listing*