1.Scope

IPL defines a flat, named, normalized, rig-agnostic parameter vector (P) that represents an upper-body signing posture — hands, arms, head, body, face, and mouth — at a single instant (one keyframe). Every dimension is explicitly defined and human-interpretable: a person can read a dimension's definition, understand what it controls, and edit it by hand. The same vector can be produced by an encoder (sign-language pose → P) or authored by a human, and decoded to a rendered avatar (and, in future revisions, to other modalities).

IPL is a representation standard, not an algorithm. It specifies what the vector means, not how any particular encoder or renderer is implemented.

1.1.Design intent (non-normative)

• Designed, not discovered. Unlike a learned latent whose axes are entangled and opaque, IPL axes are hand-designed with human definitions. Only the mapping into IPL is learned; the axes themselves are fixed by this specification.
• Interpretable. Each dimension corresponds to one human-nameable articulatory concept (cf. concept-bottleneck representations), enabling manual inspection and correction.
• Rig-agnostic. Values are normalized and skeleton-independent, so any conforming renderer can decode the same vector to its own humanoid.