MPEG-4 Facial Action Parameter

A standard for 3D face animation defined in ISO/IEC 14496-2 (MPEG-4), which parameterises a face through 68 feature points whose displacements (scaled relative to the character's own facial proportions) encode any facial expression. MPEG-4 FAP is proportion-invariant, so the same FAP vector produces the equivalent expression on faces of different sizes and shapes — a property that makes it well-suited to sign-language animation research, where researchers need to reuse facial data across different virtual signing avatars. In accessibility work, MPEG-4 FAP has been empirically shown to be sufficient for conveying grammatically required American Sign Language facial expressions such as yes-no questions (raised brows, head tilt forward), wh-questions (furrowed brows), and negation (brow furrow plus head shake).