Bodyrelative

Bodyrelative is a term used to describe data, coordinates, or frames that are defined with respect to a moving body, rather than in a fixed external or world reference frame. In this sense, a bodyrelative coordinate system is attached to a body segment (such as a torso, limb, or sensor) and rotates and translates as the body moves. This contrasts with world-relative or global coordinates, which refer to a fixed environment regardless of the subject’s movement.

In practice, bodyrelative representations are common in robotics, biomechanics, computer graphics, and animation. The idea is to describe positions, orientations, or motions in a local frame that follows the body. To relate these measurements to an external environment, the bodyrelative data are transformed by applying the body’s pose (its rotation and translation in world space). For example, the position of a hand can be stored in a shoulder-centered bodyrelative frame and then converted to global coordinates using the shoulder’s orientation and position.

In motion capture and animation, bodyrelative data simplify the analysis of how parts move relative to each other and to the body as a whole. They also help maintain consistent motion when the whole subject undergoes translation or rotation, since the relative relationships between body parts are preserved. However, working with bodyrelative coordinates relies on accurate estimation of the body’s pose; errors in pose tracking or calibration can propagate into the global space during transformation.

Overall, bodyrelative representations provide a natural and robust way to study and animate movements by focusing on internal, body-centered relationships before mapping them to a larger environment.