nonrigide

Nonrigide, often written nonrigid in English, is a term used to describe objects or models that do not maintain fixed distances between all pairs of points. In contrast to rigid bodies, nonrigide objects can change shape under forces, temperature, or other stimuli. This category includes elastic deformations (reversible), plastic deformations (permanent), and time-dependent viscoelastic behavior.

From a mechanical perspective, deformable bodies are described by fields of strain and stress. Small deformations are modeled with linear elasticity and Hooke's law, while large strains require nonlinear elasticity. Common constitutive models for rubber-like or soft materials include Neo-Hookean and Mooney-Rivlin formulations.

In mathematics and computer science, nonrigid transformations do not preserve distances or angles. They are contrasted with rigid motions, which consist of rotations and translations. Nonrigid registration and morphable models align or reconstruct shapes by allowing smooth deformations, using techniques such as thin-plate splines or diffeomorphic mappings.

Applications span computer vision, medical imaging, and computer graphics, including registration of scans over time, 3D reconstruction of deformable objects, and cloth or soft-body simulation. The term is common in French-language technical literature, where it is spelled nonrigide.