Rectilineaire

Rectilineaire, or rectilinéaire in French, is an adjective and, less commonly, a noun used across several disciplines to denote straight lines or straight-line behavior. The term derives from Latin rectus “straight” and linea “line,” and appears in accented (rectilinéaire/rectiligne) and unaccented forms in French and in English adaptations.

In geometry and mathematics, rectilinear describes figures or paths composed entirely of straight segments. A rectilinear polygon has edges that are straight, and in some contexts the term is used for shapes whose sides align with a fixed set of axes. In kinematics, rectilinear motion refers to movement along a single straight line, with the direction of motion remaining constant.

In optics and photography, a rectilinear (rectilinéaire) lens or projection preserves straight lines in the captured image, so lines that are straight in the scene remain straight in the photograph. This contrasts with curvilinear projections, such as fisheye, where straight lines can appear curved, especially toward the edges. Rectilinear projection is common in standard perspective rendering and many photographic lenses, though it can introduce perspective distortions at wide angles.

In cartography and computer graphics, rectilinear projection maps three-dimensional space to a two-dimensional plane while maintaining straight-line relationships, which is useful for many technical applications but can distort shapes at extreme extents.

The term is widely used in both French and English contexts, with variations including rectiligne and rectilineaire.