Rastlines

Rastlines are a class of line patterns used in raster-based graphics, cartography, and generative art to represent bands at fixed distances from a reference curve or axis. They occur when a line is offset repeatedly by a constant spacing along its normal, producing a family of parallel-like curves that can be straight, curved, or spiraling depending on the base geometry. The term Rastline is derived from a combination of raster and line, emphasizing their use in grid-oriented workflows.

Construction and geometry: given a base curve C, defined by a parametric representation, the Rastline family consists of curves C_k obtained by moving C along its normal by a distance k times a fixed spacing d, where k is an integer. For straight base lines, Rastlines reduce to evenly spaced parallel lines; for circles, they form concentric rings; for arbitrary curves, they trace offset curves that follow the geometry of the base.

Characteristics: Rastlines exhibit uniform spacing and can be straight, curved, or complex in form. They may be closed or open and can be rendered with varying widths or colors to create hatch-like shading, texture, or depth cues in a raster image or map.

Applications: they are used in shading on topographic maps, architectural drawings, computer graphics, and generative art. Rastlines offer a method to convey elevation, density, or motion without resorting to solid fills.

History: the concept was coined by a digital artist and mathematician in the 2010s and has been described in tutorials and demonstrations on curve offsetting and hatch patterns. See also hatch patterns, crosshatching, and offset curves.