This is a continuous version of the chain code described in the previous section. is the tangent angle of a point on the boundary with reference to a fixed datum, for example the x-axis, and s is the distance along the boundary. For a closed curve the function is periodic. Horizontal straight lines in the description correspond to straight lines on the object boundary, whereas straight lines of uniform gradient in space correspond to uniform circular arcs; i.e. is changing at a constant rate. The first derivative of the curve is often used to segment a boundary into quasi-uniform segments, since boundary discontinuities result in a peak or delta function in space.