(a) Virtual shape in the polar coordinate system. (b) One of the sectors whose arc length is the sliding distance corresponding to the half spatial period of the texture. (o,-h) is the center coordinate, (x, y) is the contact point, θ ₀ is the angle between the edge of the sector and the horizontal direction, θ is the angle corresponding to the boundary between rough and smooth, r is the radius, C is the curvature, β is the half spatial period of texture, and l ₀ is the arc length.