Sketching piecewise clothoid curves

We present a novel approach to sketching 2D curves with minimally varying curvature as piecewise clothoids. A stable and efficient algorithm fits a sketched piecewise linear curve using a number of clothoid segments with G2G2 continuity based on a specified error tolerance. Further, adjacent clothoid segments can be locally blended to result in a G3G3 curve with curvature that predominantly varies linearly with arc length. We also handle intended sharp corners or G1G1 discontinuities, as independent rotations of clothoid pieces. Our formulation is ideally suited to conceptual design applications where aesthetic fairness of the sketched curve takes precedence over the precise interpolation of geometric constraints. We show the effectiveness of our results within a system for sketch-based road and robot-vehicle path design, where clothoids are already widely used.