和声：程序化绘画工具

Keep an array of every point the pointer has visited in the current drawing. On each pointer move, draw the main segment, then loop over previous points and, for those within a distance threshold, draw a faint line from the current point toward them, optionally shortened by a random factor. Use a very low alpha so overlapping lines accumulate into shading. Variants change the threshold, whether lines extend beyond the neighbour (fur), or connect only to recent points (sketchy).