    path.segmentIndexAt(ratio [, opt])

Return the index of the path segment that contains a point that lies `ratio` (normalized length) away from the beginning of the path.

The function expects `ratio` to lie between 0 and 1; values outside the range are constrained to 0 and 1, respectively. Invisible segments (e.g. Moveto segments) have no length and are therefore skipped by the algorithm. If the path contains no visible segments, `null` is returned. If the path has no segments at all, `null` is returned, as well.

The `opt` argument is optional. Two properties may be specified, `opt.precision` and `opt.segmentSubdivisions`, which determine maximum error allowed in `pointAtLength` calculations for curved segments (default precision is 3; this corresponds to maximum observed error of 0.1%). The `opt.segmentSubdivisions` property is an array of individual segments' subdivision arrays. The `path.getSegmentSubdivisions()` function may be used to obtain the `segmentSubdivisions` array. The `opt.precision` property is still necessary, however; it determines the precision of the point search algorithm in curved segments.