path.segmentIndexAtLength(length [, opt])
Return the index of the path segment that contains a point that lies
length away from the beginning of the path.
length is provided, the algorithm starts looking from the end of the path. If
length is higher than path length, the closest visible path segment is returned. 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.
opt argument is optional. Two properties may be specified,
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.