path.tangentAtLength(length [, opt])
Return a line tangent to the path at 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, a line tangent to the closest valid path endpoint is returned instead. If point at
length is a point of discontinuity (e.g. it is a point shared by two Lineto segments with different slopes), the tangent line is constructed for the earlier segment (i.e. the segment closer to the beginning of the path).
The tangent line starts at the specified point. The direction from
end is the same as the direction of the path segment at the specified point.
The algorithm skips over segments that are not differentiable. This includes all invisible segments (e.g. Moveto segments) and visible segments with zero length. If the path contains no valid segments,
null is returned. If the path has no segments at all,
null is returned, as well. The
segment.isDifferentiable() functions may be used to determine whether a given segment is valid; the
path.isDifferentiable() function may be used to determine whether the path contains at least one valid segment.
One valid segment is identified which contains the point at
length. Finding the desired point is straightforward for linear segments (see
line.pointAtLength() for reference). Finding the desired point in curved segments is more complex, as illustrated by the
curve.pointAtLength() function. A tangent line is then obtained that touches the path at the identified point (see
curve.tangentAtLength() for reference).
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.