Reduce number of points in line

Question

I'm searching for algorithms to reduce the LOD of polylines, lines (looped or not) of nodes. In simple words, I want to take hi-resolution coastline data and be able to reduce its LOD hundred- or thousandfold to render it in small-scale.

I found polygon reduction algorithms (but they require triangles) and Laplacian smoothing, but that doesn't seem exactly what I need.

Answer 1

I've modified the code in culebrón's answer, removing the need for the Vec2D/Line classes, instead handling the points as a list of tuples.

The code is slightly less tidy, but shorter, and a bit quicker (for 900 points, the original code took 2966ms, and this version takes 500ms - still a bit slower than I'd like, but an improvement)

def _vec2d_dist(p1, p2):
    return (p1[0] - p2[0])**2 + (p1[1] - p2[1])**2


def _vec2d_sub(p1, p2):
    return (p1[0]-p2[0], p1[1]-p2[1])


def _vec2d_mult(p1, p2):
    return p1[0]*p2[0] + p1[1]*p2[1]


def ramerdouglas(line, dist):
    """Does Ramer-Douglas-Peucker simplification of a curve with `dist`
    threshold.

    `line` is a list-of-tuples, where each tuple is a 2D coordinate

    Usage is like so:

    >>> myline = [(0.0, 0.0), (1.0, 2.0), (2.0, 1.0)]
    >>> simplified = ramerdouglas(myline, dist = 1.0)
    """

    if len(line) < 3:
        return line

    (begin, end) = (line[0], line[-1]) if line[0] != line[-1] else (line[0], line[-2])

    distSq = []
    for curr in line[1:-1]:
        tmp = (
            _vec2d_dist(begin, curr) - _vec2d_mult(_vec2d_sub(end, begin), _vec2d_sub(curr, begin)) ** 2 / _vec2d_dist(begin, end))
        distSq.append(tmp)

    maxdist = max(distSq)
    if maxdist < dist ** 2:
        return [begin, end]

    pos = distSq.index(maxdist)
    return (ramerdouglas(line[:pos + 2], dist) + 
            ramerdouglas(line[pos + 1:], dist)[1:])