geojson-vt/src/simplify.js

Slice GeoJSON data into vector tiles efficiently

'use strict';

module.exports = simplify;

// calculate simplification data using optimized Douglas-Peucker algorithm

function simplify(points, tolerance) {

    var sqTolerance = tolerance * tolerance,
        len = points.length,
        first = 0,
        last = len - 1,
        stack = [],
        i, maxSqDist, sqDist, index;

    points[first][2] = 1;
    points[last][2] = 1;

    while (last) {

        maxSqDist = 0;

        for (i = first + 1; i < last; i++) {
            sqDist = getSqSegDist(points[i], points[first], points[last]);

            if (sqDist > maxSqDist) {
                index = i;
                maxSqDist = sqDist;
            }
        }

        if (maxSqDist > sqTolerance) {
            points[index][2] = maxSqDist;
            stack.push(first, index, index, last);
        }

        last = stack.pop();
        first = stack.pop();
    }
}

// square distance from a point to a segment
function getSqSegDist(p, a, b) {

    var x = a[0], y = a[1],
        bx = b[0], by = b[1],
        px = p[0], py = p[1],
        dx = bx - x,
        dy = by - y;

    if (dx !== 0 || dy !== 0) {

        var t = ((px - x) * dx + (py - y) * dy) / (dx * dx + dy * dy);

        if (t > 1) {
            x = bx;
            y = by;

        } else if (t > 0) {
            x += dx * t;
            y += dy * t;
        }
    }

    dx = px - x;
    dy = py - y;

    return dx * dx + dy * dy;
}