geojson-vt/src/clip.js

Slice GeoJSON data into vector tiles efficiently

'use strict';

module.exports = clip;

/* clip features between two axis-parallel lines:
 *     |        |
 *  ___|___     |     /
 * /   |   \____|____/
 *     |        |
 */

function clip(features, scale, k1, k2, axis, intersect) {

    k1 /= scale;
    k2 /= scale;

    var clipped = [];

    for (var i = 0; i < features.length; i++) {

        var feature = features[i],
            geometry = feature.geometry,
            type = feature.type,
            min, max;

        if (feature.min) {
            min = feature.min[axis];
            max = feature.max[axis];

            if (min >= k1 && max <= k2) { // trivial accept
                clipped.push(feature);
                continue;
            } else if (min > k2 || max < k1) continue; // trivial reject
        }

        var slices = type === 1 ?
                clipPoints(geometry, k1, k2, axis) :
                clipGeometry(geometry, k1, k2, axis, intersect, type === 3);

        if (slices.length) {
            clipped.push({
                geometry: slices,
                type: type,
                tags: features[i].tags || null
            });
        }
    }

    return clipped.length ? clipped : null;
}

function clipPoints(geometry, k1, k2, axis) {
    var slice = [];

    for (var i = 0; i < geometry.length; i++) {
        var a = geometry[i],
            ak = a[axis];

        if (ak >= k1 && ak <= k2) slice.push(a);
    }
    return slice;
}

function clipGeometry(geometry, k1, k2, axis, intersect, closed) {

    var slices = [];

    for (var i = 0; i < geometry.length; i++) {

        var ak = 0,
            bk = 0,
            b = null,
            points = geometry[i],
            area = points.area,
            dist = points.dist,
            len = points.length,
            a, j;

        var slice = [];

        for (j = 0; j < len - 1; j++) {
            a = b || points[j];
            b = points[j + 1];
            ak = bk || a[axis];
            bk = b[axis];

            if (ak < k1) {

                if ((bk > k2)) { // ---|-----|-->
                    slice.push(intersect(a, b, k1), intersect(a, b, k2));
                    if (!closed) slice = newSlice(slices, slice, area, dist);

                } else if (bk >= k1) slice.push(intersect(a, b, k1)); // ---|-->  |

            } else if (ak > k2) {

                if ((bk < k1)) { // <--|-----|---
                    slice.push(intersect(a, b, k2), intersect(a, b, k1));
                    if (!closed) slice = newSlice(slices, slice, area, dist);

                } else if (bk <= k2) slice.push(intersect(a, b, k2)); // |  <--|---

            } else {

                slice.push(a);

                if (bk < k1) { // <--|---  |
                    slice.push(intersect(a, b, k1));
                    if (!closed) slice = newSlice(slices, slice, area, dist);

                } else if (bk > k2) { // |  ---|-->
                    slice.push(intersect(a, b, k2));
                    if (!closed) slice = newSlice(slices, slice, area, dist);
                }
                // | --> |
            }
        }

        a = points[len - 1];
        ak = a[axis];
        if (ak >= k1 && ak <= k2) slice.push(a);

        var sliceLen = slice.length;

        // close the polygon if its endpoints are not the same after clipping
        if (closed && slice[0] !== slice[sliceLen - 1]) slice.push(slice[0]);

        // add the final slice
        newSlice(slices, slice, area, dist);
    }

    return slices;
}

function newSlice(slices, slice, area, dist) {
    if (slice.length) {
        slice.area = area;
        slice.dist = dist;
        slices.push(slice);
    }
    return [];
}