@mapgis/geojson-vt
Version:
Slice GeoJSON data into vector tiles efficiently
201 lines (166 loc) • 6.39 kB
JavaScript
import createFeature from './feature.js';
/* clip features between two vertical or horizontal axis-parallel lines:
* | |
* ___|___ | /
* / | \____|____/
* | |
*
* k1 and k2 are the line coordinates
* axis: 0 for x, 1 for y
* minAll and maxAll: minimum and maximum coordinate value for all features
*/
export default function clip(features, scale, k1, k2, axis, minAll, maxAll, options) {
k1 /= scale;
k2 /= scale;
if (minAll >= k1 && maxAll < k2) return features; // trivial accept
else if (maxAll < k1 || minAll >= k2) return null; // trivial reject
const clipped = [];
for (const feature of features) {
const geometry = feature.geometry;
let type = feature.type;
const min = axis === 0 ? feature.minX : feature.minY;
const max = axis === 0 ? feature.maxX : feature.maxY;
if (min >= k1 && max < k2) { // trivial accept
clipped.push(feature);
continue;
} else if (max < k1 || min >= k2) { // trivial reject
continue;
}
let newGeometry = [];
if (type === 'Point' || type === 'MultiPoint') {
clipPoints(geometry, newGeometry, k1, k2, axis);
} else if (type === 'LineString') {
clipLine(geometry, newGeometry, k1, k2, axis, false, options.lineMetrics);
} else if (type === 'MultiLineString') {
clipLines(geometry, newGeometry, k1, k2, axis, false);
} else if (type === 'Polygon') {
clipLines(geometry, newGeometry, k1, k2, axis, true);
} else if (type === 'MultiPolygon') {
for (const polygon of geometry) {
const newPolygon = [];
clipLines(polygon, newPolygon, k1, k2, axis, true);
if (newPolygon.length) {
newGeometry.push(newPolygon);
}
}
}
if (newGeometry.length) {
if (options.lineMetrics && type === 'LineString') {
for (const line of newGeometry) {
clipped.push(createFeature(feature.id, type, line, feature.tags));
}
continue;
}
if (type === 'LineString' || type === 'MultiLineString') {
if (newGeometry.length === 1) {
type = 'LineString';
newGeometry = newGeometry[0];
} else {
type = 'MultiLineString';
}
}
if (type === 'Point' || type === 'MultiPoint') {
type = newGeometry.length === 3 ? 'Point' : 'MultiPoint';
}
clipped.push(createFeature(feature.id, type, newGeometry, feature.tags));
}
}
return clipped.length ? clipped : null;
}
function clipPoints(geom, newGeom, k1, k2, axis) {
for (let i = 0; i < geom.length; i += 3) {
const a = geom[i + axis];
if (a >= k1 && a <= k2) {
addPoint(newGeom, geom[i], geom[i + 1], geom[i + 2]);
}
}
}
function clipLine(geom, newGeom, k1, k2, axis, isPolygon, trackMetrics) {
let slice = newSlice(geom);
const intersect = axis === 0 ? intersectX : intersectY;
let len = geom.start;
let segLen, t;
for (let i = 0; i < geom.length - 3; i += 3) {
const ax = geom[i];
const ay = geom[i + 1];
const az = geom[i + 2];
const bx = geom[i + 3];
const by = geom[i + 4];
const a = axis === 0 ? ax : ay;
const b = axis === 0 ? bx : by;
let exited = false;
if (trackMetrics) segLen = Math.sqrt(Math.pow(ax - bx, 2) + Math.pow(ay - by, 2));
if (a < k1) {
// ---|--> | (line enters the clip region from the left)
if (b > k1) {
t = intersect(slice, ax, ay, bx, by, k1);
if (trackMetrics) slice.start = len + segLen * t;
}
} else if (a > k2) {
// | <--|--- (line enters the clip region from the right)
if (b < k2) {
t = intersect(slice, ax, ay, bx, by, k2);
if (trackMetrics) slice.start = len + segLen * t;
}
} else {
addPoint(slice, ax, ay, az);
}
if (b < k1 && a >= k1) {
// <--|--- | or <--|-----|--- (line exits the clip region on the left)
t = intersect(slice, ax, ay, bx, by, k1);
exited = true;
}
if (b > k2 && a <= k2) {
// | ---|--> or ---|-----|--> (line exits the clip region on the right)
t = intersect(slice, ax, ay, bx, by, k2);
exited = true;
}
if (!isPolygon && exited) {
if (trackMetrics) slice.end = len + segLen * t;
newGeom.push(slice);
slice = newSlice(geom);
}
if (trackMetrics) len += segLen;
}
// add the last point
let last = geom.length - 3;
const ax = geom[last];
const ay = geom[last + 1];
const az = geom[last + 2];
const a = axis === 0 ? ax : ay;
if (a >= k1 && a <= k2) addPoint(slice, ax, ay, az);
// close the polygon if its endpoints are not the same after clipping
last = slice.length - 3;
if (isPolygon && last >= 3 && (slice[last] !== slice[0] || slice[last + 1] !== slice[1])) {
addPoint(slice, slice[0], slice[1], slice[2]);
}
// add the final slice
if (slice.length) {
newGeom.push(slice);
}
}
function newSlice(line) {
const slice = [];
slice.size = line.size;
slice.start = line.start;
slice.end = line.end;
return slice;
}
function clipLines(geom, newGeom, k1, k2, axis, isPolygon) {
for (const line of geom) {
clipLine(line, newGeom, k1, k2, axis, isPolygon, false);
}
}
function addPoint(out, x, y, z) {
out.push(x, y, z);
}
function intersectX(out, ax, ay, bx, by, x) {
const t = (x - ax) / (bx - ax);
addPoint(out, x, ay + (by - ay) * t, 1);
return t;
}
function intersectY(out, ax, ay, bx, by, y) {
const t = (y - ay) / (by - ay);
addPoint(out, ax + (bx - ax) * t, y, 1);
return t;
}