leaflet/src/geometry/PolyUtil.js

JavaScript library for mobile-friendly interactive maps

/*
 * L.PolyUtil contains utility functions for polygons (clipping, etc.).
 */

L.PolyUtil = {};

/*
 * Sutherland-Hodgeman polygon clipping algorithm.
 * Used to avoid rendering parts of a polygon that are not currently visible.
 */
L.PolyUtil.clipPolygon = function (points, bounds, round) {
	var clippedPoints,
	    edges = [1, 4, 2, 8],
	    i, j, k,
	    a, b,
	    len, edge, p,
	    lu = L.LineUtil;

	for (i = 0, len = points.length; i < len; i++) {
		points[i]._code = lu._getBitCode(points[i], bounds);
	}

	// for each edge (left, bottom, right, top)
	for (k = 0; k < 4; k++) {
		edge = edges[k];
		clippedPoints = [];

		for (i = 0, len = points.length, j = len - 1; i < len; j = i++) {
			a = points[i];
			b = points[j];

			// if a is inside the clip window
			if (!(a._code & edge)) {
				// if b is outside the clip window (a->b goes out of screen)
				if (b._code & edge) {
					p = lu._getEdgeIntersection(b, a, edge, bounds, round);
					p._code = lu._getBitCode(p, bounds);
					clippedPoints.push(p);
				}
				clippedPoints.push(a);

			// else if b is inside the clip window (a->b enters the screen)
			} else if (!(b._code & edge)) {
				p = lu._getEdgeIntersection(b, a, edge, bounds, round);
				p._code = lu._getBitCode(p, bounds);
				clippedPoints.push(p);
			}
		}
		points = clippedPoints;
	}

	return points;
};