iD/modules/svg/path.js

import * as d3 from 'd3';

export function svgPath(projection, graph, isArea) {

    // Explanation of magic numbers:
    // "padding" here allows space for strokes to extend beyond the viewport,
    // so that the stroke isn't drawn along the edge of the viewport when
    // the shape is clipped.
    //
    // When drawing lines, pad viewport by 5px.
    // When drawing areas, pad viewport by 65px in each direction to allow
    // for 60px area fill stroke (see ".fill-partial path.fill" css rule)

    var cache = {},
        padding = isArea ? 65 : 5,
        viewport = projection.clipExtent(),
        paddedExtent = [
            [viewport[0][0] - padding, viewport[0][1] - padding],
            [viewport[1][0] + padding, viewport[1][1] + padding]
        ],
        clip = d3.geoIdentity().clipExtent(paddedExtent).stream,
        project = projection.stream,
        path = d3.geoPath()
            .projection({stream: function(output) { return project(clip(output)); }});

    return function(entity) {
        if (entity.id in cache) {
            return cache[entity.id];
        } else {
            return cache[entity.id] = path(entity.asGeoJSON(graph));
        }
    };
}