mapbox-gl/src/symbol/get_anchors.js

A WebGL interactive maps library

'use strict';

const interpolate = require('../util/interpolate');
const Anchor = require('../symbol/anchor');
const checkMaxAngle = require('./check_max_angle');

module.exports = getAnchors;

function getAnchors(line, spacing, maxAngle, shapedText, shapedIcon, glyphSize, boxScale, overscaling, tileExtent) {

    // Resample a line to get anchor points for labels and check that each
    // potential label passes text-max-angle check and has enough froom to fit
    // on the line.

    const angleWindowSize = shapedText ?
        3 / 5 * glyphSize * boxScale :
        0;

    const labelLength = Math.max(
        shapedText ? shapedText.right - shapedText.left : 0,
        shapedIcon ? shapedIcon.right - shapedIcon.left : 0);

    // Is the line continued from outside the tile boundary?
    const isLineContinued = line[0].x === 0 || line[0].x === tileExtent || line[0].y === 0 || line[0].y === tileExtent;

    // Is the label long, relative to the spacing?
    // If so, adjust the spacing so there is always a minimum space of `spacing / 4` between label edges.
    if (spacing - labelLength * boxScale  < spacing / 4) {
        spacing = labelLength * boxScale + spacing / 4;
    }

    // Offset the first anchor by:
    // Either half the label length plus a fixed extra offset if the line is not continued
    // Or half the spacing if the line is continued.

    // For non-continued lines, add a bit of fixed extra offset to avoid collisions at T intersections.
    const fixedExtraOffset = glyphSize * 2;

    const offset = !isLineContinued ?
        ((labelLength / 2 + fixedExtraOffset) * boxScale * overscaling) % spacing :
        (spacing / 2 * overscaling) % spacing;

    return resample(line, offset, spacing, angleWindowSize, maxAngle, labelLength * boxScale, isLineContinued, false, tileExtent);
}


function resample(line, offset, spacing, angleWindowSize, maxAngle, labelLength, isLineContinued, placeAtMiddle, tileExtent) {

    const halfLabelLength = labelLength / 2;
    let lineLength = 0;
    for (let k = 0; k < line.length - 1; k++) {
        lineLength += line[k].dist(line[k + 1]);
    }

    let distance = 0,
        markedDistance = offset - spacing;

    let anchors = [];

    for (let i = 0; i < line.length - 1; i++) {

        const a = line[i],
            b = line[i + 1];

        const segmentDist = a.dist(b),
            angle = b.angleTo(a);

        while (markedDistance + spacing < distance + segmentDist) {
            markedDistance += spacing;

            const t = (markedDistance - distance) / segmentDist,
                x = interpolate(a.x, b.x, t),
                y = interpolate(a.y, b.y, t);

            // Check that the point is within the tile boundaries and that
            // the label would fit before the beginning and end of the line
            // if placed at this point.
            if (x >= 0 && x < tileExtent && y >= 0 && y < tileExtent &&
                    markedDistance - halfLabelLength >= 0 &&
                    markedDistance + halfLabelLength <= lineLength) {
                const anchor = new Anchor(x, y, angle, i)._round();

                if (!angleWindowSize || checkMaxAngle(line, anchor, labelLength, angleWindowSize, maxAngle)) {
                    anchors.push(anchor);
                }
            }
        }

        distance += segmentDist;
    }

    if (!placeAtMiddle && !anchors.length && !isLineContinued) {
        // The first attempt at finding anchors at which labels can be placed failed.
        // Try again, but this time just try placing one anchor at the middle of the line.
        // This has the most effect for short lines in overscaled tiles, since the
        // initial offset used in overscaled tiles is calculated to align labels with positions in
        // parent tiles instead of placing the label as close to the beginning as possible.
        anchors = resample(line, distance / 2, spacing, angleWindowSize, maxAngle, labelLength, isLineContinued, true, tileExtent);
    }

    return anchors;
}