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Create a gradient line using an expression

Create a gradient line using an expression

Use the line-gradient paint property and an expression to visualize distance from the starting point of a line.

Create a gradient line using an expression
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Create a gradient line using an expression</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no" />
<script src="https://cdn.jsdelivr.net/npm/@goongmaps/goong-js@1.0.9/dist/goong-js.js"></script>
<link href="https://cdn.jsdelivr.net/npm/@goongmaps/goong-js@1.0.9/dist/goong-js.css" rel="stylesheet" />
<style>
	body { margin: 0; padding: 0; }
    #map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>

<div id="map"></div>

<script>
    goongjs.accessToken = 'wnicbAmnNkoMHNYUKWnlFHezV189FjmMwkNJ7hKW';
    var map = (window.map = new goongjs.Map({
        container: 'map',
        style: 'https://tiles.goong.io/assets/goong_map_web.json',
        center: [-77.035, 38.875],
        zoom: 12
    }));

    var geojson = {
        'type': 'FeatureCollection',
        'features': [
            {
                'type': 'Feature',
                'properties': {},
                'geometry': {
                    'coordinates': [
                        [-77.044211, 38.852924],
                        [-77.045659, 38.860158],
                        [-77.044232, 38.862326],
                        [-77.040879, 38.865454],
                        [-77.039936, 38.867698],
                        [-77.040338, 38.86943],
                        [-77.04264, 38.872528],
                        [-77.03696, 38.878424],
                        [-77.032309, 38.87937],
                        [-77.030056, 38.880945],
                        [-77.027645, 38.881779],
                        [-77.026946, 38.882645],
                        [-77.026942, 38.885502],
                        [-77.028054, 38.887449],
                        [-77.02806, 38.892088],
                        [-77.03364, 38.892108],
                        [-77.033643, 38.899926]
                    ],
                    'type': 'LineString'
                }
            }
        ]
    };

    map.on('load', function () {
        // 'line-gradient' can only be used with GeoJSON sources
        // and the source must have the 'lineMetrics' option set to true
        map.addSource('line', {
            type: 'geojson',
            lineMetrics: true,
            data: geojson
        });

        // the layer must be of type 'line'
        map.addLayer({
            type: 'line',
            source: 'line',
            id: 'line',
            paint: {
                'line-color': 'red',
                'line-width': 14,
                // 'line-gradient' must be specified using an expression
                // with the special 'line-progress' property
                'line-gradient': [
                    'interpolate',
                    ['linear'],
                    ['line-progress'],
                    0,
                    'blue',
                    0.1,
                    'royalblue',
                    0.3,
                    'cyan',
                    0.5,
                    'lime',
                    0.7,
                    'yellow',
                    1,
                    'red'
                ]
            },
            layout: {
                'line-cap': 'round',
                'line-join': 'round'
            }
        });
    });
</script>
</body>
</html>