javascript - OpenLayers 3: Offset stroke style

Question

I have LineString features styled by a single stroke. Some of these features represent two things, so in addition to the initial stroke color, I would like to have a parallel line next to it in a different color. This would be a no-brainer if ol/style/Stroke had an offset property like ol/style/Image does, but sadly, it does not. If I created offset geometries for those features, they wouldn't be visible from a zoomed out level. What are my options?

Answer 1

You can offset the geometries based on resolution

var style = function(feature, resolution) {
var colors = ['green', 'yellow', 'red'];
var width = 4;
var styles = [];
for (var line = 0; line < colors.length; line++) {
    var dist = width * resolution * (line - (colors.length-1)/2);
    var geom = feature.getGeometry();
    var coords = [];
    var counter = 0;
    geom.forEachSegment(function(from, to) {
        var angle = Math.atan2(to[1] - from[1], to[0] - from[0]);
        var newFrom = [
            Math.sin(angle) * dist + from[0],
            -Math.cos(angle) * dist + from[1]
        ];
        var newTo = [
            Math.sin(angle) * dist + to[0],
            -Math.cos(angle) * dist + to[1]
        ];
        coords.push(newFrom);
        coords.push(newTo);
        if (coords.length > 2) {
            var intersection = math.intersect(coords[counter], coords[counter+1], coords[counter+2], coords[counter+3]);
            coords[counter+1] = (intersection) ? intersection : coords[counter+1];
            coords[counter+2] = (intersection) ? intersection : coords[counter+2];
            counter += 2;
        }
    });
    styles.push(
        new ol.style.Style({
            geometry: new ol.geom.LineString(coords),
            stroke: new ol.style.Stroke({
                color: colors[line],
                width: width
            })
        })
    );
}
return styles;
};


var raster = new ol.layer.Tile({
  source:  new ol.source.OSM() 
});

var source = new ol.source.Vector();

var vector = new ol.layer.Vector({
  source: source,
  style: style
});

var map = new ol.Map({
  layers: [raster, vector],
  target: 'map',
  view: new ol.View({
center: [-11000000, 4600000],
zoom: 4
  })
});

map.addInteraction(new ol.interaction.Draw({
  source: source,
  type: 'LineString'
}));

html, body, .map {
    margin: 0;
    padding: 0;
    width: 100%;
    height: 100%;
}

<link href="https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/css/ol.css" rel="stylesheet" />
<script src="https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/build/ol.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/mathjs/5.4.1/math.min.js"></script>
<div id="map" class="map"></div>