Openlayers: Gradient glow on LineString

Question

I have a layer that renders LineStrings and am trying to apply a glow effect to the lines. The Style I created uses a custom renderer to create a stroke with a gradient perpendicular to each line segment:

const glow_style = new Style({
  renderer: (_coords, state) => {
    const ctx = state.context;
    const coords = _coords as Coordinate[];
    ctx.lineWidth = 25;
    for (let i = 1; i < coords.length; i++) {
      const start = coords[i - 1];
      const end = coords[i];
      const [grd_start, grd_end] = getPerpendicularPoints(start, end, ctx.lineWidth);
      const grd = ctx.createLinearGradient(grd_start[0], grd_start[1], grd_end[0], grd_end[1]);
      grd.addColorStop(0, '#ffffff00');
      grd.addColorStop(0.5, 'white');
      grd.addColorStop(1, '#ffffff00');
      ctx.strokeStyle = grd;
      ctx.beginPath();
      ctx.moveTo(start[0], start[1]);
      ctx.lineTo(end[0], end[1]);
      ctx.stroke();
    }
  }
});

This style works for completely straight lines, but breaks down at corners, because the gradient doesn't connect nicely between line segments. If ctx.lineCap is left as butt , the gradient is discontiguous around corners. If it's set to round , the segments touch, but the gradient becomes discontinuous because of overlapping. Here are examples of each:

What options do I have for creating a smooth gradient along the entire LineString?

Answer 1

It would be simpler to render the entire linestring without breaking it into segments by drawing lines of decreasing width. For a smooth gradient the opacity of each line should be defined as the fraction of the remaining transparency. It could even be done as an OpenLayers style array:

var steps = 13;
var styles = [];
for (var i = 0; i < steps; i++) {
    styles.push(
        new ol.style.Style({
            stroke: new ol.style.Stroke({
                color: [255, 255, 255, 1/(steps - i)],
                width: (steps-i)*2 - 1
            })
        })
    );
}