Setting Vector Feature Fill Opacity when you have a hexadecimal color

Question

I'm trying to set the fill opacity of a vector feature in OL3 and can't figure out how to do it if I have a hexadecimal value...I've seen examples with rgba. Any suggestions?

Here's what I have:

style : function(feature, resolution) {
          return [new ol.style.Style(
          {
            stroke : new ol.style.Stroke(
            {
              color : feature.get('color'),  
              width : 2
            }),
            fill : new ol.style.Fill(
            {
              color : feature.get('color'), //'#FF0000'
              opacity : 0.2   // I thought this would work, but it's not an option
            })
          })]
        }

Answer 1

You can use the ol.color.asArray function. That function converts color strings to color arrays.

So this is what you can use:

var hexColor = feature.get('color');
var color = ol.color.asArray(hexColor);
color = color.slice();
color[3] = 0.2;  // change the alpha of the color

slice() is used to create a new color array. This is to avoid corrupting the "color strings to color arrays" cache that the ol.color.asArray function maintains.

See http://openlayers.org/en/master/apidoc/ol.color.html?unstable=true#asArray.