Using OpenLayers 3 forEachFeatureAtPixel to specify layer?

I must first state that my skills at javascript are about the same as my skills in Spanish. I know barely enough to get myself into trouble and then I go looking for help. anyway I have an application that has a few geoJSON layers in it. I have created a map.forEachFeatureAtPixel function to update a div with some information.

The problem is it looks at all of the layers and I just want it to look at one. this way I can create another function to get some different attributes from another layer. I have looked over the documentation and I see the opt_layerFilter option but I dont know how to apply it (due to my lack of Javascript knowledge). If someone could give me a hint here hopefully I will then be able to read the documentation and stop asking these questions.

function:

var displayFeatureInfo = function(pixel ) {
  var feature = map.forEachFeatureAtPixel(pixel, function(feature, layer) {
    return feature;
    });


  var info = document.getElementById('info');
  if (feature) {
    info.innerHTML =  '
' + feature.get('Name') +
     '
' + '
API: 
' + feature.get('API') + '
'
     + '
Status: 
' + feature.get('Status') +  '
' +
     '
Legal : 
' + feature.get('Legal') +  '
' +
     '
Site Name: 
' + feature.get('Site_Name') +  '
'+'
';
  } else {
    info.innerHTML = ' ';

  }

  if (feature !== highlight) {
    if (highlight) {
      featureOverlay.removeFeature(highlight);
    }
    if (feature) {
      featureOverlay.addFeature(feature);
    }
    highlight = feature;

  }

}; 

Answer

You can use the opt_layerFilter argument to forEachFeatureAtPixel. See the documentation at http://openlayers.org/en/master/apidoc/ol.Map.html#forEachFeatureAtPixel.

For example, if vectorLayer is a reference to the layer in which you want to detect features you will use the following:

var feature = map.forEachFeatureAtPixel(pixel, function(feature, layer) {
  return feature;
}, null, function(layer) {
  return layer === vectorLayer;

});

With this "filter function" layers that are not vectorLayer will be filtered out.