text: var key = 'As1HiMj1PvLPlqc_gtM7AqZfBL8ZL3VrjaS3zIb22Uvb9WKhuJObROC-qUpa81U5';
  
  var roads = new ol.layer.Tile({
    source: new ol.source.BingMaps({key: key, imagerySet: 'Road'})
  });
  
  var imagery = new ol.layer.Tile({
    source: new ol.source.BingMaps({key: key, imagerySet: 'Aerial'})
  });
  
  var container = document.getElementById('map');
  
  var map = new ol.Map({
    layers: [roads, imagery],
    target: container,
    view: new ol.View({
      center: ol.proj.fromLonLat([-109, 46.5]),
      zoom: 6
    })
  });
  
  var radius = 75;
  document.addEventListener('keydown', function(evt) {
    if (evt.which === 38) {
      radius = Math.min(radius + 5, 150);
      map.render();
      evt.preventDefault();
    } else if (evt.which === 40) {
      radius = Math.max(radius - 5, 25);
      map.render();
      evt.preventDefault();
    }
  });
  
  // get the pixel position with every move
  var mousePosition = null;
  
  container.addEventListener('mousemove', function(event) {
    mousePosition = map.getEventPixel(event);
    map.render();
  });
  
  container.addEventListener('mouseout', function() {
    mousePosition = null;
    map.render();
  });
  
  // before rendering the layer, do some clipping
  imagery.on('precompose', function(event) {
    var ctx = event.context;
    var pixelRatio = event.frameState.pixelRatio;
    ctx.save();
    ctx.beginPath();
    if (mousePosition) {
      // only show a circle around the mouse
      ctx.arc(mousePosition[0] * pixelRatio, mousePosition[1] * pixelRatio,
          radius * pixelRatio, 0, 2 * Math.PI);
      ctx.lineWidth = 5 * pixelRatio;
      ctx.strokeStyle = 'rgba(0,0,0,0.5)';
      ctx.stroke();
    }
    ctx.clip();
  });
  
  // after rendering the layer, restore the canvas context
  imagery.on('postcompose', function(event) {
    var ctx = event.context;
    ctx.restore();
  });
  