openlayers/examples/layer-spy.js

goog.require('ol.Map');
goog.require('ol.View');
goog.require('ol.layer.Tile');
goog.require('ol.proj');
goog.require('ol.source.BingMaps');

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();
});