Layer Spy Example

View a portion of one layer over another

Layer rendering can be manipulated in precompose and postcompose event listeners. These listeners get an event with a reference to the Canvas rendering context. In this example, the precompose listener sets a clipping mask around the most recent mouse position, giving you a spyglass effect for viewing one layer over another.

Move around the map to see the effect. Use the ↑ up and ↓ down arrow keys to adjust the spyglass size.

spy, image manipulation

Related API documentation:

Copy Edit

var key = 'Ak-dzM4wZjSqTlzveKz5u0d4IQ4bRzVI309GxmkgSVr1ewS6iPSrOvOKhA-CJlm3';

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 map = new ol.Map({
  layers: [roads, imagery],
  target: 'map',
  view: new ol.View({
    center: ol.proj.fromLonLat([-109, 46.5]),
    zoom: 6
  })
});

// get the pixel position with every move
var mousePosition = null;
$(map.getViewport()).on('mousemove', function(evt) {
  mousePosition = map.getEventPixel(evt.originalEvent);
  map.render();
}).on('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();
});

<!DOCTYPE html>
<html>
<head>
<title>Layer Spy Example</title>
<script src="https://code.jquery.com/jquery-1.11.2.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="http://openlayers.org/en/v3.9.0/css/ol.css" type="text/css">
<script src="http://openlayers.org/en/v3.9.0/build/ol.js"></script>

</head>
<body>
<div class="container-fluid">

<div class="row-fluid">
  <div class="span12">
    <div id="map" class="map"></div>
  </div>
</div>

</div>
<script>
var key = 'Ak-dzM4wZjSqTlzveKz5u0d4IQ4bRzVI309GxmkgSVr1ewS6iPSrOvOKhA-CJlm3';

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 map = new ol.Map({
  layers: [roads, imagery],
  target: 'map',
  view: new ol.View({
    center: ol.proj.fromLonLat([-109, 46.5]),
    zoom: 6
  })
});

var radius = 75;
$(document).keydown(function(evt) {
  if (evt.which === 38) {
    radius = Math.min(radius + 5, 150);
    map.render();
  } else if (evt.which === 40) {
    radius = Math.max(radius - 5, 25);
    map.render();
  }
});

// get the pixel position with every move
var mousePosition = null;
$(map.getViewport()).on('mousemove', function(evt) {
  mousePosition = map.getEventPixel(evt.originalEvent);
  map.render();
}).on('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();
});

</script>
</body>
</html>