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.
<!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>