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I have a simple map with a static pixel image layer:
<!DOCTYPE html>
<html>
<head>
<title>Simple Map</title>
<link rel="stylesheet" href="http://openlayers.org/en/v3.13.0/css/ol.css" type="text/css">
<script src="http://openlayers.org/en/v3.13.0/build/ol.js"></script>
// reference to jquery here
</head>
<body>
<div id="map" class="map"></div>
<script>
var extent = [0, 0, 2000000, 2000000];
var projection = new ol.proj.Projection({
code: 'xkcd-image',
units: 'pixels',
extent: extent
});
var map = new ol.Map({
layers: [
new ol.layer.Tile({
source: new ol.source.OSM()
})
],
target: 'map',
view: new ol.View({
center: [0, 0],
zoom: 2
})
});
image = new ol.layer.Image({
source: new ol.source.ImageStatic({
url: 'http://imgs.xkcd.com/comics/online_communities.png',
projection: projection,
imageExtent: extent
})
});
map.addLayer(image);
image.on('singleclick', function(evt) {
var xy = evt.pixel;
console.log(xy);
var canvasContext = $('.ol-unselectable')[0].getContext('2d');
var pixelAtClick = canvasContext.getImageData(xy[0], xy[1], 1, 1).data;
var red = pixelAtClick[0]; // green is [1] , blue is [2] , alpha is [4]
});
</script>
</body>
</html>
When clicking on the image I want to get the color of the pixel that I clicked on. As far as I understand the raster source example (http://openlayers.org/en/v3.13.0/examples/raster.html), this is only possible with raster sources so I converted the image into a raster source. (When I add that raster source to the layer, I get the message that this operation is insecure, so I still use the image to show on the map.)
Here (How to get a pixel's color value from an Openlayers 3 layer?) the color is read from evt.context. However, with me evt.context is undefined.
Addendum: There might be several image layers overlaying each other. I need to get the color from a single specific image layer.
So this isn't ideal but might send you along the right lines:
You can use the pixel xy of the click event to query the canvas object that openlayers creates and puts your map data onto.
This assumes that you are using jQuery and that the canvas is the first DOM element using the class '.ol-unselectable'.
Hope it helps.