ol.source.Geoportail = function(key, layer, options)
{  if (!options) options={};
   var matrixIds = new Array();
   var resolutions = new Array();
   var size = ol.extent.getWidth(ol.proj.get('EPSG:3857').getExtent()) /256;
   for (var z=0; z <= (options.maxZoom ? options.maxZoom:18) ; z++) 
   {  matrixIds[z] = z ; 
      resolutions[z] = size / Math.pow(2, z)
   }
   var attr = [ ol.source.Geoportail.prototype.attribution ];
   if (options.attributions) attr.push(options.attributions);
   ol.source.WMTS.call (this, 
   {  url: "http://wxs.ign.fr/" + key + "/wmts",
      layer: layer,
      matrixSet: "PM",
      format: options.format ? options.format:"image/jpeg",
      projection: "EPSG:3857",
      tileGrid: new ol.tilegrid.WMTS
      ({ origin: [-20037508, 20037508],
         resolutions: resolutions,
         matrixIds: matrixIds
      }),
      style: options.style ? options.style:"normal",
      attributions: attr
   });	
};
ol.inherits (ol.source.Geoportail, ol.source.WMTS);
// Attribution standard
ol.source.Geoportail.prototype.attribution = new ol.Attribution
({  html: '<a href="http://www.geoportail.gouv.fr/">Géoportail</a>'
    +'&copy; <a href="http://www.ign.fr/">IGN-France</a>'
});