title: openlayers 3 - Load TileLayer with highest zoom level tiles but display at all zoom levels
body: <p>Is is possible to load a Tile layer from a TileImage source with tiles in its highest resolution (zoom) for a specific extent and have these tiles show for all zoom levels.  I have high resolution imagery I wish to show as a layer above another Tile layer such as Bing or OSM.</p>
  
  <p>Example source</p>
  
  <pre><code>var source = new ol.source.TileImage({
  projection: 'EPSG:900913',
  tileGrid: new ol.tilegrid.TileGrid({
  projection: 'EPSG:900913',
  extent: projectionExtent,
  //extent: ol.proj.transformExtent([-83.7893967, 42.851353, -83.7855987, 42.849371], "EPSG:4326", "EPSG:900913"),
  resolutions: resolutions,
  tileSize:[pictometryImageSize, pictometryImageSize]
  }),
  tileUrlFunction: function(tileCoord, pixelRatio, projection){
  var zoom = tileCoord[0];
  
  console.log(zoom)
  
  if(zoom == 21){
      var tileGrid = this.getTileGrid();
      var center = ol.proj.transform(ol.extent.getCenter(tileGrid.getTileCoordExtent(tileCoord)),"EPSG:900913", "EPSG:4326");
      return 'www.myrealtileurl;
  }
  //return undefined;
  //return 'https://dummy.com';
  },
  minZoom: 0,
  maxZoom: 21
  </code></pre>
  
  <p>});</p>