openlayers - Clipping TileLayer with georeferenced polygon (clipping mask)

I would like to add a mask to clip layers (Tile) below in a way that only the part bounded by a ol.Feature (polygon or multipolygon) is visible and the rest is hidden (left white). The polygon is stored in a DB so I need a way to do that programatically.

Does anybody have some clues or examples to achieve this?

• ol.Overlay?


• ol.layer.Vector filled with white outside of the features

Ideally: - add / remove vector features and interactions should update the mask - swichting layer below the mask is possible

I'm not looking for a working code but more inspirations and ideas to start doing that in a "ol3" way.

Answer

This can easily be done adding an Image layer with an ImageVector source on top of the tile layer. The ImageVector layer's precompose and postcompose hooks are used to change the global canvas composite operation:

var clipLayer = new ol.layer.Image({
  source: new ol.source.ImageVector({
    source: new ol.source.Vector({

      // contains your clip polygon
    }),
    style: new ol.style.Style({
      fill: new ol.style.Fill({
        color: 'black'
      })
    })
  })
});
clipLayer.on('precompose', function(e) {

  e.context.globalCompositeOperation = 'destination-in';
});
clipLayer.on('postcompose', function(e) {
  e.context.globalCompositeOperation = 'source-over';
});

Working example: http://jsfiddle.net/2gy96vbv/