Layer Swipe OpenLayers 5 - How to change the left side?

Question

I used the Layer Swipe for an app with OL5, like this example and it work perfectly for compare the baselayer with a new layer on the right side. But how to set a layer for the left side to ? I could change baselayer, but it appear behind the layer on right side, and I don't want.

var anneeCouche = "2016_20cm";

var _rasterSource = new ol.source.TileWMS({
  url: 'http://serveradress/geoserver/raster/wms?',
  params: {LAYERS: 'raster:ortho'+anneeCouche}
});

function changeDateRight(annee) {
  if (annee === '2013') {
    var anneeCouche = "2013_20cm";
  } else {
    var anneeCouche = "2016_20cm";
  };
  _rasterSource.updateParams({LAYERS: 'raster:ortho'+anneeCouche});
};

var _rightLayer = new ol.layer.Tile({
  title: 'Ortho',
  source: _rasterSource
});

_rightLayer.on('precompose', function(event) {
    var ctx = event.context;
    var width = ctx.canvas.width * (swipe.value / 100);
    ctx.save();
    ctx.beginPath();
    ctx.rect(width, 0, ctx.canvas.width - width, ctx.canvas.height);
    ctx.clip();
});

_rightLayer.on('postcompose', function(event) {
    var ctx = event.context;
    ctx.restore();
});

Answer 1

You would need to change the clipping rectangle to the left side of the swipe

ctx.rect(0, 0, width, ctx.canvas.height);