code: function blendTile( tiledImage, tile, x, y, level, levelOpacity, currentTime ){
      var blendTimeMillis = 1000 * tiledImage.blendTime,
          deltaTime,
          opacity;
  
      if ( !tile.blendStart ) {
          tile.blendStart = currentTime;
      }
  
      deltaTime   = currentTime - tile.blendStart;
      opacity     = blendTimeMillis ? Math.min( 1, deltaTime / ( blendTimeMillis ) ) : 1;
  
      if ( tiledImage.alwaysBlend ) {
          opacity *= levelOpacity;
      }
  
      tile.opacity = opacity;
  
      tiledImage.lastDrawn.push( tile );
  
      if ( opacity === 1 ) {
          setCoverage( tiledImage.coverage, level, x, y, true );
          tiledImage._hasOpaqueTile = true;
      } else if ( deltaTime < blendTimeMillis ) {
          return true;
      }
  
      return false;
  }
docs: @private
  @inner
  Updates the opacity of a tile according to the time it has been on screen
  to perform a fade-in.
  Updates coverage once a tile is fully opaque.
  Returns whether the fade-in has completed.
  
  @param {OpenSeadragon.TiledImage} tiledImage
  @param {OpenSeadragon.Tile} tile
  @param {Number} x
  @param {Number} y
  @param {Number} level
  @param {Number} levelOpacity
  @param {Number} currentTime
  @returns {Boolean}