code: function projection(c) {
        var zoom = c.zoom,
            max = zoom < 0 ? 1 : 1 << zoom,
            column = c.column % max,
            row = c.row;
        if (column < 0) column += max;
        return {
          locationPoint: function(l) {
            var c = po.map.locationCoordinate(l),
                k = Math.pow(2, zoom - c.zoom);
            return {
              x: tileSize.x * (k * c.column - column),
              y: tileSize.y * (k * c.row - row)
            };
          }
        };
      }
docs: tile-specific projection