function changeContentData(uint256 _x, uint256 _y, bytes32 _contentData)
    public
  {
    Pixel storage pixel = pixelByCoordinate[_x][_y];

    require(msg.sender == pixel.seller);

    pixel.contentData = _contentData;

    emit UpdateContentData(
      pixel.id,
      pixel.seller,
      _x,
      _y,
      _contentData
  );

  }