ERC-721, A Standard Interface for NFT

interface ERC165 {
    function supportsInterface(bytes4 interfaceID) external view returns (bool);
}

contract ERC721Impl is ERC721, ERC165 {

    mapping (bytes4 => bool) supportedInterfaces;

    constructor() public {
        supportedInterfaces[0x80ac58cd] = true;
    }

    function supportsInterface(bytes4 interfaceID) external view returns (bool){
        return supportedInterfaces[interfaceID];
    }

    this.balanceOf.selector ^
    this.ownerOf.selector ^
    bytes4(keccak256("safeTransferFrom(address,address,uint256)"))^
    bytes4(keccak256("safeTransferFrom(address,address,uint256,bytes)"))^
    this.transferFrom.selector ^
    this.approve.selector ^
    this.setApprovalForAll.selector ^
    this.getApproved.selector ^
    this.isApprovedForAll.selector

/// @dev This emits when ownership of any NFT changes by any mechanism.
///  This event emits when NFTs are created (`from` == 0) and destroyed
///  (`to` == 0). Exception: during contract creation, any number of NFTs
///  may be created and assigned without emitting Transfer. At the time of
///  any transfer, the approved address for that NFT (if any) is reset to none.
event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);

/// @dev This emits when the approved address for an NFT is changed or
///  reaffirmed. The zero address indicates there is no approved address.
///  When a Transfer event emits, this also indicates that the approved
///  address for that NFT (if any) is reset to none.
event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

/// @dev This emits when an operator is enabled or disabled for an owner.
///  The operator can manage all NFTs of the owner.
event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);

/// @notice Count all NFTs assigned to an owner
/// @dev NFTs assigned to the zero address are considered invalid, and this
///  function throws for queries about the zero address.
/// @param _owner An address for whom to query the balance
/// @return The number of NFTs owned by `_owner`, possibly zero
function balanceOf(address _owner) external view returns (uint256);

/// @notice Find the owner of an NFT
/// @dev NFTs assigned to zero address are considered invalid, and queries
///  about them do throw.
/// @param _tokenId The identifier for an NFT
/// @return The address of the owner of the NFT
function ownerOf(uint256 _tokenId) external view returns (address);

/// @notice Transfers the ownership of an NFT from one address to another address
/// @dev This works identically to the other function with an extra data parameter,
///  except this function just sets data to "".
/// @param _from The current owner of the NFT
/// @param _to The new owner
/// @param _tokenId The NFT to transfer
function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;

/// @notice Transfer ownership of an NFT -- THE CALLER IS RESPONSIBLE
///  TO CONFIRM THAT `_to` IS CAPABLE OF RECEIVING NFTS OR ELSE
///  THEY MAY BE PERMANENTLY LOST
/// @dev Throws unless `msg.sender` is the current owner, an authorized
///  operator, or the approved address for this NFT. Throws if `_from` is
///  not the current owner. Throws if `_to` is the zero address. Throws if
///  `_tokenId` is not a valid NFT.
/// @param _from The current owner of the NFT
/// @param _to The new owner
/// @param _tokenId The NFT to transfer
function transferFrom(address _from, address _to, uint256 _tokenId) external payable;

/// @notice Change or reaffirm the approved address for an NFT
/// @dev The zero address indicates there is no approved address.
///  Throws unless `msg.sender` is the current NFT owner, or an authorized
///  operator of the current owner.
/// @param _approved The new approved NFT controller
/// @param _tokenId The NFT to approve
function approve(address _approved, uint256 _tokenId) external payable;

/// @notice Enable or disable approval for a third party ("operator") to manage
///  all of `msg.sender`'s assets
/// @dev Emits the ApprovalForAll event. The contract MUST allow
///  multiple operators per owner.
/// @param _operator Address to add to the set of authorized operators
/// @param _approved True if the operator is approved, false to revoke approval
function setApprovalForAll(address _operator, bool _approved) external;

/// @notice Get the approved address for a single NFT
/// @dev Throws if `_tokenId` is not a valid NFT.
/// @param _tokenId The NFT to find the approved address for
/// @return The approved address for this NFT, or the zero address if there is none
function getApproved(uint256 _tokenId) external view returns (address);

/// @notice Query if an address is an authorized operator for another address
/// @param _owner The address that owns the NFTs
/// @param _operator The address that acts on behalf of the owner
/// @return True if `_operator` is an approved operator for `_owner`, false otherwise
function isApprovedForAll(address _owner, address _operator) external view returns (bool);

/// @dev Note: the ERC-165 identifier for this interface is 0x150b7a02.
interface ERC721TokenReceiver {
    /// @notice Handle the receipt of an NFT
    /// @dev The ERC721 smart contract calls this function on the recipient
    ///  after a `transfer`. This function MAY throw to revert and reject the
    ///  transfer. Return of other than the magic value MUST result in the
    ///  transaction being reverted.
    ///  Note: the contract address is always the message sender.
    /// @param _operator The address which called `safeTransferFrom` function
    /// @param _from The address which previously owned the token
    /// @param _tokenId The NFT identifier which is being transferred
    /// @param _data Additional data with no specified format
    /// @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
    ///  unless throwing
    function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4);
}

/// @title ERC-721 Non-Fungible Token Standard, optional metadata extension
/// @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
///  Note: the ERC-165 identifier for this interface is 0x5b5e139f.
interface ERC721Metadata /* is ERC721 */ {
    /// @notice A descriptive name for a collection of NFTs in this contract
    function name() external view returns (string _name);

    /// @notice An abbreviated name for NFTs in this contract
    function symbol() external view returns (string _symbol);

    /// @notice A distinct Uniform Resource Identifier (URI) for a given asset.
    /// @dev Throws if `_tokenId` is not a valid NFT. URIs are defined in RFC
    ///  3986. The URI may point to a JSON file that conforms to the "ERC721
    ///  Metadata JSON Schema".
    function tokenURI(uint256 _tokenId) external view returns (string);
}

{
    "title": "Asset Metadata",
    "type": "object",
    "properties": {
        "name": {
            "type": "string",
            "description": "Identifies the asset to which this NFT represents"
        },
        "description": {
            "type": "string",
            "description": "Describes the asset to which this NFT represents"
        },
        "image": {
            "type": "string",
            "description": "A URI pointing to a resource with mime type image/* representing the asset to which this NFT represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive."
        }
    }
}