Format of metadata json for ERC721

Question

I deployed a simple NFT contract based on the nibbstack ERC721 implementation

For testing purposes I hardcoded the ipfs link into the contract. The mint function looks like this, for testing purposes everyone who send 100 Wei can get an NFT

function mint() public payable {
         require(msg.value == 100, "Send 100 Wei");
         super._mint(msg.sender, tokenId);
         super._setTokenUri(tokenId, nfts[0]);
         tokenId++;
    }

The ipfs are hardcoded like this:

string[] nfts = ["https://ipfs.io/ipfs/QmUFbUjAifv9GwJo7ufTB5sccnrNqELhDMafoEmZdPPng7"];

the ipfs link points to a json file that looks like this (I picked it from the ipfs documentation):

{
    "name": "NFT Art",
    "description": "This image shows the true nature of NFT.",
    "image": "https://ipfs.io/ipfs/QmQEVVLJUR1WLN15S49rzDJsSP7za9DxeqpUzWuG4aondg",
}

However, format must be wrong, I can never view the image. Here is the opensea site for an example deployment on the Goerli testnet:

Answer 1

Checkout the ERC721 Metadata JSON Schema from the ERC721 token standard

Answer 2

You didn't add tokenURI(tokenId) nor _baseURI() functions.

If you are using smart contract libs like @openzeppelin, you will find that this function tokenURI(tokenId) returns a URL of the NFT of the given tokenId.

You need to modify (override) it to return the link for the NFT, I will leave an example.

string constant TOKEN_URI = "https://ipfs.io/ipfs/QmUFbUjAifv9GwJo7ufTB5sccnrNqELhDMafoEmZdPPng7";

function tokenURI(uint256 /* tokenId */) public pure override returns (string memory) {
    return TOKEN_URI;
}

This is just a simple demo, you can use the tokenId that is passed as a parameter to the function to return different URLs for different NFTs.