Overview of ERC721

Let’s begin with an overview of EIP-721, the standard that defines NFT contracts.

ERC721 is a variant of ERC20. The main difference between them is that ERC721 tokens are non-fungible, that is: one token is not identical to another. To distinguish ERC721 tokens, each of them has a unique ID, which is almost always the counter at which a token was minted. ERC721 tokens also have an extended concept of ownership: the owner of each token is tracked and stored in the contract. This means that only distinct tokens, identified by token IDs, can be transferred (or approved for transfer).

What Uniswap V3 liquidity positions and NFTs have in common is this non-fungibility: NFTs and liquidity positions are not interchangeable and are identified by unique IDs. It’s this similarity that will allow us to merge the two concepts.

The biggest difference between ERC20 and ERC721 is the tokenURI function in the latter. NFT tokens, which are implemented as ERC721 smart contracts, have linked assets that are stored externally, not on the blockchain. To link token IDs to images (or sounds, or anything else) stored outside of the blockchain, ERC721 defines the tokenURI function. The function is expected to return a link to a JSON file that defines NFT token metadata, e.g.:

    "name": "Thor's hammer",
    "description": "Mjölnir, the legendary hammer of the Norse god of thunder.",
    "image": "https://game.example/item-id-8u5h2m.png",
    "strength": 20

(This example is taken from the ERC721 documentation on OpenZeppelin)

Such JSON file defines the name of a token, the description of a collection, the link to the image of a token, and the properties of a token.

Alternatively, we may store JSON metadata and token images on-chain. This is very expensive of course (saving data on-chain is the most expensive operation in Ethereum), but we can make it cheaper if we store templates. All tokens within a collection have similar metadata (mostly identical but image links and properties are different for each token) and visuals. For the latter, we can use SVG, which is an HTML-like format, and HTML is a good templating language.

When storing JSON metadata and SVG on-chain, the tokenURI function, instead of returning a link, would return JSON metadata directly, using the data URI scheme to encode it. SVG images would also be inlined, it won’t be necessary to make external requests to download token metadata and images.