eip	title	description	author	discussions-to	status	type	category	created
<to be assigned>	Identify NFT Content Type	NFT contracts to provide a contentType getter so clients can identify the type of NFT and how to fetch metadata	Louis Garoche <louis@garoche.me>, Iain Nash <iain@iain.in>	https://www.nftstandards.wtf	Draft	Standards Track	ERC	2021-08-30

Simple Summary

Define an extension to the EIP1155, EIP721 standard supporting metadata to also support linking content in a rich way both on- and off-chain.

Abstract

Define a standardized way to identify the underlying content and associated data of an NFT through an on-chain getter.

A content getter field queried by clients (wallets, marketplaces...) to allow for underlying integrations into programs and tools along with rich integrations into applications and programs that can support rich content.

This sort of extension has been already realized for many NFT projects using metadataURI, getSVGData, getMidiData, contentURI, and contentHash getter functions. If these concerns could be combined this can unlock:

1. on-chain "exotic" media types: generating or storing MIDI sound natively even though no field exists in either 721 or 1155 metadata for markdown

2. off-chain program file types or executables: the ability to make an NFT of image/tiff file type and give users a standard way to quickly see the canonical data they are purchasing.

3. off-chain "exotic" media types reference: NFTs made with obsolete file formats can be referenced for provenance or enthusasists here.

4. off-chain content verification hashes: content stored in IPFS / Arweave can be verified with a SHA256 hash and allow for the NFT contract to allow owners to update the URI (see Zora's cryptomedia)

5. on-chain ascii art such as Autoglyphs that have a core component on-chain but no standard way to expose this to users. Platforms could render this inline or include a download link.

This greatly expands the utility of the NFT ability to make unique and broad forms of data in standard files more clear and usable.

Motivation

The ERC721/1155 standard defines a tokenURI/uri getter on the contract, pointing to a JSON document containing the NFT metadata according to a conventional schema. This works for many audio/image/video projects but this scheme falls short when a creator wants to combine on-chain and off-chain data. Even for fully on-chain data, building JSON documents from the smart contract code is quite costly and reading can be complicated.
Every project has its own metadata requirements, and this EIP aims to unleash the creativity of NFT creators, by providing a common base to describe NFT content information.

Specification

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Smart contracts implementing this ERC MUST implement the following interface.

pragma solidity ^0.8.0;

struct ContentData {
    /// Mime type of the content as a string "video/mp4"
    string mime;
    /// String of the content itself from an on-chain source
    string content;
    /// URI Referencing the content remotely
    string uri;
    /// SHA256 content hash
    bytes32 hash;
}

/// Interface to show content getter for token
interface ITokenContent {
    /// @param tokenId: token id to retrieve content for
    /// @return ContentData: struct of content information
    function content(uint256 tokenId) external returns (ContentData memory);
}

It is RECOMMENDED that Smart contracts implementing this ERC also implement the ERC-165 interface, with the interface id ???, which is the result of bytes4(keccak256('content(string)').

The mime field is required if content is provided.
If uri is provided, hash and mime fields are strongly recommended but not required. The uri field MUST conform to the RFC 3986 URI standard.

At least one of uri or content are required to be provided.

Rationale

This design is inspired from the IANA Media Types (formerly known as MIME types), the same way an operating system recognizes supported formats with the available installed software, wallet or marketplace clients can check if they "support" a given NFT. The document does not intend to describe all possible NFT formats but provide an effective way to identify the NFT type.

Creators are free to imagine new content types that provide additional on-chain or off-chain metadata content. The standard also provides a simple way of linking content for NFTs in a off-chain or on-chain method that is not reliant on metadata. This method can be used for linking large content types such as AI models, on-chain SVG that may break normal gas limits, or even code to the NFT. Most platforms should show these links as source full-resolution files or download links instead of attempting to render them.

Type value

A struct being returned from the NFT will not much additional gas overhead and any field omitted can simply be left blank.

This proposal provides support for both on-chain and off-chain content to compliment the provided existing NFT metadata. This also allows full provenance for on-chain HTML or text content that may need to be interperted by a metadata server or external file (such as metadata on IPFS).

By allowing arbitrary mime files with content, platforms can decide what to render from content as opposed to metadata. Content here can be text/markdown, for instance, and supporting platforms can render the markdown file alongside the metadata if recognized.

Additional example:

A NFT solely consisting of HTML content can be created with the following implementation:

pragma solidity ^0.8.0;

import "./IERCContentType.sol";

abstract contract HtmlNFT is IERCContentType {
    
    /**
        @dev On-chain HTML rendering
    */
    function content(uint256 tokenId) public view override returns (ContentData) {
        return ContentData({
          mime: 'text/html',
          content: getHTMLContent(tokenId),
          hash: bytes32(0x0),
          uri: '',
        });
    }
}

Backwards Compatibility

This EIP does not introduce any backward compatibility with existing NFT standards. It extends these standards by adding the possibility of having additional metadata.

NFT creators should keep the existing ERC721/1155 metadata URI schemes on top of new content types to compliment this standard and provide metadata associated with the content.

Common questions

1. Why use a struct instead of multiple getter functions: it allows for easier fetching (one fetch call vs up to 4) from clients and more gas-efficent code for deployment

2. What is this inspired by: This is inspired by the larvalab's Autoglyphs project, returning the ascii art in the given metadata URI, this would be a perfect use case for that type of on-chain data. Zora's cryptomedia that seperates the content uri and hash allowing for updatable URIs and more rich support of different content-types.

Reference Implementation

See the example in the Specification section.

Security Considerations

Clients that decide to support and implement executable content (binary, javascript...) must ensure that proper security measures are in place before running any code on the client.

Copyright

Copyright and related rights waived via CC0.