Simple Onchain NFTs
Many NFTs are dependent on offchain metadata and images. Some use immutable storage locations, such as IPFS. Others use traditional web locations, and many of these allow the owner of the contract to modify the URL returned by a contract when a site or user attempts to retrieve the location of the token art and metadata. This power isn't inherently bad, because we probably want someone to be able to fix the contract if the storage location goes down. However, it does introduce a requirement to trust the contract owner.
In this tutorial, we'll show you how to do this to create a simple NFT that is fully onchain. This contract is used in our tutorials for Thirdweb and Unreal - NFT Items and the Coinbase Smart Wallet.
The result of this tutorial is used in other tutorials. Below, you can find the complete contract and ABI. Feel free to use it if you're working on one of those and don't want to get sidetracked.
Objectives
By the end of this tutorial you should be able to:
- Programmatically generate and return json metadata for ERC-721 tokens
- Deterministically construct unique SVG art in a smart contract
- Generate deterministic, pseudorandom numbers
Prerequisites
ERC-721 Tokens
This tutorial assumes that you are able to write, test, and deploy your own ERC-721 tokens using the Solidity programming language. If you need to learn that first, check out our content in Base Camp or the sections specific to ERC-721 Tokens!
Vector Art
You'll need some familiarity with the SVG art format and a basic level of ability to edit and manipulate vector art. If you don't have this, find an artist friend and collaborate!
Building the Contract
Start by setting up an [OpenZeppelin ERC-721] contract. You'll need to set up a mintTo
function that accepts the address that should receive the NFT.
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.24;
import "hardhat/console.sol";
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
contract RandomColorNFT is ERC721 {
uint public counter;
constructor() ERC721("RandomColorNFT", "RCNFT") {
}
function mintTo(address _to) public {
counter++;
_safeMint(_to, counter);
}
}
With the Smart Wallet, msg.sender
is the users custodial address - where you want to send the NFT. This is not always the case with account abstraction. In some other implementations, msg.sender
is the smart contract address, even if the user signs in with an EOA. Regardless, it's becoming a common practice to pass the address you want the NFT to go to explicitly.
Onchain Metadata
Rather than pointing to a json
file on the traditional internet, you can put your metadata directly in the contract. To do so, first import some helper libraries:
import "@openzeppelin/contracts/utils/Base64.sol";
import "@openzeppelin/contracts/utils/Strings.sol";
Next, override
the functions for _baseURI
and tokenURI
to return base 64 encoded json metadata with the appropriate information:
function _baseURI() internal pure override returns (string memory) {
return "data:application/json;base64,";
}
function tokenURI(uint _tokenId) public view override returns (string memory) {
if(_tokenId > counter) {
revert InvalidTokenId(_tokenId);
}
string memory json = Base64.encode(
bytes(
string(
abi.encodePacked(
'{"name": "',
name(),
' #: ',
Strings.toString(_tokenId),
'","description": "Random colors are pretty or boring!", "image": "image": "data:image/svg+xml;base64,',
// TODO...,
'"}'
)
)
)
);
return string(abi.encodePacked(_baseURI(), json));
}
Be very careful setting up the single and double quotes above and be sure to test this function to make sure the result is valid json metadata. An error here will break the NFT and it won't show up correctly in wallets or marketplaces!
Onchain SVG Image
For this NFT, the art will consist of a simple onchain SVG containing a square with a pseudo-randomly chosen color. Check out our tutorial on [Building Onchain NFTs] if you want to try something more complicated.
Start by scaffolding out a render
function:
function render(uint _tokenId) public view returns (string memory) {
return string(
abi.encodePacked(
"<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 1024 1024'>",
// TODO: add a rectangle with a random color fill
"</svg>"
)
);
}
Rectangles in SVG images are created with the [rect] element. To cover the whole background, you can set the width and height to the size of the viewbox
. Although not listed directly in the MDN page for rectangles, you can add a fill
property to add a fill color to any SVG element. You can use color names, or hex codes for colors:
<rect width="100" height="100" fill="#aabbcc" />
Generating a Random Color
Instead of a fixed color, your design calls for a unique color for each NFT. Add a function to generate this:
// Function to generate a random color hex code
function generateRandomColor() public view returns (string memory) {
// Generate a pseudo-random number using block.prevrandao
uint256 randomNum = uint256(keccak256(abi.encodePacked(block.prevrandao, block.timestamp, msg.sender)));
// Extract RGB components from the random number
bytes memory colorBytes = new bytes(3);
colorBytes[0] = bytes1(uint8(randomNum >> 16));
colorBytes[1] = bytes1(uint8(randomNum >> 8));
colorBytes[2] = bytes1(uint8(randomNum));
// Convert RGB components to hex string
string memory colorHex = string(abi.encodePacked(
"#",
toHexDigit(uint8(colorBytes[0]) >> 4),
toHexDigit(uint8(colorBytes[0]) & 0x0f),
toHexDigit(uint8(colorBytes[1]) >> 4),
toHexDigit(uint8(colorBytes[1]) & 0x0f),
toHexDigit(uint8(colorBytes[2]) >> 4),
toHexDigit(uint8(colorBytes[2]) & 0x0f)
));
return colorHex;
}
// Helper function to convert a uint8 to a hex character
function toHexDigit(uint8 d) internal pure returns (bytes1) {
if (d < 10) {
return bytes1(uint8(bytes1('0')) + d);
} else {
return bytes1(uint8(bytes1('a')) + d - 10);
}
}
Remember, randomness generated using onchain information is not fully secure. A determined attacker could manipulate a block to compromise your contract. That being said, prevrandao
is a passable solution for anything not involving a large amount of money.
Saving the Color to the NFT
You'll need to generate this color with the function, then save it in a way that it can be retrieved when the tokenURI
function is called. Add a mapping to store this relationship:
mapping (uint => string) public tokenIdToColor;
Then set the color when the token is minted:
function mintTo(address _to) public {
counter++;
_safeMint(_to, counter);
tokenIdToColor[counter] = generateRandomColor();
}
Finishing the tokenURI
Function
Update your render
function to generate the SVG.
function render(uint _tokenId) public view returns (string memory) {
return string(
abi.encodePacked(
"<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 1024 1024'>",
"<rect width='1024' height='1024' fill='",
tokenIdToColor[_tokenId],
"' />",
"</svg>"
)
);
}
Then update your tokenURI
function to use it, and return the SVG as base64 encoded data:
function tokenURI(uint _tokenId) public view override returns (string memory) {
if(_tokenId > counter) {
revert InvalidTokenId(_tokenId);
}
string memory json = Base64.encode(
bytes(
string(
abi.encodePacked(
'{"name": "',
name(),
' #: ',
Strings.toString(_tokenId),
'","description": "Random colors are pretty or boring!", "image": "data:image/svg+xml;base64,',
Base64.encode(bytes(render(_tokenId))),
'"}'
)
)
)
);
return string(abi.encodePacked(_baseURI(), json));
}
List of NFTs Owned
Most ERC-721 implementations don't contain an on-contract method to retrieve a list of all the NFTs owned by a single address. The reason for this is that it costs extra gas go manage this list, and the information can be retrieved by using read-only services that analyze blockchain data.
However, gas prices are getting lower, and adding this data to your contract will reduce your dependency on third-party APIs.
To track ownership in-contract, first import EnumerableSet
from OpenZeppelin:
import "@openzeppelin/contracts/utils/structs/EnumerableSet.sol";
Then enable it for uint
sets and add a mapping to relate addresses
to token ids.
// Inside the RandomColorNFT contract
using EnumerableSet for EnumerableSet.UintSet;
mapping (address => EnumerableSet.UintSet) tokensOwned;
Finally, utilize the _update
function to handle changes of ownership, including minting:
function _update(address to, uint256 tokenId, address auth) internal override(ERC721) returns(address) {
// Only remove the token if it is not being minted
if (tokenId != counter){
tokensOwned[auth].remove(tokenId);
}
tokensOwned[to].add(tokenId);
return super._update(to, tokenId, auth);
}
Now that you have a list of NFTs owned by an address, you can add a function to retrieve all of them. While you're at it, add the json metadata for each token. Doing so lets you get the complete list of NFTs and their metadata for just one RPC call!
function getNFftsOwned(address owner) public view returns (TokenAndMetatdata[] memory) {
TokenAndMetatdata[] memory tokens = new TokenAndMetatdata[](tokensOwned[owner].length());
for (uint i = 0; i < tokensOwned[owner].length(); i++) {
uint tokenId = tokensOwned[owner].at(i);
tokens[i] = TokenAndMetatdata(tokenId, tokenURI(tokenId));
}
return tokens;
}
Testing
Write some local tests, then [deploy] and test your contract. It can be very tricky to get all the commas, brackets, and single, and double quotes all lined up properly. The surest way to make sure it is working is to check the collection on [Testnet Opensea] or similar.
Remember, it can take a few minutes for them to register and add the collection. If the metadata or image don't show up correctly, use [Sepolia Basescan] to pull the tokenURI
and an online or console base64 decoder to decode and check the json metadata and SVG image.
Conclusion
In this lesson, you learned how to make a simple NFT that is entirely onchain. You generated an SVG with a random color, and set up the JSON metadata for your NFT -- entirely onchain! Next, check out our tutorial for Complex Onchain NFTs!
Random Color NFT Contract
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.24;
import "hardhat/console.sol";
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/utils/Base64.sol";
import "@openzeppelin/contracts/utils/Strings.sol";
import "@openzeppelin/contracts/utils/structs/EnumerableSet.sol";
contract RandomColorNFT is ERC721 {
using EnumerableSet for EnumerableSet.UintSet;
mapping (address => EnumerableSet.UintSet) tokensOwned;
uint public counter;
mapping (uint => string) public tokenIdToColor;
error InvalidTokenId(uint tokenId);
error OnlyOwner(address);
constructor() ERC721("RandomColorNFT", "RCNFT") {
}
function mintTo(address _to) public {
counter++;
_safeMint(_to, counter);
tokenIdToColor[counter] = generateRandomColor();
}
struct TokenAndMetatdata {
uint tokenId;
string metadata;
}
function getNftsOwned(address owner) public view returns (TokenAndMetatdata[] memory) {
TokenAndMetatdata[] memory tokens = new TokenAndMetatdata[](tokensOwned[owner].length());
for (uint i = 0; i < tokensOwned[owner].length(); i++) {
uint tokenId = tokensOwned[owner].at(i);
tokens[i] = TokenAndMetatdata(tokenId, tokenURI(tokenId));
}
return tokens;
}
function shuffleColor(uint _tokenId) public {
if(_tokenId > counter) {
revert InvalidTokenId(_tokenId);
}
if(ownerOf(_tokenId) != msg.sender) {
revert OnlyOwner(msg.sender);
}
tokenIdToColor[_tokenId] = generateRandomColor();
}
function _update(address to, uint256 tokenId, address auth) internal override(ERC721) returns(address) {
// Only remove the token if it is not being minted
if (tokenId != counter){
tokensOwned[auth].remove(tokenId);
}
tokensOwned[to].add(tokenId);
return super._update(to, tokenId, auth);
}
function _baseURI() internal pure override returns (string memory) {
return "data:application/json;base64,";
}
function tokenURI(uint _tokenId) public view override returns (string memory) {
if(_tokenId > counter) {
revert InvalidTokenId(_tokenId);
}
string memory json = Base64.encode(
bytes(
string(
abi.encodePacked(
'{"name": "',
name(),
' #: ',
Strings.toString(_tokenId),
'","description": "Random colors are pretty or boring!", "image": "data:image/svg+xml;base64,',
Base64.encode(bytes(render(_tokenId))),
'"}'
)
)
)
);
return string(abi.encodePacked(_baseURI(), json));
}
function render(uint _tokenId) public view returns (string memory) {
return string(
abi.encodePacked(
"<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 1024 1024'>",
"<rect width='1024' height='1024' fill='",
tokenIdToColor[_tokenId],
"' />",
"</svg>"
)
);
}
// Function to generate a random color hex code
function generateRandomColor() public view returns (string memory) {
// Generate a pseudo-random number using block.prevrandao
uint256 randomNum = uint256(keccak256(abi.encodePacked(block.prevrandao, block.timestamp, msg.sender)));
// Extract RGB components from the random number
bytes memory colorBytes = new bytes(3);
colorBytes[0] = bytes1(uint8(randomNum >> 16));
colorBytes[1] = bytes1(uint8(randomNum >> 8));
colorBytes[2] = bytes1(uint8(randomNum));
// Convert RGB components to hex string
string memory colorHex = string(abi.encodePacked(
"#",
toHexDigit(uint8(colorBytes[0]) >> 4),
toHexDigit(uint8(colorBytes[0]) & 0x0f),
toHexDigit(uint8(colorBytes[1]) >> 4),
toHexDigit(uint8(colorBytes[1]) & 0x0f),
toHexDigit(uint8(colorBytes[2]) >> 4),
toHexDigit(uint8(colorBytes[2]) & 0x0f)
));
return colorHex;
}
// Helper function to convert a uint8 to a hex character
function toHexDigit(uint8 d) internal pure returns (bytes1) {
if (d < 10) {
return bytes1(uint8(bytes1('0')) + d);
} else {
return bytes1(uint8(bytes1('a')) + d - 10);
}
}
}
{
"address": "0x59c35beE5eAdeEDDc2c34d419459243bD70AFD72",
"abi": [
{
"inputs": [],
"stateMutability": "nonpayable",
"type": "constructor"
},
{
"inputs": [
{
"internalType": "address",
"name": "sender",
"type": "address"
},
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
},
{
"internalType": "address",
"name": "owner",
"type": "address"
}
],
"name": "ERC721IncorrectOwner",
"type": "error"
},
{
"inputs": [
{
"internalType": "address",
"name": "operator",
"type": "address"
},
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "ERC721InsufficientApproval",
"type": "error"
},
{
"inputs": [
{
"internalType": "address",
"name": "approver",
"type": "address"
}
],
"name": "ERC721InvalidApprover",
"type": "error"
},
{
"inputs": [
{
"internalType": "address",
"name": "operator",
"type": "address"
}
],
"name": "ERC721InvalidOperator",
"type": "error"
},
{
"inputs": [
{
"internalType": "address",
"name": "owner",
"type": "address"
}
],
"name": "ERC721InvalidOwner",
"type": "error"
},
{
"inputs": [
{
"internalType": "address",
"name": "receiver",
"type": "address"
}
],
"name": "ERC721InvalidReceiver",
"type": "error"
},
{
"inputs": [
{
"internalType": "address",
"name": "sender",
"type": "address"
}
],
"name": "ERC721InvalidSender",
"type": "error"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "ERC721NonexistentToken",
"type": "error"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "InvalidTokenId",
"type": "error"
},
{
"inputs": [
{
"internalType": "address",
"name": "",
"type": "address"
}
],
"name": "OnlyOwner",
"type": "error"
},
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"internalType": "address",
"name": "owner",
"type": "address"
},
{
"indexed": true,
"internalType": "address",
"name": "approved",
"type": "address"
},
{
"indexed": true,
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "Approval",
"type": "event"
},
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"internalType": "address",
"name": "owner",
"type": "address"
},
{
"indexed": true,
"internalType": "address",
"name": "operator",
"type": "address"
},
{
"indexed": false,
"internalType": "bool",
"name": "approved",
"type": "bool"
}
],
"name": "ApprovalForAll",
"type": "event"
},
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"internalType": "address",
"name": "from",
"type": "address"
},
{
"indexed": true,
"internalType": "address",
"name": "to",
"type": "address"
},
{
"indexed": true,
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "Transfer",
"type": "event"
},
{
"inputs": [
{
"internalType": "address",
"name": "to",
"type": "address"
},
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "approve",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "owner",
"type": "address"
}
],
"name": "balanceOf",
"outputs": [
{
"internalType": "uint256",
"name": "",
"type": "uint256"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [],
"name": "counter",
"outputs": [
{
"internalType": "uint256",
"name": "",
"type": "uint256"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [],
"name": "generateRandomColor",
"outputs": [
{
"internalType": "string",
"name": "",
"type": "string"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "getApproved",
"outputs": [
{
"internalType": "address",
"name": "",
"type": "address"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "owner",
"type": "address"
}
],
"name": "getNFftsOwned",
"outputs": [
{
"components": [
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
},
{
"internalType": "string",
"name": "metadata",
"type": "string"
}
],
"internalType": "struct RandomColorNFT.TokenAndMetatdata[]",
"name": "",
"type": "tuple[]"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "owner",
"type": "address"
},
{
"internalType": "address",
"name": "operator",
"type": "address"
}
],
"name": "isApprovedForAll",
"outputs": [
{
"internalType": "bool",
"name": "",
"type": "bool"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "_to",
"type": "address"
}
],
"name": "mintTo",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [],
"name": "name",
"outputs": [
{
"internalType": "string",
"name": "",
"type": "string"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "ownerOf",
"outputs": [
{
"internalType": "address",
"name": "",
"type": "address"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "_tokenId",
"type": "uint256"
}
],
"name": "render",
"outputs": [
{
"internalType": "string",
"name": "",
"type": "string"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "from",
"type": "address"
},
{
"internalType": "address",
"name": "to",
"type": "address"
},
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "safeTransferFrom",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "from",
"type": "address"
},
{
"internalType": "address",
"name": "to",
"type": "address"
},
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
},
{
"internalType": "bytes",
"name": "data",
"type": "bytes"
}
],
"name": "safeTransferFrom",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "operator",
"type": "address"
},
{
"internalType": "bool",
"name": "approved",
"type": "bool"
}
],
"name": "setApprovalForAll",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "_tokenId",
"type": "uint256"
}
],
"name": "shuffleColor",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
},
{
"inputs": [
{
"internalType": "bytes4",
"name": "interfaceId",
"type": "bytes4"
}
],
"name": "supportsInterface",
"outputs": [
{
"internalType": "bool",
"name": "",
"type": "bool"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [],
"name": "symbol",
"outputs": [
{
"internalType": "string",
"name": "",
"type": "string"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "",
"type": "uint256"
}
],
"name": "tokenIdToColor",
"outputs": [
{
"internalType": "string",
"name": "",
"type": "string"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "uint256",
"name": "_tokenId",
"type": "uint256"
}
],
"name": "tokenURI",
"outputs": [
{
"internalType": "string",
"name": "",
"type": "string"
}
],
"stateMutability": "view",
"type": "function"
},
{
"inputs": [
{
"internalType": "address",
"name": "from",
"type": "address"
},
{
"internalType": "address",
"name": "to",
"type": "address"
},
{
"internalType": "uint256",
"name": "tokenId",
"type": "uint256"
}
],
"name": "transferFrom",
"outputs": [],
"stateMutability": "nonpayable",
"type": "function"
}
]
}