post: onchain nft tutorial

_posts/2024-09-23-onchain-nft-tutorial.md

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+---
+title: Building a fully onchain NFT
+date: 2024-09-23
+author: m
+tags: [starknet]
+---
+
+Let's build a fully onchain NFT. It will look something like this:
+
+<img src="/assets/img/nouns_glasses.svg" alt="Nouns Glasses" />
+
+To achieve this, there are three standards we need to stitch together - Data URI, Opensea Metadata Standard and SVG. We'll have a look at them individually first.
+
+### Data URI
+
+A [Data URI](https://en.wikipedia.org/wiki/Data_URI_scheme) is a way how to represent "stuff" inline, in a single string that web browsers and libraries understand. In our case, the "stuff" we need to represent is a JSON object, so our Data URI will look something like this:
+
+```json
+data:application/json,{"json":"object"}
+```
+
+### Opensea Metadata Standard
+
+What shape should the JSON in our Data URI be? EIP-721 does specify an optional metadata extension, but it's rather clunky. A de facto standard followed by many is [this one from Opensea](https://docs.opensea.io/docs/metadata-standards#metadata-structure) so it's the one we're going to use. At minimum then, our JSON will have `name`, `description` and `image` properties. Here's how it will look as a Data URI:
+
+```json
+data:application/json,{"name":"Our collection name","description":"Description of the particular item","image":"TODO"}
+```
+
+### SVG
+
+Last thing we need to figure out is how to put an image inside a JSON. Enter, Scalable Vector Graphics. [SVG](https://developer.mozilla.org/en-US/docs/Web/SVG) is a XML-based (i.e. textual) format to do 2D graphics. A small green square in SVG looks like this in code:
+
+```xml
+<svg width="50" height="50" xmlns="http://www.w3.org/2000/svg">
+  <rect x="0" y="0" width="50" height="50" fill="green" />
+</svg>
+```
+
+And like this when rendered:
+
+<img src="/assets/img/green_square.svg" alt="Green square" />
+
+SVG may look simple, but it is a rather powerful standard (it can do animations!) and it's widely supported. Every modern browser has a full SVG rendering engine.
+
+This `<svg>...</svg>` string is what we want to put as the content of the `image` attribute from the Opensea Metadata Standard. However, the spec says that `image` is an "URL" of the image and not the image itself. Well, we already know how to solve that - we'll encode the SVG in a Data URI! So the small green square will become this:
+
+```text
+data:image/svg+xml,<svg width='50' height='50' xmlns='http://www.w3.org/2000/svg'><rect x='0' y='0' width='50' height='50' fill='green' /></svg>
+```
+
+We've only replaced the double quotes with single quotes because SVG permits for using single quotes for arguments but JSON requires double quotes to represent strings.
+
+Note there are alternatives to using raw SVG to build onchain NFTs. A great example is [Blobert](https://github.com/BibliothecaDAO/codename-bobby-realms) that uses base64 encoded PNGs inside `<image>` tags.
+
+### Code
+
+Now that we've got the theory covered, let's get cooking.
+
+We can bootstrap our coding efforts using a template from the wonderful [Cairo Wizard from OpenZeppelin](https://wizard.openzeppelin.com/cairo#erc721). This deals with the boring part of a NFT implementation. We need to modify the template slightly. Instead of embedding the ERC721MixinImpl we'll only use the ERC721Impl and do the IERC721Metadata functions (`name`, `symbol` and most importantly `token_uri`) ourselves. That also means we can delete the ERC721 initializer from the constructor.
+
+The `token_uri` is the part we're interested in. This function must return a string representing the whole NFT combining the techniques described above. Because the string contains double quotes (`"`) and curly braces (`{}`), we have to escape them properly in Cairo. That means using `\"` for a literal double quote and `{{` or `}}` for curlies in the `format!` macro.
+
+As usual, the [code is open source](https://github.com/milancermak/nouns-glasses), so go check it out.
+
+### Free mint
+
+Not only is the whole code freely available, minting the NFT is free as well. Since this is a educational project, there's no frontend, you'll have to use Starkscan to call the `mint` function. On the [contract page](https://starkscan.co/contract/0x03e4b9cd4a127040fd29a0373bd62ca6a3135cc6a9b3a10588ce664ac6478e9c#read-write-contract-sub-write), connect your wallet (1) and then execute the `mint` function (2). There's no limit to the number of times you can mint, but note all images from a single address will look the same as the address is used to generate the rim color.
+
+Happy minting.