r/place on Reddit is a fun event and I thought it could be nice to bring it to Stellar

What it does

It allows to set the color of a pixel on an image. Each interaction is recorded in the blockchain, one can replay the events to build the full picture.

It provides

  • the smart contract with basic authorisation and authentication,
  • the webapp (front and back end)
  • and an SDK (with an example) to allow others build on top of the contract.

Challenges we ran into

Calling the contract, with the right types from javascript was the main challenge. Also, adding the footprint to the transaction gave me some good thinking moment. I finally came to this helper method which allows to use the same method call to build the XDR to simulateTransaction or sendTransaction.

Hopefully could help others too:

// Add the footprint to the transaction
function decorateFootprint(t, footprint) {
    if (footprint) {
        let source = new SorobanClient.Account(t.source, `${parseInt(t.sequence) - 1}`);
        let op = SorobanClient.Operation.invokeHostFunction({
            function: t.operations[0].function,
            parameters: t.operations[0].parameters,
            footprint: SorobanClient.xdr.LedgerFootprint.fromXDR(footprint, 'base64'),

        return new SorobanClient.TransactionBuilder(source, {
            fee: '1000',
            networkPassphrase: SorobanClient.Networks.FUTURENET,

    return t;

// Example to call a contract method
function my_function(contractId, source, accountId, footprint) {
    let contract = new SorobanClient.Contract(contractId)
        let t = new SorobanClient.TransactionBuilder(source, {
        fee: '100',
        networkPassphrase: SorobanClient.Networks.FUTURENET,

    return decorateFootprint(t, footprint);


I wanted to add a more user-friendly way to connect to the Stellar blockchain and to be able to test the dApp without technical settings. The discord authentication that create a Stellar account while the end-user keeps its secret key is a nice addition to the project.

Note: the key is not stored securely

What we learned

A lot of new things but the biggest step is related to the ScVal type building

What's next for PixelWar

This smart contract should be deployed through a deployer contract to provide better experience. That would allow anyone to easily launch its own pixel war event.

Built With

Share this project: