Million Ether Homepage

Storing Colors in a Pixel Grid

Million Ether Page Essential Smart Contract

Tip: Mining only when there are transactions

Tip: Using Remix IDE

Getting Started

Getting The Pixels and Ethereum Events

Building a DApp UI

Drawing in HTML Canvas

Using Websockets with Web3

Web3 and Events Over WebSockets

Drawing Pixels from the Blockchain

Viewing Pixels

Buying An Image -- Reading Pixels

Buying An Image -- Sending Transactions

Pixel bidding and structs

Payable Contracts, payable Functions

Contract Owner and require

Payable color pixel function

Buying Pixels

Sending funds from a contract

The safer way to send funds from a contract

The Pull Payment Pattern - Withdraw payments when outbid

Million Ether Homepage Summary

Auctioning Pixels

solidity

In this course, we'll build a DApp inspired by [The Million Dollar Homepage](https://en.wikipedia.org/wiki/The_Million_Dollar_Homepage). We'll build a 1 million pixel page, and each pixel can be colored by placing the highest bid.

In this course we'll walk through every step of writing a smart contract, deploying it, hosting auctions, and connecting it to the browser through web3. 

If you're looking to build real, full-stack ethereum Dapps, then this is a perfectly-sized example.

We'll build a DApp inspired by the Million Dollar Homepage, powered by an auction on the Ethereum blockchain

We have PixelChanged events coming over Websockets and we need to draw them on our canvas.

What we need to do is capture each event, parse the color string, and then write the pixel to the canvas.

First, take a look at the event that comes through. The x, y, and color are all strings. We want them all to be integers.

Actually, x and y are plain integers and we need to parse color into three integers: R G B, each one being between 0 and 255. 

Let's write a function that, given an event, will parse these values and write the pixel.

// * We haven't written `writePixel` yet, but we well
// * We also need to write parseColor

function writePixelWithEvent(event) {
  let { x, y, color } = event.returnValues;
  writePixel(
    parseInt(x),
    parseInt(y),
    parseColor(color)
  );
}

 will take this raw color string, split the bytes, and convert it to a three-element array: each element corresponding to a channel red, green, blue. You don't need to worry too much about how this function works. Just know that it returns a three-element array.

 this particular pixel. What we're going to do is read the current image data, find the index for this pixel, and update its color. 

---
title: Drawing Pixels from the Blockchain


slug: 'drawing-pixels-from-the-blockchain'
thumbnail: https://embed-ssl.wistia.com/deliveries/7d1b19effa31b750310a7090acb8f49215939a40.jpg

privateVideoUrl: https://fullstack.wistia.com/medias/8yptpb25jv



---

We have PixelChanged events coming over Websockets and we need to draw them on our canvas.

What we need to do is capture each event, parse the color string, and then write the pixel to the canvas.

Let's take each step at a time.

First, take a look at the event that comes through. The x, y, and color are all strings. We want them all to be integers.

Actually, x and y are plain integers and we need to parse color into three integers: R G B, each one being between 0 and 255. 

Let's write a function that, given an event, will parse these values and write the pixel.

```javascript
// * We haven't written `writePixel` yet, but we well
// * We also need to write parseColor

function writePixelWithEvent(event) {
  let { x, y, color } = event.returnValues;
  writePixel(
    parseInt(x),
    parseInt(y),
    parseColor(color)
  );
}
```

`parseColor` will take this raw color string, split the bytes, and convert it to a three-element array: each element corresponding to a channel red, green, blue. You don't need to worry too much about how this function works. Just know that it returns a three-element array.

```javascript
function parseColor(rawColor) {
  let rgb = [];
  for (let i = 0; i < rawColor.length; i += 2) {
    let chunk = rawColor.substring(i, i + 2);
    if (chunk !== '0x') {
      rgb.push(parseInt(chunk, 16).toString(10));
    }
  }
  return rgb;
}
```



Now we need to _write_ this particular pixel. What we're going to do is read the current image data, find the index for this pixel, and update its color.

Learn

The newline Guide to Building Your First GraphQL Server with Node and TypeScript

Teach

Amelia Wattenberger

Author of Fullstack D3

Community

Million Ether Homepage

WELCOMEIntroduction

Introduction:

Start Here

Module 1: Getting Started

Lesson 1: (3:23)

Million Ether Homepage

Lesson 2: (3:18)

Storing Colors in a Pixel Grid

Lesson 3: (4:29)

Million Ether Page Essential Smart Contract

Lesson 4: (1:56)

Tip: Mining only when there are transactions

Lesson 5: (7:08)

Tip: Using Remix IDE

Module 2: Viewing Pixels

Lesson 1: (6:08)

Getting The Pixels and Ethereum Events

Lesson 2: (7:15)

Gas

Lesson 3: (1:01)

Building a DApp UI

Lesson 4: (3:09)

Drawing in HTML Canvas

Lesson 5: (1:15)

Using Websockets with Web3

Lesson 6: (3:09)

Web3 and Events Over WebSockets

Lesson 7: (3:41)

Drawing Pixels from the Blockchain

Module 3: Buying Pixels

Lesson 1: (1:57)

Buying An Image -- Reading Pixels

Lesson 2: (4:51)

Buying An Image -- Sending Transactions

Lesson 3: (1:25)

Pixel bidding and structs

Lesson 4: (2:48)

Payable Contracts, payable Functions

Lesson 5: (3:35)

Contract Owner and require

Lesson 6: (2:38)

Payable color pixel function

Module 4: Auctioning Pixels

Lesson 1: (4:03)

Sending funds from a contract

Lesson 2: (3:26)

The safer way to send funds from a contract

Lesson 3: (4:37)

The Pull Payment Pattern - Withdraw payments when outbid

Lesson 4: (6:46)

Million Ether Homepage Summary

Drawing Pixels from the Blockchain