Canvas を使用した画像操作

Setting up Canvas

Canvas is an image manipulation tool that allows you to modify images with code. We'll explore how to use this module in a slash command to make a profile command. But first, you must go through the intense labor of installing Canvas. It's highly recommended that you use a Linux distribution for this because it'll be much easier to install on.

This guide is last tested with `canvas^2.6.0`, so make sure you have this or a similar version after installation. :::

Be sure that you're familiar with things like [async/await](/additional-info/async-await.md) and [object destructuring](/additional-info/es6-syntax.md#object-destructuring) before continuing, as we'll be making use of features like these. :::

Installation

Windows

You will need a package called Windows Build Tools. To install this, open Powershell as Administrator. You then can install it with your preferred package manager. It is also bundled with Chocolatey, should you choose that installation path.

npm install --global --production windows-build-tools
yarn global add --production windows-build-tools
pnpm add --global --production windows-build-tools
choco install -y python2 gtk-runtime microsoft-build-tools libjpeg-turbo

::::

Afterward, you should follow the instructions detailed hereopen in new window. Additionally, make sure Node and Cairo are both either 32-bit or 64-bit; having a 32-bit version of one and a 64-bit version of the other will cause errors.

If you are still unable to install Canvas, you might want to consider installing Microsoft Visual Studio 2015open in new window.

Other distributions

You can run one of the commands listed hereopen in new window to install the necessary tools Canvas needs.

Package installation

After installing all the necessary software, run the following command in your terminal:

npm install canvas
yarn add canvas
pnpm add canvas

Getting started

Here is the base code you'll be using to get started:

const { Client, Intents, MessageAttachment } = require('discord.js');
const Canvas = require('canvas');

const client = new Client({ intents: [Intents.FLAGS.GUILDS] });

client.once('ready', () => {
    console.log('Ready!');
});

client.on('interactionCreate', interaction => {
    if (!interaction.isCommand()) return;

    if (interaction.commandName === 'profile') {
        // ...
    }
});

client.login('your-token-goes-here');
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Remember to register the slash commands before continuing on with this section of the guide. You can view how to do that [here](/interactions/slash-commands.md#registering-slash-commands). :::

Basic image loading

The end goal will be to display the user's avatar and nickname.

After importing the Canvas module and initializing it, you should load the images. With Canvas, you have to specify where the image comes from first, naturally, and then specify how it gets loaded into the actual Canvas using context, which you will use to interact with Canvas.

`canvas` works almost identical to HTML5 Canvas. You can read the HTML5 Canvas tutorials on [w3Schools](https://www.w3schools.com/html/html5_canvas.asp) and [MDN](https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API) for more information later! :::

client.on('interactionCreate', async interaction => {
    if (!interaction.isCommand()) return;

    if (interaction.commandName === 'profile') {
        // Create a 700x250 pixel canvas and get its context
        // The context will be used to modify the canvas
        const canvas = Canvas.createCanvas(700, 250);
        const context = canvas.getContext('2d');
        // ...
    }
});




 
 
 
 



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Now, you need to load the image you want to use into Canvas. You can use node-canvas's loadImage() utility methodopen in new window to load images from local directories or URLs.

We'll be using this imageopen in new window as the background in the welcome image, but you can use whatever you want. Be sure to download the file, name it wallpaper.jpg, and save it inside the same directory as your main bot file.

client.on('interactionCreate', async interaction => {
    // ...
    const context = canvas.getContext('2d');

    const background = await Canvas.loadImage('./wallpaper.jpg');

    // This uses the canvas dimensions to stretch the image onto the entire canvas
    context.drawImage(background, 0, 0, canvas.width, canvas.height);

    // Use the helpful Attachment class structure to process the file for you
    const attachment = new MessageAttachment(canvas.toBuffer(), 'profile-image.png');

    interaction.reply({ files: [attachment] });
});




 
 
 
 
 
 
 
 
 

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Basic canvas preview

If you get an error such as `Error: error while reading from input stream`, then the file's provided path was incorrect. :::

Manipulating images

Next, let's place a border around the image for the sake of demonstration purposes.

client.on('interactionCreate', async interaction => {
    // ...
    context.drawImage(background, 0, 0, canvas.width, canvas.height);

    // Set the color of the stroke
    context.strokeStyle = '#0099ff';

    // Draw a rectangle with the dimensions of the entire canvas
    context.strokeRect(0, 0, canvas.width, canvas.height);
    // ...
});




 
 
 
 
 


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Image

A bit plain, right? Fear not, for you have a bit more to do until you reach completion. Since this guide page's goal is focused more on actual code than design, let's place a basic square-shaped avatar for now on the left side of the image. In the interest of coverage, you will also make it a circle afterward.

client.on('interactionCreate', async interaction => {
    // ...
    context.strokeRect(0, 0, canvas.width, canvas.height);

    const avatar = await Canvas.loadImage(interaction.user.displayAvatarURL({ format: 'jpg' }));

    // Draw a shape onto the main canvas
    context.drawImage(avatar, 25, 0, 200, canvas.height);
    // ...
});




 
 
 
 
 

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Image

It works well, but the avatar image itself seems a bit stretched out. Let's remedy that.

client.on('interactionCreate', async interaction => {
    // ...
    const avatar = await Canvas.loadImage(interaction.user.displayAvatarURL({ format: 'jpg' }));

    // Move the image downwards vertically and constrain its height to 200, so that it's square
    context.drawImage(avatar, 25, 25, 200, 200);
    // ...
});




 
 


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Image

The purpose of this small section is to demonstrate that working with Canvas is essentially a hit-and-miss workflow where you fiddle with properties until they work just right.

Since we covered how to load external images and fix dimensions, let's turn the avatar into a circle to improve the image's overall style.

client.on('interactionCreate', async interaction => {
    // ...
    context.strokeRect(0, 0, canvas.width, canvas.height);

    // Pick up the pen
    context.beginPath();

    // Start the arc to form a circle
    context.arc(125, 125, 100, 0, Math.PI * 2, true);

    // Put the pen down
    context.closePath();

    // Clip off the region you drew on
    context.clip();
    // ...
});




 
 
 
 
 
 
 
 
 
 
 


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Image

You can read more about `context.arc()` on [w3schools](https://www.w3schools.com/tags/canvas_arc.asp) or [MDN](https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/arc). :::

Adding in text

Now, let's quickly go over adding text to your image. This will help make the purpose of this image apparent since currently, it's just an avatar floating on a starry background that comes out of nowhere.

client.on('interactionCreate', async interaction => {
    // ...
    context.strokeRect(0, 0, canvas.width, canvas.height);

    // Select the font size and type from one of the natively available fonts
    context.font = '60px sans-serif';

    // Select the style that will be used to fill the text in
    context.fillStyle = '#ffffff';

    // Actually fill the text with a solid color
    context.fillText(interaction.member.displayName, canvas.width / 2.5, canvas.height / 1.8);
    // ...
});




 
 
 
 
 
 
 
 


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Image

If you get an error like `Fontconfig error: Cannot load default config file`, it means you do not have any fonts installed on your system. On Linux, you can run the following command to fix this: `sudo apt-get install fontconfig`. This might also need to be installed if you see boxes where the text should be. As for Windows, you will need to find a way to install fonts. :::

You may have noticed or considered that if a member's username is too long, then the output won't be quite nice. This is because the text overflows out of the canvas, and you don't have any measures in place for that. Let's take care of this issue!

// Pass the entire Canvas object because you'll need access to its width and context
const applyText = (canvas, text) => {
    const context = canvas.getContext('2d');

    // Declare a base size of the font
    let fontSize = 70;

    do {
        // Assign the font to the context and decrement it so it can be measured again
        context.font = `${fontSize -= 10}px sans-serif`;
        // Compare pixel width of the text to the canvas minus the approximate avatar size
    } while (context.measureText(text).width > canvas.width - 300);

    // Return the result to use in the actual canvas
    return context.font;
};

client.on('interactionCreate', async interaction => {
    // ...
    context.strokeRect(0, 0, canvas.width, canvas.height);

    // Assign the decided font to the canvas
    context.font = applyText(canvas, interaction.member.displayName);
    context.fillStyle = '#ffffff';
    context.fillText(interaction.member.displayName, canvas.width / 2.5, canvas.height / 1.8);
    // ...
});
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 





 
 
 
 


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Before adjustment:

Before adjustment

After adjustment:

After adjustment

Let's move the welcome text inside the image itself instead of adding it outside as a nice finishing touch.

client.on('interactionCreate', async interaction => {
    // ...
    context.strokeRect(0, 0, canvas.width, canvas.height);

    // Slightly smaller text placed above the member's display name
    context.font = '28px sans-serif';
    context.fillStyle = '#ffffff';
    context.fillText('Profile', canvas.width / 2.5, canvas.height / 3.5);

    // Add an exclamation point here and below
    context.font = applyText(canvas, `${interaction.member.displayName}!`);
    context.fillStyle = '#ffffff';
    context.fillText(`${interaction.member.displayName}!`, canvas.width / 2.5, canvas.height / 1.8);
    // ...
});




 
 
 
 

 
 
 
 


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Final result

And that's it! We have covered the basics of image manipulation, text generation, and loading from a remote source.

Resulting code

If you want to compare your code to the code we've constructed so far, you can review it over on the GitHub repository here open in new window.

Last Updated: 2022/4/7 12:28:23