canvas

What is canvas?

The canvas npm package is a powerful library that allows Node.js users to draw 2D graphics on the fly using a Canvas API similar to the one provided by browsers. It can be used for creating images, manipulating graphics, and generating dynamic visual content.

What are canvas's main functionalities?

Drawing shapes

This feature allows you to draw basic shapes such as rectangles, circles, and lines on the canvas.

const { createCanvas } = require('canvas');
const canvas = createCanvas(200, 200);
const ctx = canvas.getContext('2d');

// Draw a rectangle
ctx.fillStyle = 'green';
ctx.fillRect(10, 10, 150, 100);

Text manipulation

With the canvas package, you can add text to your images, apply fonts, styles, and rotation.

const { createCanvas } = require('canvas');
const canvas = createCanvas(200, 200);
const ctx = canvas.getContext('2d');

// Draw text
ctx.font = '30px Impact';
ctx.rotate(0.1);
ctx.fillText('Awesome!', 50, 100);

Image manipulation

This feature allows you to load images, draw them onto the canvas, and manipulate their appearance.

const { createCanvas, loadImage } = require('canvas');
const canvas = createCanvas(200, 200);
const ctx = canvas.getContext('2d');

loadImage('image.png').then((image) => {
  ctx.drawImage(image, 50, 50, 150, 100);
});

Pixel manipulation

Canvas allows direct pixel manipulation for effects like inversion, brightness, contrast, and more.

const { createCanvas } = require('canvas');
const canvas = createCanvas(200, 200);
const ctx = canvas.getContext('2d');

// Draw a rectangle
ctx.fillStyle = 'red';
ctx.fillRect(0, 0, 50, 50);

// Get pixel data
const imageData = ctx.getImageData(0, 0, 50, 50);

// Manipulate pixels
for (let i = 0; i < imageData.data.length; i += 4) {
  imageData.data[i] = 255 - imageData.data[i]; // Invert red
  imageData.data[i+1] = 255 - imageData.data[i+1]; // Invert green
  imageData.data[i+2] = 255 - imageData.data[i+2]; // Invert blue
}

// Put the image data back onto the canvas
ctx.putImageData(imageData, 0, 0);

Other packages similar to canvas

fabric

Fabric.js is a powerful and rich graphics library, similar to canvas, but it runs in the browser. It provides interactive object model on top of canvas element and also has SVG-to-canvas (and canvas-to-SVG) parser.

konva

Konva.js is an HTML5 2D canvas library for desktop and mobile applications. It extends the 2D context by enabling canvas interactivity and much more. It is similar to canvas but with additional features like event handling, layering, filtering, caching, and animations.

pixijs

PixiJS is a 2D webGL renderer with a seamless canvas fallback that can be used to create rich interactive graphics, cross-platform applications, and games. Unlike canvas, PixiJS is focused on webGL and provides a faster rendering for complex scenes.

p5

p5.js is a client-side library that enables creative coding with a focus on making coding accessible for artists, designers, educators, and beginners. It is similar to canvas but emphasizes an easy-to-use API and includes a full set of drawing functionality.

README

node-canvas

Node canvas is a Cairo backed Canvas implementation for NodeJS.

Authors

• TJ Holowaychuk (visionmedia)

Installation

$ npm install canvas

Unless previously installed you'll need Cairo. For system-specific installation view the Wiki.

Screencasts

• Introduction

Example

var Canvas = require('canvas')
  , canvas = new Canvas(200,200)
  , ctx = canvas.getContext('2d');

ctx.font = '30px Impact';
ctx.rotate(.1);
ctx.fillText("Awesome!", 50, 100);

var te = ctx.measureText('Awesome!');
ctx.strokeStyle = 'rgba(0,0,0,0.5)';
ctx.beginPath();
ctx.lineTo(50, 102);
ctx.lineTo(50 + te.width, 102);
ctx.stroke();

console.log('<img src="' + canvas.toDataURL() + '" />');

Non-Standard API

node-canvas extends the canvas API to provide interfacing with node, for example streaming PNG data, converting to a Buffer instance, etc. Among the interfacing API, in some cases the drawing API has been extended for SSJS image manipulation / creation usage, however keep in mind these additions may fail to render properly within browsers.

Image#src=Buffer

node-canvas adds Image#src=Buffer support, allowing you to read images from disc, redis, etc and apply them via ctx.drawImage(). Below we draw scaled down squid png by reading it from the disk with node's I/O.

fs.readFile(__dirname + '/images/squid.png', function(err, squid){
  if (err) throw err;
  img = new Image;
  img.src = squid;
  ctx.drawImage(img, 0, 0, img.width / 4, img.height / 4);
});

Below is an example of a canvas drawing it-self as the source several time:

var img = new Image;
img.src = canvas.toBuffer();
ctx.drawImage(img, 0, 0, 50, 50);
ctx.drawImage(img, 50, 0, 50, 50);
ctx.drawImage(img, 100, 0, 50, 50);

Image#dataMode

node-canvas adds Image#dataMode support, which can be used to opt-in to mime data tracking of images (currently only JPEGs).

When mime data is tracked, in PDF mode JPEGs can be embedded directly into the output, rather than being re-encoded into PNG. This can drastically reduce filesize, and speed up rendering.

var img = new Image;
img.dataMode = Image.MODE_IMAGE; // Only image data tracked
img.dataMode = Image.MODE_MIME; // Only mime data tracked
img.dataMode = Image.MODE_MIME | Image.MODE_IMAGE; // Both are tracked

If image data is not tracked, and the Image is drawn to an image rather than a PDF canvas, the output will be junk. Enabling mime data tracking has no benefits (only a slow down) unless you are generating a PDF.

Canvas#createPNGStream()

To create a PNGStream simply call canvas.createPNGStream(), and the stream will start to emit data events, finally emitting end when finished. If an exception occurs the error event is emitted.

var fs = require('fs')
  , out = fs.createWriteStream(__dirname + '/text.png')
  , stream = canvas.createPNGStream();

stream.on('data', function(chunk){
  out.write(chunk);
});

stream.on('end', function(){
  console.log('saved png');
});

Currently only sync streaming is supported, however we plan on supporting async streaming as well (of course :) ). Until then the Canvas#toBuffer(callback) alternative is async utilizing eio_custom().

Canvas#createJPEGStream()

You can likewise create a JPEGStream by calling canvas.createJPEGStream() with some optional parameters; functionality is otherwise identical to createPNGStream(). See examples/crop.js for an example.

Canvas#toBuffer()

A call to Canvas#toBuffer() will return a node Buffer instance containing all of the PNG data.

canvas.toBuffer();

Canvas#toBuffer() async

Optionally we may pass a callback function to Canvas#toBuffer(), and this process will be performed asynchronously, and will callback(err, buf).

canvas.toBuffer(function(err, buf){

});

Canvas#toDataURL() async

Optionally we may pass a callback function to Canvas#toDataURL(), and this process will be performed asynchronously, and will callback(err, str).

canvas.toDataURL(function(err, str){

});

or specify the mime type:

canvas.toDataURL('image/png', function(err, str){

});

CanvasRenderingContext2d#patternQuality

Given one of the values below will alter pattern (gradients, images, etc) render quality, defaults to good.

• fast
• good
• best

CanvasRenderingContext2d#textDrawingMode

Can be either path or glyph. Using glyph is much faster than path for drawing, and when using a PDF context will embed the text natively, so will be selectable and lower filesize. The downside is that cairo does not have any subpixel precision for glyph, so this will be noticeably lower quality for text positioning in cases such as rotated text. Also, strokeText in glyph will act the same as fillText, except using the stroke style for the fill.

Defaults to path.

This property is tracked as part of the canvas state in save/restore.

Global Composite Operations

In addition to those specified and commonly implemented by browsers, the following have been added:

• multiply
• screen
• overlay
• hard-light
• soft-light
• hsl-hue
• hsl-saturation
• hsl-color
• hsl-luminosity

Anti-Aliasing

Set anti-aliasing mode

• default
• none
• gray
• subpixel

For example:

ctx.antialias = 'none';

PDF Support

Basic PDF support was added in 0.11.0. Make sure to install cairo with --enable-pdf=yes for the PDF backend. node-canvas must know that it is creating a PDF on initialization, using the "pdf" string:

var canvas = new Canvas(200, 500, 'pdf');

An additional method .addPage() is then available to create multiple page PDFs:

ctx.font = '22px Helvetica';
ctx.fillText('Hello World', 50, 80);
ctx.addPage();

ctx.font = '22px Helvetica';
ctx.fillText('Hello World 2', 50, 80);
ctx.addPage();

ctx.font = '22px Helvetica';
ctx.fillText('Hello World 3', 50, 80);
ctx.addPage();

Benchmarks

Although node-canvas is extremely new, and we have not even begun optimization yet it is already quite fast. For benchmarks vs other node canvas implementations view this gist, or update the submodules and run $ make benchmark yourself.

Contribute

Want to contribute to node-canvas? patches for features, bug fixes, documentation, examples and others are certainly welcome. Take a look at the issue queue for existing issues.

Examples

Examples are placed in ./examples, be sure to check them out! most produce a png image of the same name, and others such as live-clock.js launch an http server to be viewed in the browser.

Testing

If you have not previously, init git submodules:

$ git submodule update --init

Build node-canvas:

$ node-waf configure build

Unit tests:

$ make test

Visual tests:

$ make test-server

Versions

Tested with and designed for:

• node 0.4.2
• cairo 1.8.6

For node 0.2.x node-canvas <= 0.4.3 may be used, 0.5.0 and above are designed for node 0.4.x only.

License

(The MIT License)

Copyright (c) 2010 LearnBoost <[e-mail]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.