node-canvas

This is the documentation for the unreleased version 2.0

For the current version 1.x documentation, see the v1.x branch

Canvas graphics API backed by Cairo

node-canvas is a Cairo backed Canvas implementation for NodeJS.

Authors

• TJ Holowaychuk (tj)
• Nathan Rajlich (TooTallNate)
• Rod Vagg (rvagg)
• Juriy Zaytsev (kangax)

Installation

$ npm install canvas

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

Currently the minimum version of node required is 4.0.0

You can quickly install the dependencies by using the command for your OS:

OS | Command ----- | ----- OS X | brew install pkg-config cairo pango libpng jpeg giflib Ubuntu | sudo apt-get install libcairo2-dev libjpeg8-dev libpango1.0-dev libgif-dev build-essential g++ Fedora | sudo yum install cairo cairo-devel cairomm-devel libjpeg-turbo-devel pango pango-devel pangomm pangomm-devel giflib-devel Solaris | pkgin install cairo pango pkg-config xproto renderproto kbproto xextproto Windows | Instructions on our wiki

El Capitan users: If you have recently updated to El Capitan and are experiencing trouble when compiling, run the following command: xcode-select --install. Read more about the problem on Stack Overflow.

Screencasts

• Introduction

Example

var Canvas = require('canvas')
  , Image = Canvas.Image
  , 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#pngStream()

To create a PNGStream simply call canvas.pngStream(), 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.pngStream();

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#jpegStream() and Canvas#syncJPEGStream()

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

Note: At the moment, jpegStream() is the same as syncJPEGStream(), both are synchronous

var stream = canvas.jpegStream({
    bufsize: 4096 // output buffer size in bytes, default: 4096
  , quality: 75 // JPEG quality (0-100) default: 75
  , progressive: false // true for progressive compression, default: false
});

Canvas#toBuffer()

A call to Canvas#toBuffer() will return a node Buffer instance containing image data.

// PNG Buffer, default settings
var buf = canvas.toBuffer();

// PNG Buffer, zlib compression level 3 (from 0-9), faster but bigger
var buf2 = canvas.toBuffer(undefined, 3, canvas.PNG_FILTER_NONE);

// ARGB32 Buffer, native-endian
var buf3 = canvas.toBuffer('raw');
var stride = canvas.stride;
// In memory, this is `canvas.height * canvas.stride` bytes long.
// The top row of pixels, in ARGB order, left-to-right, is:
var topPixelsARGBLeftToRight = buf3.slice(0, canvas.width * 4);
var row3 = buf3.slice(2 * canvas.stride, 2 * canvas.stride + canvas.width * 4);

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() sync and async

The following syntax patterns are supported:

var dataUrl = canvas.toDataURL(); // defaults to PNG
var dataUrl = canvas.toDataURL('image/png');
canvas.toDataURL(function(err, png){ }); // defaults to PNG
canvas.toDataURL('image/png', function(err, png){ });
canvas.toDataURL('image/jpeg', function(err, jpeg){ }); // sync JPEG is not supported
canvas.toDataURL('image/jpeg', {opts...}, function(err, jpeg){ }); // see Canvas#jpegStream for valid options
canvas.toDataURL('image/jpeg', quality, function(err, jpeg){ }); // spec-following; quality from 0 to 1

Canvas.registerFont for bundled fonts

It can be useful to use a custom font file if you are distributing code that uses node-canvas and a specific font. Or perhaps you are using it to do automated tests and you want the renderings to be the same across operating systems regardless of what fonts are installed.

To do that, you should use Canvas.registerFont.

You need to call it before the Canvas is created

Canvas.registerFont('comicsans.ttf', {family: 'Comic Sans'});

var canvas = new Canvas(500, 500),
  ctx = canvas.getContext('2d');

ctx.font = '12px "Comic Sans"';
ctx.fillText(250, 10, 'Everyone hates this font :(');

The second argument is an object with properties that resemble the CSS properties that are specified in @font-face rules. You must specify at least family. weight, and style are optional (and default to "normal").

CanvasRenderingContext2D#patternQuality

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

• fast
• good
• best
• nearest
• bilinear

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.

CanvasRenderingContext2D#filter

Like patternQuality, but applies to transformations effecting more than just patterns. Defaults to good.

• fast
• good
• best
• nearest
• bilinear

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();

SVG support

Just like PDF support, make sure to install cairo with --enable-svg=yes. You also need to tell node-canvas that it is working on SVG upon its initialization:

var canvas = new Canvas(200, 500, 'svg');
// Use the normal primitives.
fs.writeFile('out.svg', canvas.toBuffer());

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

Install the node modules:

$ npm install

Build node-canvas:

$ node-gyp rebuild

Unit tests:

$ make test

Visual tests:

$ make test-server

License

(The MIT License)

Copyright (c) 2010 LearnBoost, and contributors <[email protected]>

Copyright (c) 2014 Automattic, Inc and contributors <[email protected]>

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.