node-canvas

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This is the documentation for version 2.0.0-alpha

Alpha versions of 2.0 can be installed using npm install canvas@next.

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

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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

By default, binaries for macOS, Linux and Windows will be downloaded. If you want to build from source, use npm install --build-from-source.

Currently the minimum version of node required is 4.0.0

Compiling

If you don't have a supported OS or processor architecture, or you use --build-from-source, the module will be compiled on your system. Unless previously installed you'll need Cairo and Pango. For system-specific installation view the Wiki.

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

OS	Command
OS X	Using Homebrew: brew install pkg-config cairo pango libpng jpeg giflib Using MacPorts: port install pkgconfig cairo pango libpng jpeg giflib
Ubuntu	sudo apt-get install libcairo2-dev libjpeg-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

Mac OS X v10.11+: If you have recently updated to Mac OS X v10.11+ and are experiencing trouble when compiling, run the following command: xcode-select --install. Read more about the problem on Stack Overflow.

Screencasts

• Introduction

Example

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

// Write "Awesome!"
ctx.font = '30px Impact'
ctx.rotate(0.1)
ctx.fillText('Awesome!', 50, 100)

// Draw line under text
var text = ctx.measureText('Awesome!')
ctx.strokeStyle = 'rgba(0,0,0,0.5)'
ctx.beginPath()
ctx.lineTo(50, 102)
ctx.lineTo(50 + text.width, 102)
ctx.stroke()

// Draw cat with lime helmet
loadImage('examples/images/lime-cat.jpg').then((image) => {
  ctx.drawImage(image, 50, 0, 70, 70)

  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.

const { Image } = require('canvas');
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:

const { Image } = require('canvas');
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.

const { Image } = require('canvas');
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(options)

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

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

stream.pipe(out);

out.on('finish', function(){
  console.log('The PNG file was created.');
});

To encode indexed PNGs from canvases with pixelFormat: 'A8' or 'A1', provide an options object:

var palette = new Uint8ClampedArray([
  //r    g    b    a
    0,  50,  50, 255, // index 1
   10,  90,  90, 255, // index 2
  127, 127, 255, 255
  // ...
]);
canvas.pngStream({
  palette: palette,
  backgroundIndex: 0 // optional, defaults to 0
})

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
  , disableChromaSubsampling: false // true to disable 2x2 subsampling of the chroma components, 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

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 registerFont().

You need to call it before the Canvas is created

const { registerFont, createCanvas } = require('canvas');
registerFont('comicsans.ttf', {family: 'Comic Sans'});

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

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

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. If you are building cairo from source, be sure to use --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 = createCanvas(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 = createCanvas(200, 500, 'svg');
// Use the normal primitives.
fs.writeFile('out.svg', canvas.toBuffer());

SVG Image Support

If librsvg is on your system when node-canvas is installed, node-canvas can render SVG images within your canvas context. Note that this currently works by simply rasterizing the SVG image using librsvg.

var img = new Image;
img.src = './example.svg';
ctx.drawImage(img, 0, 0, 100, 100);

Image pixel formats (experimental)

node-canvas has experimental support for additional pixel formats, roughly following the Canvas color space proposal.

var canvas = createCanvas(200, 200);
var ctx = canvas.getContext('2d', {pixelFormat: 'A8'});

By default, canvases are created in the RGBA32 format, which corresponds to the native HTML Canvas behavior. Each pixel is 32 bits. The JavaScript APIs that involve pixel data (getImageData, putImageData) store the colors in the order {red, green, blue, alpha} without alpha pre-multiplication. (The C++ API stores the colors in the order {alpha, red, green, blue} in native-endian ordering, with alpha pre-multiplication.)

These additional pixel formats have experimental support:

• RGB24 Like RGBA32, but the 8 alpha bits are always opaque. This format is always used if the alpha context attribute is set to false (i.e. canvas.getContext('2d', {alpha: false})). This format can be faster than RGBA32 because transparency does not need to be calculated.
• A8 Each pixel is 8 bits. This format can either be used for creating grayscale images (treating each byte as an alpha value), or for creating indexed PNGs (treating each byte as a palette index) (see the example using alpha values with fillStyle and the example using imageData).
• RGB16_565 Each pixel is 16 bits, with red in the upper 5 bits, green in the middle 6 bits, and blue in the lower 5 bits, in native platform endianness. Some hardware devices and frame buffers use this format. Note that PNG does not support this format; when creating a PNG, the image will be converted to 24-bit RGB. This format is thus suboptimal for generating PNGs. ImageData instances for this mode use a Uint16Array instead of a Uint8ClampedArray.
• A1 Each pixel is 1 bit, and pixels are packed together into 32-bit quantities. The ordering of the bits matches the endianness of the platform: on a little-endian machine, the first pixel is the least- significant bit. This format can be used for creating single-color images. Support for this format is incomplete, see note below.
• RGB30 Each pixel is 30 bits, with red in the upper 10, green in the middle 10, and blue in the lower 10. (Requires Cairo 1.12 or later.) Support for this format is incomplete, see note below.

Notes and caveats:

• Using a non-default format can affect the behavior of APIs that involve pixel data:

  • context2d.createImageData The size of the array returned depends on the number of bit per pixel for the underlying image data format, per the above descriptions.
  • context2d.getImageData The format of the array returned depends on the underlying image mode, per the above descriptions. Be aware of platform endianness, which can be determined using node.js's os.endianness() function.
  • context2d.putImageData As above.

• A1 and RGB30 do not yet support getImageData or putImageData. Have a use case and/or opinion on working with these formats? Open an issue and let us know! (See #935.)

• A1, A8, RGB30 and RGB16_565 with shadow blurs may crash or not render properly.

• The ImageData(width, height) and ImageData(Uint8ClampedArray, width) constructors assume 4 bytes per pixel. To create an ImageData instance with a different number of bytes per pixel, use new ImageData(new Uint8ClampedArray(size), width, height) or new ImageData(new Uint16ClampedArray(size), width, height).

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 <dev@learnboost.com>

Copyright (c) 2014 Automattic, Inc and contributors <dev@automattic.com>

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.