When you resize images using modern browsers' canvas low quality interpolation algorythms are used by default. That's why we wrote pica.

Monthly Downloads: 0
Programming language: JavaScript
License: MIT License
Tags: Image Processing     Image     Scale     Canvas     Lanczos     Resize    
Latest version: v5.1.1

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pica - high quality image resize in browser

Build Status NPM version

Resize images in browser without pixelation and reasonably fast. Autoselect the best of available technologies: webworkers, webassembly, createImageBitmap, pure JS.


With pica you can:

  • Reduce upload size for large images, saving upload time.
  • Saves server resources on image processing.
  • Generate thumbnails in browser.
  • ...

Note. Old browsers may need Promise polyfill to work. For example, lie.

Prior to use

Here is a short list of problems you can face:

  • Loading image:
    • Due to JS security restrictions, you can process images from the same domain or local files only. If you load images from remote domain use proper Access-Control-Allow-Origin header.
    • iOS has a memory limits for canvas elements, that may cause problems in some cases, more details.
    • If you plan to show images on screen after load, you should parse exif header to get proper orientation. Images can be rotated.
  • Saving image:
    • Some ancient browsers do not support canvas.toBlob() method. Use pica.toBlob(), it includes required shim.
    • It's a good idea to keep exif data, to avoid rotation info loss. The most simple way is to cut original header and glue it to resized result. Look here for examples.
  • Quality
    • JS canvas does not support access to info about gamma correction. Bitmaps have 8 bits per channel. That causes some quality loss, because with gamma correction precision could be 12 bits per channel.
    • Precision loss will not be noticeable for ordinary images like kittens, selfies and so on. But we don't recommend this library for resizing professional quality images.


node.js (to develop, build via browserify and so on):

npm install pica

Transforms plugins for build via browserify:

npm install babelify @babel/core @babel/preset-env


bower install pica

Attention!. Compiled files are in /dist folder! If you wish to load module in node.js style as require('pica') - your project MUST be compiled with browserify to properly use Web Workers. In other case - use require('pica/dist/pica').

Webpack notice

If you use Webpack to bundle your application, you probably need to define a resolve alias into your webpack config, like this:

  resolve: {
    alias: {
      // Use compiled pica files from /dist folder
      pica: 'pica/dist/pica.js',

After that, you will be able to use pica as usual:

import Pica from 'pica';
const pica = Pica();
pica.resize(img, canvas).then(...);


const pica = require('pica')();

// Resize from Canvas/Image to another Canvas
pica.resize(from, to, {
  unsharpAmount: 80,
  unsharpRadius: 0.6,
  unsharpThreshold: 2
.then(result => console.log('resize done!'));

// Resize & convert to blob
pica.resize(from, to)
  .then(result => pica.toBlob(result, 'image/jpeg', 0.90))
  .then(blob => console.log('resized to canvas & created blob!'));


new Pica(config)

Create resizer instance with given config (optional):

  • tile - tile width/height. Images are processed by regions, to restrict peak memory use. Default 1024.
  • features - list of features to use. Default is [ 'js', 'wasm', 'ww' ]. Can be [ 'js', 'wasm', 'cib', 'ww' ] or [ 'all' ]. Note, resize via createImageBitmap() ('cib') disabled by default due problems with quality.
  • idle - cache timeout, ms. Webworkers create is not fast. This option allow reuse webworkers effectively. Default 2000.
  • concurrency - max webworkers pool size. Default is autodetected CPU count, but not more than 4.

Important! Latest browsers may support resize via createImageBitmap. You can try this feature by enabling chrome://flags/#enable-experimental-canvas-features in Chrome AND enabling cib in pica options:

const pica = require('pica')({ features: [ 'js', 'wasm', 'ww', 'cib' ] });

But, as you can see in demo, result is still pixelated. So:

  • createImageBitmap() is used for non-blocking image decode (when available)
  • It's resize feature is blocked by default pica config. Enable it only on your own risk. Result with enabled cib will depend on your browser. Result without cib will be predictable and good.

.resize(from, to, options) -> Promise

Resize image from one canvas (or image) to another. Sizes are taken from source and destination objects.

  • from - source canvas or image.
  • to - destination canvas, its size is supposed to be non-zero.
  • options - quality (number) or object:
    • quality - 0..3. Default = 3 (lanczos, win=3).
    • alpha - use alpha channel. Default = false.
    • unsharpAmount - >=0, in percents. Default = 0 (off). Usually between 50 to 100 is good.
    • unsharpRadius - 0.5..2.0. By default it's not set. Radius of Gaussian blur. If it is less than 0.5, Unsharp Mask is off. Big values are clamped to 2.0.
    • unsharpThreshold - 0..255. Default = 0. Threshold for applying unsharp mask.
    • cancelToken - Promise instance. If defined, current operation will be terminated on rejection.

Result is Promise, resolved with to on success.

(!) If you need to process multiple images, do it sequentially to optimize CPU & memory use. Pica already knows how to use multiple cores (if browser allows).

.toBlob(canvas, mimeType [, quality]) -> Promise

Convenience method, similar to canvas.toBlob(), but with promise interface & polyfill for old browsers.

.resizeBuffer(options) -> Promise

Supplementary method, not recommended for direct use. Resize Uint8Array with raw RGBA bitmap (don't confuse with jpeg / png / ... binaries). It does not use tiles & webworkers. Left for special cases when you really need to process raw binary data (for example, if you decode jpeg files "manually").

  • options:
    • src - Uint8Array with source data.
    • width - src image width.
    • height - src image height.
    • toWidth - output width, >=0, in pixels.
    • toHeight - output height, >=0, in pixels.
    • quality - 0..3. Default = 3 (lanczos, win=3).
    • alpha - use alpha channel. Default = false.
    • unsharpAmount - >=0, in percents. Default = 0 (off). Usually between 50 to 100 is good.
    • unsharpRadius - 0.5..2.0. Radius of Gaussian blur. If it is less than 0.5, Unsharp Mask is off. Big values are clamped to 2.0.
    • unsharpThreshold - 0..255. Default = 0. Threshold for applying unsharp mask.
    • dest - Optional. Output buffer to write data, if you don't wish pica to create new one.

Result is Promise, resolved with resized rgba buffer.

What is "quality"

Pica has presets to adjust speed/quality ratio. Simply use quality option param:

  • 0 - Box filter, window 0.5px
  • 1 - Hamming filter, window 1.0px
  • 2 - Lanczos filter, window 2.0px
  • 3 - Lanczos filter, window 3.0px

In real world you will never need to change default (max) quality. All this variations were implemented to better understand resize math :)

Unsharp mask

After scale down image can look a bit blured. It's good idea to sharpen it a bit. Pica has built-in "unsharp mask" filter (off by default). Set unsharpAmount to positive number to activate the filter.

Filter's parameters are similar to ones from Photoshop. We recommend to start with unsharpAmount = 80, unsharpRadius = 0.6 and unsharpThreshold = 2. There is a correspondence between UnsharpMask parameters in popular graphics software.

Browser support

We didn't have time to test all possible combinations, but in general:

Note. Though you can run this package on node.js, browsers are the main target platform. On server side we recommend to use sharp.


You can find these links useful:

Support pica

You can support this project via Tidelift subscription.