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.
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-Originheader. - iOS has resource limits for canvas size & image size. Look here for details and possible solutions.
- If you plan to show images on screen after load, you should parse
exifheader to get proper orientation. Images can be rotated.
- 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
- Saving image:
- Some ancient browsers do not support
canvas.toBlob()method. Usepica.toBlob(), it includes required shim. - It's a good idea to keep
exifdata, to avoid rotation info loss. The most simple way is to cut original header and glue it to resized result. Look here for examples.
- Some ancient browsers do not support
- 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 picabower:
bower install picaAttention!. 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').
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', 90))
.then(blob => console.log('resized to canvas & created blob!'));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 viacreateImageBitmap()('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
cibwill depend on your browser. Result withoutcibwill be predictable and good.
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.
- 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 termitated on rejection.
- quality - 0..3. Default =
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).
Convenience method, similar to canvas.toBlob(), but with
promise interface & polyfill for old browsers.
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.
- toHeigh - output height.
- 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
picato create new one.
Result is Promise, resolved with resized rgba buffer.
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 :)
Ifter 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.
We didn't have time to test all possible combinations, but in general:
- Top level API should work in all browsers, supporting canvas and typed arrays.
- Webworkers, WebAssembly and createImageBitmap are not required, but they will be used if available.
- If you plan to use only pure math core, then typed arrays support will be enougth.
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:
- discussions on stackoverflow: 1, 2, 3.
- chromium skia sources: image_operations.cc, convolver.cc.