255 lines
9.5 KiB
Markdown
255 lines
9.5 KiB
Markdown
pica - high quality image resize in browser
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===========================================
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[](https://github.com/nodeca/pica/actions/workflows/ci.yml)
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[](https://www.npmjs.org/package/pica)
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> Resize images in browser without pixelation and reasonably fast.
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> Autoselect the best of available technologies: webworkers,
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> webassembly, createImageBitmap, pure JS.
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[__demo__](http://nodeca.github.io/pica/demo/)
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With pica you can:
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- Reduce upload size for large images, saving upload time.
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- Saves server resources on image processing.
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- Generate thumbnails in browser.
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- ...
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**Note. If you need File/Blob resize (from form's file input), consider use
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[image-blob-reduce](https://github.com/nodeca/image-blob-reduce).** It has
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additional machinery to process orientation, keep EXIF metadata and so on.
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Migration from pica v6 to pica v7
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---------------------------------
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Multiply `unsharpAmount` by 2, divide `unsharpThreshold` by 2, example:
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- `pica@6`: `pica.resize(a, b, { unsharpAmount: 80, unsharpThreshold: 2 })`
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- `pica@7`: `pica.resize(a, b, { unsharpAmount: 160, unsharpThreshold: 1 })`
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Prior to use
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------------
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Here is a short list of problems you can face:
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- Loading image:
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- Due to JS security restrictions, you can process images
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from the same domain or local files only. If you load images from
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remote domain use proper `Access-Control-Allow-Origin` header.
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- iOS has a memory limits for canvas elements, that may cause
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problems in some cases, [more details](https://github.com/nodeca/pica/wiki/iOS-Memory-Limit).
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- If your source data is jpeg image, it can be rotated. Consider use
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[image-blob-reduce](https://github.com/nodeca/image-blob-reduce).
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- Saving image:
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- Some ancient browsers do not support `canvas.toBlob()` method.
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Use `pica.toBlob()`, it includes required shim.
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- For jpeg source, it's a good idea to keep `exif` data. Consider use
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[image-blob-reduce](https://github.com/nodeca/image-blob-reduce).
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- Quality
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- JS canvas does not support access to info about gamma correction.
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Bitmaps have 8 bits per channel. That causes some quality loss,
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because with gamma correction precision could be 12 bits per
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channel.
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- Precision loss will not be noticeable for ordinary images like
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kittens, selfies and so on. But we don't recommend this library
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for resizing professional quality images.
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Install
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-------
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```sh
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npm install pica
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```
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Use
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---
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```js
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const pica = require('pica')();
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// Resize from Canvas/Image to another Canvas
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pica.resize(from, to, {
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unsharpAmount: 160,
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unsharpRadius: 0.6,
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unsharpThreshold: 1
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})
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.then(result => console.log('resize done!'));
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// Resize & convert to blob
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pica.resize(from, to)
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.then(result => pica.toBlob(result, 'image/jpeg', 0.90))
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.then(blob => console.log('resized to canvas & created blob!'));
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```
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API
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---
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### new Pica(config)
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Create resizer instance with given config (optional):
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- __tile__ - tile width/height. Images are processed by regions,
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to restrict peak memory use. Default 1024.
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- __features__ - list of features to use. Default is
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`[ 'js', 'wasm', 'ww' ]`. Can be `[ 'js', 'wasm', 'cib', 'ww' ]`
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or `[ 'all' ]`.
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- __idle__ - cache timeout, ms. Webworkers create is not fast.
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This option allow reuse webworkers effectively. Default 2000.
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- __concurrency__ - max webworkers pool size. Default is autodetected
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CPU count, but not more than 4.
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- __createCanvas__ - function which returns a new canvas, used internally
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by pica.
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Default returns a [\<canvas\> element](https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API),
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but this function could return an [OffscreenCanvas](https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas)
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instead (to run pica in a Service Worker). Function signature: createCanvas(width: number, height: number): Canvas
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__Important!__ Latest browsers may support resize via [createImageBitmap](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/createImageBitmap).
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This feature is supported (`cib`) but disabled by default and not recommended
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for use. So:
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- `createImageBitmap()` is used for non-blocking image decode (when available,
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without downscale).
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- It's resize feature is blocked by default pica config. Enable it only on your
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own risk. Result with enabled `cib` will depend on your browser. Result
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without `cib` will be predictable and good.
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### .resize(from, to, options) -> Promise
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Resize image from one canvas (or image) to another. Sizes are
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taken from source and destination objects.
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- __from__ - source, can be `Canvas`, `Image` or `ImageBitmap`.
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- __to__ - destination canvas, its size is supposed to be non-zero.
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- __options__ - quality (number) or object:
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- __quality__ - 0..3. Default = `3` (lanczos, win=3).
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- __alpha__ - use alpha channel. Default = `false`.
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- __unsharpAmount__ - >=0, in percents. Default = `0` (off). Usually
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between 100 to 200 is good.
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- __unsharpRadius__ - 0.5..2.0. By default it's not set. Radius of Gaussian
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blur. If it is less than 0.5, Unsharp Mask is off. Big values are clamped
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to 2.0.
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- __unsharpThreshold__ - 0..255. Default = `0`. Threshold for
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applying unsharp mask.
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- __cancelToken__ - Promise instance. If defined, current
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operation will be terminated on rejection.
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Result is Promise, resolved with `to` on success.
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__(!)__ If you need to process multiple images, do it
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sequentially to optimize CPU & memory use. Pica already knows
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how to use multiple cores (if browser allows).
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### .toBlob(canvas, mimeType [, quality]) -> Promise
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Convenience method, similar to `canvas.toBlob()`, but with
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promise interface & polyfill for old browsers.
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### .resizeBuffer(options) -> Promise
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Supplementary method, not recommended for direct use. Resize
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Uint8Array with raw RGBA bitmap (don't confuse with
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jpeg / png / ... binaries). It does not use tiles & webworkers.
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Left for special cases when you really need to process raw
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binary data (for example, if you decode jpeg files "manually").
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- __options:__
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- __src__ - Uint8Array with source data.
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- __width__ - src image width.
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- __height__ - src image height.
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- __toWidth__ - output width, >=0, in pixels.
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- __toHeight__ - output height, >=0, in pixels.
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- __quality__ - 0..3. Default = `3` (lanczos, win=3).
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- __alpha__ - use alpha channel. Default = `false`.
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- __unsharpAmount__ - >=0, in percents. Default = `0` (off).
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Usually between 50 to 100 is good.
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- __unsharpRadius__ - 0.5..2.0. Radius of Gaussian blur.
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If it is less than 0.5, Unsharp Mask is off. Big values are
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clamped to 2.0.
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- __unsharpThreshold__ - 0..255. Default = `0`. Threshold
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for applying unsharp mask.
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- __dest__ - Optional. Output buffer to write data,
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if you don't wish `pica` to create new one.
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Result is Promise, resolved with resized rgba buffer.
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### What is "quality"
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Pica has presets to adjust speed/quality ratio.
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Simply use `quality` option param:
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- 0 - Box filter, window 0.5px
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- 1 - Hamming filter, window 1.0px
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- 2 - Lanczos filter, window 2.0px
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- 3 - Lanczos filter, window 3.0px
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In real world you will never need to change default (max)
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quality. All this variations were implemented to better
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understand resize math :)
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### Unsharp mask
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After scale down image can look a bit blured. It's good idea to sharpen it
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a bit. Pica has built-in "unsharp mask" filter (off by default).
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Set `unsharpAmount` to positive number to activate the filter.
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Filter's parameters are similar to ones from Photoshop.
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We recommend to start with `unsharpAmount = 160`,
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`unsharpRadius = 0.6` and `unsharpThreshold = 1`.
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There is [a correspondence between UnsharpMask parameters
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in popular graphics software](https://github.com/nodeca/pica/wiki/Unsharp-mask-params-in-popular-softare).
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Browser support
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----------------
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We didn't have time to test all possible combinations, but in general:
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- Top level API should work in all browsers,
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supporting [canvas](http://caniuse.com/#feat=canvas)
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and [typed arrays](http://caniuse.com/#feat=typedarrays).
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- [Webworkers](http://caniuse.com/#feat=webworkers),
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[WebAssembly](http://webassembly.org/) and
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[createImageBitmap](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/createImageBitmap)
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are not required, but they will be used if available.
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- If you plan to use only pure math core,
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then [typed arrays support](http://caniuse.com/#feat=typedarrays) will be enough.
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__Note.__ Though you can run this package on `node.js`, browsers
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are the main target platform. On server side we recommend to use
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[sharp](https://github.com/lovell/sharp).
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References
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----------
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You can find these links useful:
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- discussions on stackoverflow:
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[1](http://stackoverflow.com/questions/943781/),
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[2](http://stackoverflow.com/questions/18922880/),
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[3](http://stackoverflow.com/questions/2303690/).
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- chromium skia sources:
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[image_operations.cc](http://src.chromium.org/svn/trunk/src/skia/ext/image_operations.cc),
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[convolver.cc](http://src.chromium.org/svn/trunk/src/skia/ext/convolver.cc).
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pica for enterprise
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-------------------
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Available as part of the Tidelift Subscription.
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The maintainers of pica and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source packages you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact packages you use. [Learn more.](https://tidelift.com/subscription/pkg/npm-pica?utm_source=npm-pica&utm_medium=referral&utm_campaign=enterprise&utm_term=repo)
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