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https://github.com/photopea/UPNG.js

Fast and advanced PNG (APNG) decoder and encoder (lossy / lossless)
https://github.com/photopea/UPNG.js

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Fast and advanced PNG (APNG) decoder and encoder (lossy / lossless)

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# UPNG.js

A small, fast and advanced PNG / APNG encoder and decoder. It is the main PNG engine for [Photopea image editor](https://www.photopea.com).



* [Examples of PNGs minified by UPNG.js](https://blog.photopea.com/png-minifier-inside-photopea.html#examples)

* [Try UPNG.js in Photopea](https://www.photopea.com) - open an image and press File - Save for web, play with the Quality

* [UPNG.Photopea.com](http://upng.photopea.com) - a separate minifier app, that uses UPNG.js

* Support us by [making a donation](https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=ivan%40kuckir%2ecom&lc=CZ&item_name=UPNG%2ejs&currency_code=USD&bn=PP%2dDonationsBF%3abtn_donateCC_LG%2egif%3aNonHosted).



Download and include the `UPNG.js` file in your code.



## Encoder



UPNG.js supports APNG and the interface expects "frames". Regular PNG is just a single-frame animation (single-item array).



#### `UPNG.encode(imgs, w, h, cnum, [dels])`

* `imgs`: array of frames. A frame is an ArrayBuffer containing the pixel data (RGBA, 8 bits per channel)

* `w`, `h` : width and height of the image

* `cnum`: number of colors in the result;  0: all colors (lossless PNG)

* `dels`: array of millisecond delays for each frame (only when 2 or more frames)

* returns an ArrayBuffer with binary data of a PNG file



UPNG.js can do a lossy minification of PNG files, similar to [TinyPNG](https://tinypng.com/) and other tools. It performed quantization with [k-means algorithm](https://en.wikipedia.org/wiki/K-means_clustering) in the past, but now we use [K-d trees](https://en.wikipedia.org/wiki/K-d_tree).



Lossy compression is allowed by the last parameter `cnum`. Set it to zero for a lossless compression, or write the number of allowed colors in the image. Smaller values produce smaller files. **Or just use 0 for lossless / 256 for lossy.**



    // Read RGBA from canvas and encode with UPNG

    var dta = ctx.getImageData(0,0,200,300).data;  // ctx is Context2D of a Canvas

    //  dta = new Uint8Array(200 * 300 * 4);       // or generate pixels manually

    var png = UPNG.encode([dta.buffer], 200, 300, 0);   console.log(new Uint8Array(png));



#### `UPNG.encodeLL(imgs, w, h, cc, ac, depth, [dels])` - low-level encode

* `imgs`: array of frames. A frame is an ArrayBuffer containing the pixel data (corresponding to following parameters)

* `w`, `h` : width and height of the image

* `cc`, `ac`: number of color channels (1 or 3) and alpha channels (0 or 1)

* `depth`: bit depth of pixel data (1, 2, 4, 8, 16)

* `dels`: array of millisecond delays for each frame (only when 2 or more frames)

* returns an ArrayBuffer with binary data of a PNG file



This function does not do any optimizations, it just stores what you give it. There are two cases when it is useful:

* saving 16-bit colors (note, that PNG is big-endian, unlike Uint16Array in JS)

* your image is too large, and "expanding" to 8-bit RGBA would use too much memory (e.g. 4-bit grayscale 50,000 x 50,000 = 1.25 GB, 8-bit RGBA would be 10 GB)



## Decoder



Supports all color types (including Grayscale and Palettes), all channel depths (1, 2, 4, 8, 16), interlaced images etc. Opens PNGs which other libraries can not open (tested with [PngSuite](http://www.schaik.com/pngsuite/)).



#### `UPNG.decode(buffer)`

* `buffer`: ArrayBuffer containing the PNG file

* returns an image object with following properties:

* * `width`: the width of the image

* * `height`: the height of the image

* * `depth`: number of bits per channel

* * `ctype`: color type of the file (Truecolor, Grayscale, Palette ...)

* * `frames`: additional info about frames (frame delays etc.)

* * `tabs`: additional chunks of the PNG file

* * `data`: pixel data of the image



PNG files may have a various number of channels and a various color depth. The interpretation of `data` depends on the current color type and color depth (see the [PNG specification](https://www.w3.org/TR/PNG/)).



#### `UPNG.toRGBA8(img)`

* `img`: PNG image object (returned by UPNG.decode())

* returns an array of frames. A frame is ArrayBuffer of the image in RGBA format, 8 bits per channel.



### Example

    var img  = UPNG.decode(buff);        // put ArrayBuffer of the PNG file into UPNG.decode

    var rgba = UPNG.toRGBA8(img)[0];     // UPNG.toRGBA8 returns array of frames, size: width * height * 4 bytes.



PNG format uses the Inflate algorithm. Right now, UPNG.js calls [Pako.js](https://github.com/nodeca/pako) for the Inflate and Deflate method.



## Quantizer



UPNG.js contains a very good Quantizer of 4-component 8-bit vectors (i.e. pixels). It can be used to generate nice color palettes (e.g. Photopea uses UPNG.js to make palettes for GIF images).



Quantization consists of two important steps: Finding a nice palette and Finding the closest color in the palette for each sample (non-trivial for large palettes). UPNG perfroms both steps.



    var res  = UPNG.quantize(data, psize);



* `data`: ArrayBuffer of samples (byte length is a multiple of four)

* `psize` : Palette size (how many colors you want to have)



The result object "res" has following properties:



* `abuf`: ArrayBuffer corresponding to `data`, where colors are remapped by a palette

* `inds`: Uint8Array : the index of a color for each sample (only when `psize`<=256)

* `plte`: Array : the Palette - a list of colors, `plte[i].est.q` and `plte[i].est.rgba` is the color value



### FAQ



- To get one common palette for multiple images (e.g. frames of the animation), concatenate them into one array `data`.

- When working with less than four components, set the remaining components to a constant value (e.g. to zero)

- When working with transparency, premultiply color components by transparency (otherwise, rgba(1,1,1,0) would be closer to rgba(1,1,1,1) than to rgba(0,0,0,0) - transparent mapped to white instead of transparent)