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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, or get it from NPM:
+
+```sh
+npm install upng-js
+```
+
+## 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 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 performs color quantization using the [k-means algorithm](https://en.wikipedia.org/wiki/K-means_clustering).
+
+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.**
+
+## 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.