Fast and advanced PNG (APNG) decoder and encoder (lossy / lossless)
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UPNG.js Add files via upload Nov 23, 2017
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A small, fast and advanced PNG / APNG encoder and decoder. It is the main PNG engine for Photopea image editor.

Download and include the UPNG.js file in your code, or get it from NPM:

npm install upng-js


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 and other tools. It performs color quantization using the k-means algorithm.

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.


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).


  • 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).


  • 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.


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 for the Inflate and Deflate method.