hkingz / upng.js

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

UPNG.js

A small, fast and advanced PNG encoder and decoder. It is the main PNG engine for Photopea image editor.

• Examples of PNGs minified by UPNG.js
• Try UPNG.js in Photopea - open an image and press File - Save for web, play with the Quality
• Support us by making a donation.

Encoder

UPNG.encode(rgba, w, h, cnum)

• rgba: 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)
• 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.

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).

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 ...)
• • 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).

UPNG.toRGBA8(img)

• img: PNG image object (returned by UPNG.decode())
• returns Uint8Array of the image in RGBA format, 8 bits per channel (ready to use in ctx.putImageData() etc.)

PNG format uses the Inflate algorithm. Right now, UPNG.js calls Pako.js for the Inflate and Deflate method.