png-js

This is a modified version of png.js that is intended to be used for palette swapping within the browser.

NOTE: Only supports indexed PNGs for obvious reasons. All others will cause IndexedPNG to throw during construction.

Usage

The following shows the process of loading a PNG file and rendering it to a canvas with its palette data reversed.

import { IndexedPNG }   from '@kettek/png-js/src'
import { readFile   }   from 'fs'

readFile("my_png.png", (err, data) => {
    if (err) throw err
    
    // Create an IndexedPNG object from our data.
    let png = new IndexedPNG(data)
    
    // Preemptively decode PNG data (optional).
    png.decode()
    
    // Convert our PNG data into an ImageData, usable by a Canvas. Passed options provide a reversed palette and a clipping.
    png.toImageData({
      palette: png.decodePalette().reverse(),
      clip: {
        x: 16,
        y: 16,
        w: 32,
        h: 32
      }
    })
    .then(imageData => {
      // This presumes at least one canvas element exists on the page.
      document.getElementsByTagName('canvas')[0]
      .getContext('2d')
      .putImageData(imageData, 0, 0)
    })
    .catch(err => {
        throw err
    })
})

Types

IndexedPNG

IndexedPNG(Buffer)

processes PNG data but does not decode it.

async decodePixels()

Decodes the PNG pixel data and returns it.

decodePalette()

Decodes the PNG palette and returns it as a Palette.

async decode()

Decodes the palette and the pixel data of the PNG data passed into the constructor. Calls decodePixels() and decodePalette().

ImageData async toImageData(ImageDataOptions)

Returns ImageData. Calls decode() automatically if PNG has not been decoded.

ImageDataOptions

Object containing options to apply during toImageData().

Property	Type	Description
palette	Palette	Palette data to use for decoding.
clip	Clip	Clipping options to use for decoding.

Palette

A Buffer or Array containing the palette RGBA palette data.

let palette = [
    255, 0  ,  0  , 255, // Index 0
    0  , 255,  0  , 255, // Index 1
    0  , 0  ,  255, 255, // Index 2
    ...
]

Clip

Options to clip pixel data.

Property	Type	Defaults	Description
x	Number	0	Starting X coordinate.
y	Number	0	Starting Y coordinate.
w	Number	width - x	Width of clip.
h	Number	height - y	Height of clip.

let clip = {
    x: 16,
    y: 16,
    w: 32,
    h: 32,
}