pngjs

What is pngjs?

The pngjs npm package is a pure JavaScript implementation for PNG encoding and decoding in Node.js. It allows for reading and writing PNG images in a non-blocking and streaming way, which is useful for applications that need to handle image data directly.

What are pngjs's main functionalities?

Reading PNG files

This code demonstrates how to read a PNG file using pngjs. It creates a read stream from a file, pipes it through the PNG decoder, and logs the image's width and height once it's parsed.

const fs = require('fs');
const PNG = require('pngjs').PNG;

fs.createReadStream('input.png')
  .pipe(new PNG())
  .on('parsed', function() {
    console.log('Image width:', this.width, 'Image height:', this.height);
  });

Writing PNG files

This code snippet shows how to create a new PNG image with a specific width and height, fill it with white color, and then write it to a file. It demonstrates the package's ability to write PNG files.

const fs = require('fs');
const PNG = require('pngjs').PNG;
const png = new PNG({ width: 10, height: 10, filterType: -1 });

for (let y = 0; y < png.height; y++) {
  for (let x = 0; x < png.width; x++) {
    let idx = (png.width * y + x) << 2;

    png.data[idx] = 255; // red
    png.data[idx + 1] = 255; // green
    png.data[idx + 2] = 255; // blue
    png.data[idx + 3] = 255; // alpha (opacity)
  }
}

png.pack().pipe(fs.createWriteStream('output.png'));

Other packages similar to pngjs

sharp

Sharp is a high-performance Node.js image processing library. It allows for converting large images in common formats to smaller, web-friendly JPEG, PNG, WebP, and AVIF images of varying dimensions. Compared to pngjs, sharp supports a wider range of image formats and provides more comprehensive image processing capabilities, but it relies on native dependencies.

jimp

Jimp is an image processing library for Node.js, written entirely in JavaScript, with zero native dependencies. It can perform a variety of operations such as resize, crop, rotate, and color manipulation. While Jimp supports PNG among other formats, it offers broader functionality beyond just PNG encoding and decoding, making it more versatile than pngjs for general image processing tasks.

README

About

Simple PNG encoder/decoder for Node.js with no native dependencies.

Installation

$ npm install pngjs

Example

var fs = require('fs'),
    PNG = require('pngjs').PNG;

fs.createReadStream('in.png')
    .pipe(new PNG({
        filterType: 4
    }))
    .on('parsed', function() {

        for (var y = 0; y < this.height; y++) {
            for (var x = 0; x < this.width; x++) {
                var idx = (this.width * y + x) << 2;

                // invert color
                this.data[idx] = 255 - this.data[idx];
                this.data[idx+1] = 255 - this.data[idx+1];
                this.data[idx+2] = 255 - this.data[idx+2];

                // and reduce opacity
                this.data[idx+3] = this.data[idx+3] >> 1;
            }
        }

        this.pack().pipe(fs.createWriteStream('out.png'));
    });

For more examples see examples folder.

Documentation

As input any color type is accepted (grayscale, rgb, palette, grayscale with alpha, rgb with alpha) but 8 bit per sample (channel) is the only supported bit depth. Interlaced mode is not supported.

Supported ancillary chunks

• gAMA - gamma,
• tRNS - transparency (but only for paletted image)

Class: PNG

PNG is readable and writable Stream.

Options

• width - use this with height if you want to create png from scratch
• height - as above
• checkCRC - whether parser should be strict about checksums in source stream (default: true)
• deflateChunkSize - chunk size used for deflating data chunks, this should be power of 2 and must not be less than 256 and more than 32*1024 (default: 32 kB)
• deflateLevel - compression level for delate (default: 9)
• filterType - png filtering method for scanlines (default: -1 => auto)

Event "metadata"

function(metadata) { } Image's header has been parsed, metadata contains this information:

• width image size in pixels
• height image size in pixels
• palette image is paletted
• color image is not grayscale
• alpha image contains alpha channel

Event: "parsed"

function(data) { } Input image has been completly parsed, data is complete and ready for modification.

Event: "error"

function(error) { }

png.parse(data, [callback])

Parses PNG file data. Alternatively you can stream data to instance of PNG.

Optional callback is once called on error or parsed. The callback gets two arguments (err, data).

Returns this for method chaining.

png.pack()

Starts converting data to PNG file Stream.

Returns this for method chaining.

png.bitblt(dst, sx, sy, w, h, dx, dy)

Helper for image manipulation, copies rectangle of pixels from current image (sx, sy, w, h) to dst image (at dx, dy).

Returns this for method chaining.

Property: width

Width of image in pixels

Property: height

Height of image in pixels

Property: data

Buffer of image pixel data. Every pixel consists 4 bytes: R, G, B, A (opacity).

Property: gamma

Gamma of image (0 if not specified)

Changelog

0.3.0-alpha - 23 Aug 2012

• Processing data as Streams, not complete Buffers of data

0.2.0-alpha - 21 Aug 2012

• Input added palette, grayscale, no alpha support
• Better scanline filter selection

0.1.0-alpha - 19 Aug 2012

• First version

License

(The MIT License)

Copyright (c) 2012 Kuba Niegowski

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.