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

pngjs

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

Based on the original pngjs with the follow enhancements.

• Support for reading 1,2,4 & 16 bit files
• Support for reading interlace files
• Support for reading tTRNS transparent colours
• Support for writing colortype 2 (RGB) and colortype 6 (RGBA)
• Sync interface as well as async
• API compatible with pngjs and node-pngjs

Known lack of support for:

• Extended PNG e.g. Animation
• Writing in different formats, colortype 0 (greyscale), colortype 3 (indexed color), colortype 4 (greyscale with alpha)
• Synchronous write

Requirements

• Async - Node.js 0.10 / 0.12 / IO.js
• Sync - Node.js 0.12 / IO.js (0.10 with node-zlib-backport dependency)

Comparison Table

Name	Forked From	Sync	Async	16 Bit	1/2/4 Bit	Interlace	Gamma	Encodes	Tested
pngjs		Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
node-png	pngjs	No	Yes	No	No	No	Hidden	Yes	Manual
png-coder	pngjs	No	Yes	Yes	No	No	Hidden	Yes	Manual
pngparse		No	Yes	No	Yes	No	No	No	Yes
pngparse-sync	pngparse	Yes	No	No	Yes	No	No	No	Yes
png-async		No	Yes	No	No	No	No	Yes	Yes
png-js		No	Yes	No	No	No	No	No	No

Native C++ node decoders:

• png
• png-sync (sync version of above)
• pixel-png
• png-img

Tests

Tested using PNG Suite. We read every file into pngjs, output it in standard 8bit colour, synchronously and asynchronously, then compare the original with the newly saved images.

To run the tests, fetch the repo (tests are not distributed via npm) and install with npm i, run npm test.

The only thing not converted is gamma correction - this is because multiple vendors will do gamma correction differently, so the tests will have different results on different browsers.

In addition we use a tolerance of 3 for 16 bit images in PhantomJS because PhantomJS seems to have non-compliant rules for downscaling 16 bit images.

Installation

$ npm install pngjs  --save

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.

Async API

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.

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)
• deflateStrategy - compression strategy for delate (default: 3)
• deflateFactory - deflate stream factory (default: zlib.createDeflate)
• filterType - png filtering method for scanlines (default: -1 => auto, accepts array of numbers 0-4)
• colorType - the output colorType - see constants. 2 = color, no alpha, 6 = color & alpha. Default currently 6, but in the future may calculate best mode.
• inputHasAlpha - whether the input bitmap has 4 bits per pixel (rgb and alpha) or 3 (rgb - no alpha).
• bgColor - an object containing red, green, and blue values between 0 and 255 that is used when packing a PNG if alpha is not to be included (default: 255,255,255)

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
• interlace image is interlaced

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. Can be String or Buffer. 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.

Example

new PNG({ filterType:4 }).parse( imageData, function(error, data)
{
	console.log(error, data)
});

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 a rectangle of pixels from current (i.e. the source) image (sx, sy, w, h) to dst image (at dx, dy).

Returns this for method chaining.

For example, the following code copies the top-left 100x50 px of in.png into dst and writes it to out.png:

var dst = new PNG({width: 100, height: 50});
fs.createReadStream('in.png')
    .pipe(new PNG())
    .on('parsed', function() {
        this.bitblt(dst, 0, 0, 100, 50, 0, 0);
        dst.pack().pipe(fs.createWriteStream('out.png'));
    });

Property: adjustGamma()

Helper that takes data and adjusts it to be gamma corrected. Note that it is not 100% reliable with transparent colours because that requires knowing the background colour the bitmap is rendered on to.

In tests against PNG suite it compared 100% with chrome on all 8 bit and below images. On IE there were some differences.

The following example reads a file, adjusts the gamma (which sets the gamma to 0) and writes it out again, effectively removing any gamma correction from the image.

fs.createReadStream('in.png')
    .pipe(new PNG())
    .on('parsed', function() {
        this.adjustGamma();
        this.pack().pipe(fs.createWriteStream('out.png'));
    });

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)

Packing a PNG and removing alpga (RGBA to RGB)

When removing the alpha channel from an image, there needs to be a background color to correctly convert each pixel's transparency to the appropriate RGB value. By default, pngjs will flatten the image against a white background. You can override this in the options:

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

fs.createReadStream('in.png')
    .pipe(new PNG({
        colorType: 2,
        bgColor: {
            red: 0,
            green: 255,
            blue: 0
        }
    }))
    .on('parsed', function() {
        this.pack().pipe(fs.createWriteStream('out.png'));
    });

Sync API

PNG.sync

PNG.sync.read(buffer)

Take a buffer and returns a PNG image. The properties on the image include the meta data and data as per the async API above.

var data = fs.readFileSync('in.png');
var png = PNG.sync.read(data);

PNG.sync.write(png)

Take a PNG image and returns a buffer. The properties on the image include the meta data and data as per the async API above.

var data = fs.readFileSync('in.png');
var png = PNG.sync.read(data);
var buffer = PNG.sync.write(png);
fs.writeFileSync('out.png', buffer);

PNG.adjustGamma(src)

Adjusts the gamma of a sync image. See the async adjustGamma.

var data = fs.readFileSync('in.png');
var png = PNG.sync.read(data);
PNG.adjustGamma(png);

Changelog

2.3.0 - 22/04/2016

• Support for sync in node 0.10

2.2.0 - 04/12/2015

• Add sync write api
• Fix newfile example
• Correct comparison table

2.1.0 - 28/10/2015

• rename package to pngjs
• added 'bgColor' option

2.0.0 - 08/10/2015

• fixes to readme
• breaking change - bitblt on the png prototype now doesn't take a unused, unnecessary src first argument

1.2.0 - 13/09/2015

• support passing colorType to write PNG's and writing bitmaps without alpha information

1.1.0 - 07/09/2015

• support passing a deflate factory for controlled compression

1.0.2 - 22/08/2015

• Expose all PNG creation info

1.0.1 - 21/08/2015

• Fix non square interlaced files

1.0.0 - 08/08/2015

• More tests
• source linted
• maintainability refactorings
• async API - exceptions in reading now emit warnings
• documentation improvement - sync api now documented, adjustGamma documented
• breaking change - gamma chunk is now written. previously a read then write would destroy gamma information, now it is persisted.

0.0.3 - 03/08/2015

• Error handling fixes
• ignore files for smaller npm footprint

0.0.2 - 02/08/2015

• Bugfixes to interlacing, support for transparent colours

0.0.1 - 02/08/2015

• Initial release, see pngjs for older changelog.

License

(The MIT License)

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.