@lunapaint/png-codec

@lunapaint/png-codec

This is a PNG decoder and encoder library for JavaScript that runs in both the browser and in Node.js. It is used in Luna Paint (an image editor for VS Code) to work with PNG files.

You can try it out on vscode.dev by installing the Luna Paint extension and opening a png file.

Features

• Performance: Just like Luna Paint, performance is a priority. This includes code splitting such that code that parses optional chunks are only loaded as needed.
• Correctness: The library has > 97% code coverage via over 7500 tests, including full coverage of PngSuite—the "official" test suite for PNG.
• Simple API: The API is a well documented TypeScript declaration file.
• Metadata: All supported metadata is exposed on the API such as text, gamma, default background color, etc.
• Readable Codebase: A big part of this was a learning exercise for me so I put some effort in to make the code as readable as possible to help others on the same journey.
• Error tolerant: Images will still load with warnings unless a critical error is hit.

Install

The supported way of installing the project is through npm:

npm install @lunapaint/png-codec

Alternatively, you could add the repo as a git submodule, or download the source from the GitHub releases page.

API

Basic usage:

import { decodePng, encodePng } from '@lunapaint/png-codec';
import * as fs from 'fs/promises';

async function decode(filepath) {
  const data = await fs.readFile(filepath);
  const decoded = await decodePng(data);
  console.log('decoded image', decoded.image.data);
  // [r, g, b, a, ...]
}

async function encode(data, width, height, filepath) {
  const encoded = await encodePng({ data, width, height });
  await fs.writeFile(filepath, encoded.data);
  console.log('encoded image', encoded.data);
  // [...binary data]
}

The full API is documented as a TypeScript .d.ts declaration file. The view the API:

• github.dev: View on the web in VS Code, which has symbol support out of the box. Try showing the Outline view and triggering the Go to Symbol in Editor command
• github.com: View the raw file in github.com.

Decoder chunk support

PNGs are made up of a fixed signature followed by a series of chunks. The following chunks can be decoded supported, with some notes provided where applicable:

Critical chunks:

Chunk	Name	Notes
IHDR	Image header
PLTE	Palette
IDAT	Image data	Full filtering and interlacing support for all bit depths (1, 2, 4, 8, 16) are supported.
IEND	Image trailer

Ancillary chunks:

Chunk	Name	Notes
bKGD	Background color
cHRM	Primary chromaticities and white point
eXIf	Exchangeable image file format	Approved 2017/7
gAMA	Image gamma	Gamma values are provided, but are not applied to the resulting image (see #11)
hIST	Image histogram
iCCP	Embedded ICC profile	Exposes the profile as a byte array
iTXt	International textual data
oFFs	Image offset	🧪 Limited testing Extension to the PNG 1.2 Specification v1.2.0
pCAL	Calibration of pixel values	🧪 Limited testing Extension to the PNG 1.2 Specification v1.2.0
pHYs	Physical pixel dimensions
sBIT	Significant bits	Since the decoded buffer uses a minimum of uint8, this is only when the significant bits are in the range of 9-15
sCAL	Physical scale of image subject	🧪 Limited testing Extension to the PNG 1.2 Specification v1.2.0
sPLT	Suggested palette
sRGB	Standard RGB colour space
sTER	Indicator of stereo image	🧪 Limited testing Extension to the PNG 1.2 Specification v1.3.0
tEXt	Textual data
tIME	Image last-modification time
tRNS	Transparency	Since this chunk modifies the resulting image, you cannot skip this chunk
zTXt	Compressed textual data

Why does this exist?

These are the main reasons:

• To deeply understand the format.
• To integrate better with Luna Paint and my other existing and future decoders that depend on the png format.
• The scope is limited, this project will eventually be "done" and need minimal maintenance.
• I didn't want to go the wasm route in Luna Paint (yet?).
• To have full control over how the code is loaded, for example Luna Paint uses dynamic imports extensively to reduce the amount of code loaded to combat slow startup times.
• As an educational resource.
• To have some fun.

Dependencies

The library has the single runtime dependency pako which provides the compression/decompression capabilities needed to read various png chunks.

References

• https://www.w3.org/TR/2003/REC-PNG-20031110
• http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html
• http://www.libpng.org/pub/png/spec/register/pngext-1.4.0-pdg.html
• http://ftp-osl.osuosl.org/pub/libpng/documents/proposals/eXIf/png-proposed-eXIf-chunk-2017-06-15.html