Overview (PNGJ 2.1.0 API)

Packages
Package	Description
ar.com.hjg.pngj	PNGJ main package
ar.com.hjg.pngj.chunks	Contains the code related to chunk management for the PNGJ library.
ar.com.hjg.pngj.pixels	Mostly related with logic specific to reading/writing pixels.

PNGJ: a pure Java library for PNG image encoding/decoding

Main features:

Very efficient in memory usage and speed.
Pure Java (requires 5 or greater).
Small and self contained. No dependencies on third party libraries, nor even on java.awt.*/javax.imageio.*
Runs on GAE (Google App Engine) and Android
Allows to read/write progressively, row by row. This means that you can process huge images without needing to load them fully in memory.
Reads all PNG color models (true color, gray, palette, all bit depths: 1-2-4-8-16, with-without alpha). Interlaced PNG is supported (though not welcomed).
Writes all PNG color models - interlaced PNG writing is not supported.
Full support for metadata (CHUNKS) handling.
Supports (since 2.0) asyncronous reading.
The format of the pixel data (read and write) is extensible (since 2.0) and very efficient (no double copies).
Supports (since 2.0) low level tweaking and extension in the reader.
Open source (Apache 2.0 licence)

Note that this is a relatively low-level library. It does not provide any high-level image processing (eg, resizing, colour conversions), nor tries to abstract the concrete image format (as BufferedImage does, for example). In particular, the default format of the scanlines (as wrapped in ImageLineInt or ImageLineByte) is not abstract, the meaning of the values depends on the image color model and bitdepth.