libpng is the official PNG reference library. It supports almost all PNG
features, is extensible, and has been extensively tested for over 23 years. The
home site for development versions (i.e., may be buggy or subject to change or
include experimental features) is https://libpng.sourceforge.io/, and the place
to go for questions about the library is the png-mng-implement mailing list.

libpng is available as ANSI C (C89) source code and requires zlib 1.0.4 or later
(1.2.5 or later recommended for performance and security reasons). The current
public release, libpng 1.6.37, fixes the use-after-free security vulnerability
noted below, as well as an ARM NEON memory leak in the palette-to-RGB(A)
expansion code (png_do_expand_palette()). 

The packaging of libpng for build2 is tracked in a Git repository at:

https://github.com/b8u/libpng