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libpng is the official PNG reference library. It supports almost all PNG features, is extensible, and has been extensively tested for over 18 years. The home site for development versions (i.e., may be buggy or subject to change or include experimental features) is http://libpng.sourceforge.net/, and the place to go for questions about the library is the png-mng-implement mailing list.
brew install libpng
This formula has no dependencies.