featherpng is a PNG image optimization and manipulation library for Java 8+,
based on the pngtastic library.
The latest code adds the ability to optimize png images using the new zopfli deflate compression algorithm. The java port is based heavily on this.
If you're willing to sacrifice compression speed in exchange for ridiculously good compression ratios, you'll want to try using the optional zopfli compressor.
Example usage:
$ mvn install
$ java -cp target/pngtastic-1.8-SNAPSHOT.jar com.googlecode.pngtastic.PngtasticOptimizer --compressor zopfli --iterations 32 --fileSuffix .min.png images/optimizer/amigaball.png
So far I'm seeing better compression ratios for my test images than even the excellent ImageOptim app produces.
Here's a taste (ordered from worst to best compression):
Pngtastic Default Compression
[pngtastic] 59.76% : 169B -> 68B ( 101B saved) - build/images/optimizer/1px.png
[pngtastic] 5.99% : 35731B -> 33590B ( 2141B saved) - build/images/optimizer/amigaball.png
[pngtastic] 0.01% :251938B ->251922B ( 16B saved) - build/images/optimizer/frymire.png
[pngtastic] 22.37% : 93167B -> 72322B (20845B saved) - build/images/optimizer/gamma.png
ImageOptim Compression
[ImageOptim] 59.8% : 169B -> 73B - build/images/optimizer/1px.png
[ImageOptim] 11.2% : 35731B -> 31729B - build/images/optimizer/amigaball.png
[ImageOptim] 8.7% :251938B ->230055B - build/images/optimizer/frymire.png
[ImageOptim] 28.5% : 93167B -> 66607B - build/images/optimizer/gamma.png
Pngtastic Zopfli Compression
[pngtastic] 61.54% : 169B -> 65B ( 104B saved) - build/images/optimizer/1px.png
[pngtastic] 12.21% : 35731B -> 31370B ( 4361B saved) - build/images/optimizer/amigaball.png
[pngtastic] 10.40% :251938B ->225749B (26189B saved) - build/images/optimizer/frymire.png
[pngtastic] 29.27% : 93167B -> 65895B (27272B saved) - build/images/optimizer/gamma.png