Other Programs

Also in the Guetzli GitHub repository under tools is the guetzli-compare.py heat map program that is created when compiling and can be used to display the differences between the original image and the edited version. The Butteraugli program is also part of Guetzli, in a broader sense, and can be built and installed separately. As far as quantization is concerned, in the Guetzli environment, you will find a number of tools that optimize PNG images and even reduce the size of ZIP and Gzip archives.

In 2013, the developers first presented an algorithm to the public that became known as Zopfli [9]. As with ZIP, Gzip, and PNG, it supports the DEFLATE output format, but does so more thoroughly, which costs more computing power and makes the process particularly interesting for once generated, often read and distributed files. Using this method, the Gimp images are reduced better than with BZIP2; however, the version created by XZ Utils [10] is often even smaller.

The Zopfli suite implements several programs (Table 1). The most universal, advdef (the AdvanceCOMP deflate compression utility), is used to recompress existing images. The current implementation requires a large amount of memory, which limits the size of the input data. The -z switch activates recompression. The -0 to -4 parameters control (increasingly) the degree of recompression.

With -i <number>, you can influence the number of iterations and, therefore, the optimization indirectly; the default is 15 rounds. Higher values improve the outcome for the -3 and -4 modes but require more computing power. Through -f (force), Advdef creates the output files, even if they are bigger than the input files. The -h option explains which options and parameters control the processing.

In 2015 the developers provided the public with a further development in Zopfli [11]: a variant of LZ77 and Huffman codings that again promises better results. The only program for compression is bro.

Conclusions

Guetzli and its relatives show that the opportunity for innovations can still be found in image editing. In addition to special visual optimizations are the useful methods developed for quantization. In particular, operators of websites that use JPEG or PNG images should check to see whether and to what extent the programs presented here are suitable for their purposes.