Okay. So my autistic brain decided that was this kind of moment.
I did a little research, and found some new toys. After many tests, I came to the conclusion that the best tool to use is
advpng (open-source GPL)
Command:
advpng -z -4 -i 10 <file>
Result:
Note: Like the previous, this test was done at commit
d28319b1. And like the previous, your find the results in the attached ZIP file.
The results of advpng on were very interesting. Alone, it is slightly more efficient than PNGout, while covering more files, 1156 files for 172976 bytes saved. However, once you make a pass with OptiPNG afterwards, the results are far more impressive: 1297 files for 1621349 bytes saved.
In the end, on the test commit, advpng+OptiPNG achieved a 3.7% gain in total PNGs size (less with the most recent commit which is already optimized, but more than 1%).
Conclusion:
I think I can bring this topic to a close with the final suggestion of using advpng+OptiPNG, and therefore possibly adding advpng to the calibre toolbox.
I also made a
Pull request with this new tool set.