i've never contributed to any mailing lists, or partaken much, so i don't know how to start this off...

hi? i've realized i can compressed a dataset of lossless .png files down to between a third- or a fourth of the initial size on disk.

that said: in compressing my own backups i find that lossless .png files converted to .xpm (not .ppm, not .bmp) before being compressed result in a compression that of 3- to 4 times smaller in size on disk than the initial .png with it's own compression --- irregardless of any 'PNG optimization' done beforehand: the resulting .xpm files remain identical in size and compress to precisely the same size.

the following commands¹ can surely be tinkered with to greater effect:

---start of shell commands---

mkdir -p xpm

magick convert image.png xpm/image.xpm

mkdwarfs -i xpm/ -o compressed.dfs -l9

---end of shell commands---

now you have a compressed image, three to four times smaller in size on disk, to inspect.

here i openly wonder how a comparison - to assert whether the resulting .xpm file and the lossless .png are indeed the same picture still - would be carried out.

likewise i see this quality of compression extends to the .xgm .xbm formats.

in sharing i wish to bring up the above observation to my best ability. insights welcome as to why this happens and if indeed the image resulting from a lossless .png to .xpm conversion is the same - if this compressed bitmap outperforms the PNG compression significantly without compromising image integrity. (do reply with saying if this is irrelavant information and or presented inadequately in any way: i don't wish to bring red herrings to this mailing list.)

ultimately this is about if a large portion of WikiMedia imagedata indeed can be compressed further by this process - in a 'Pareto improvement' kind of way.

-Ivy, 25

for interest: sources to programs referenced and my brief notes on these.

[1]

magick: https://imagemagick.org/index.php

mkdwarfs, part of the inappropriately named dwarfs toolset:

https://github.com/mhx/dwarfs/blob/main/doc/mkdwarfs.md

note that the -l flag is given the option 9 in the command - this means LZMA is used in this program.

also note that while i use 'mkdwarfs' with LZMA here, i realize the same result on any other program using LZMA or XZ occurs - like the following 'dar', more adequate for single file extraction from an archive and analyzing individual file compression values en masse.

dar: http://dar.linux.free.fr/doc/man/dar.html
unfortunately the projects' website doesn't use HTTPS, so a wayback machine link with HTTPS:

https://web.archive.org/web/20240423233825/http://dar.linux.free.fr/doc/man/dar.html

---start of referenced dar command---

dar -c output -zxz9 -R input/

---end of referenced dar command---

lastly two clarifications:

'a lossless .png file' here means an imagefile which never has been converted in a lossy way. a .jpg file converted to a .png used in this procedure produces an output taking more space on disk.

'the procedure' here refers to conversion of a lossless .png to .xpm imagefile format then compressed with either LZMA or XZ in any program.

thank you for reading.

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