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…
---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.