Apologies is this is not the place for this question.
My understanding of an appimage is that it is self contained - that is, there should be no requirement for access to any libraries beyond those included in the appimage. I have downloaded krita-4.4.2-x86_64.appimage and made it executable, but when I try to run it it complains that ZLIB_1.2.9 is not found. I am running Debian 9, which I believe has ZLIB_1.2.8 installed.
I can't simply replace my existing copy of zlib without potentially breaking other apps. I think I could find ZLIB1.2.9, install it in a private location, make a link to it, etc, but I'd first like to be clear that I understand the issue. (Also, that sounds like work. )

Either
1) I'm misunderstanding how an appimage should work, or
2) This appimage was not correctly built (perhaps the required library was on the machine on which the appimage was built, so it *did* run when tested, but under false pretenses as it were...)
3) somethings else?

In any case, I don't think this is a bug in krita itself so I did not think it should go in the regular bug reports (which I did check).
I am able to run the version of krita that Debian 9 "knows about" i.e., 3.1.1, but this seems to be so far behind the current examples that I've having trouble following them.

Any clarification or suggestions appreciated. If I can't easily resolve this I may check the flatpack, or maybe install a parallel distribution (recommendations?) that lets me run the latest stable krita.

Thanks

Appimage does not provide a fully sandboxed environment like Flatpak, while it may contain any libraries, it's up to the packagers to decide which ones to include.
So yes, Krita Appimages do have a minimum system requirement.

I'm not sure why zlib is not included, or why the required version seems higher than what the build environment based on Ubuntu 16.04 provides, maybe it is an oversight.
It may work to create a launcher script that adds missing libraries via LD_LIBRARY_PATH environment variable to the search path of the linker, but I've never tried that myself.

Flatpak is certainly an easy way to get it working on older distros, but it's a lot heavier, it's almost like installing a minimal Linux distribution in a chroot environment. And it has the downside that it'll lack a few Qt patches that Appimages have to address some annoyances and performance.

> And it has the downside that it'll lack a few Qt patches that Appimages have to address some annoyances and performance.

I believe right now flatpak does patch Qt. And it does interact with normal system resources, too. I might be wrong in that, though.

I tried flatpak. It apparently installed krita with no reported issues, but then failed silently when I tried to run krita. I was not able to even re-install the krita 3.1 from the Debian 9 repository until I purged all the remains of flatpak from my system. I also tried the windows version of krita in a Win7 virtual box that I have, and that failed with a missing dll problem. My impression is that most linux users run krita on Ubuntu, or perhaps Mint. I guess a next thing to try might be to install one of those as dual boot or in a virtual machine and see how far I get.

I've just noticed this topic.
I have Debian 10 and it has zlib 1.2.11 so it looks like you need to update your installation. Debian 10 will soon be old-stable.
Debian 9 will soon be out of support, I think.

"Yes, But."
There have been numerous reports up upgrading 9 to 10 breaking things in an historically unprecedented way, and I'm reluctant to go down that rabbit hole. As long as 9 works for me I'm not eager to change, but Krita may add one more reason.

It broke access to my network drives so I had to set them up again and the default/forced installation of AppArmor with standard policies broke my non-standard way of doing something, so I had to disable and then remove AppArmor. (I could have learned about, found and edited AppArmor policies but I couldn't be bothered.)

One possibility would be to backport zlib 1.2.11 from the Debian 10 repo, watching out for dependencies. The safest way to do anything like that is to use GParted Live to make a copy of your system partitions for later restoration if things get broken.

Debian Testing (Bullseye) is close to becoming the stable release (11) so you could try a brand new installation with that and you'd be good for another four years. Some people run Debian Testing as their daily driver and say they've been happy with it for years.

Having spent many years chasing technical details on multiple systems, all I really wanted now was a more or less current version of Krita running so I could pursue a less technical activity. At this point I have (gasp!) installed Windows 7 on an otherwise unused drive, and boot from one or the other. The Windows install of Krita seems to be complete and working, the Huion tablet I got also. Now I just have to learn how to use it. I'll muddle through with Debian 9 for my daily tasks until it gives me grief, and deal with that when it happens.