How to import new lensfun correction file to darktable installed using snap

Question

System:

• Ubuntu 18.04
• Darktable installed using snap
  • darktable version 2.6.2
  • darktable snap version 38 (ls -la /snap/darktable/current)

Issue

• https://github.com/lensfun/lensfun/blob/master/data/db/slr-sigma.xml has already vignetting correction for my favorite lens "Sigma 17-50mm f/2.8 EX DC OS HSM"
• my snap version (/snap/darktable/current/usr/share/lensfun/slr-sigma.xml) is probably based on lensfun 0.3.2, which does not have it.
• /snap/darktable is read only squashfs in snap, so simple override does not work :(
  • I would prefer to keep using snap for darktable because it still gets faster updates than Ubuntu LTS.

Already tried:

• based on https://photo.stackexchange.com/questions/61942/how-to-add-lensfun-0-3-0-db-to-darktable-on-ubuntu I tried to put the newer slr-sigma.xml to following two locations, but without success:
  • ~/.local/share/lensfun
  • ~/snap/darktable/common/.local/share/lensfun

Answer 1

Thanks for your follow-up. I would like to add some information I found on top of your findings:

snap local directory can be found in

~/snap/darktable/current/.local/share/lensfun

I also found that only creating your own config file was not enough. You have to copy everythin from the snap directory to the local configuration directory, because it replaces the snap database:

cp /snap/darktable/current/usr/share/lensfun/version_1/*.xml ~/snap/darktable/current/.local/share/lensfun/

Be careful not only with the syntax of the file (different from v2 to v1) but with the content. The v2 profile for my lens did not match its actual focal length and apparently darktable/lensfun 0.3.2 do not like it.

Answer 2

It was more complicated than I thought as there were two issues:

First issue:

Finding the right directory - strace darktable did not work, but strace is also a first class citizen in snap, so following helped

snap run --strace darktable 2> /tmp/err

searching for lensfun in the output gives you quite some directories which can be used, for example:

/home/$USERNAME/snap/darktable/38/lensfun
/home/$USERNAME/snap/darktable/38/.local/share/lensfun

unfortunately I have not found any which would not be version specific so work may be redone when darktable is updated by snap.

Second isssue:

The lensfun version of library inside the snap, can not parse the new format of slr-sigma.xml, so I had to take the original /snap/darktable/current/usr/share/lensfun/slr-sigma.xml and copy it to /home/$USERNAME/snap/darktable/38/lensfun/ and only add the vignetting section for my lens. (luckily there was a hint of xml parsing error)