What do I need to do to use LensFun Profiles?

I currently use the LocalLab build and want to start using LensFun profiles to correct distortion. I have not downloaded or otherwise installed LensFun. What do I need to to and where do I need to install the profiles? What do I do to delete the profiles I do not need?

@RichardRegal
You can have a look at Lens/Geometry - RawPedia

I cannot answer for Linux and Apple
For the windows builds , the database is located in ./share/lensfun.

I did before I posted here and it told me absolutely nothing, hence my questions.
(a) Is there a separate program I need to download and install or does RT make use of the profiles natively?
(b) Where is /usr/share/lensfun/ located as I can’t see in on my disk? Is it in appdata, users, or somewhere else?
(c) Do I need to tell RT where to find the profiles if I go the $HOME[my login]/.local/share/lensfun/myLensfun route?
(d) Do I just need the profiles for my camera and lenses or do I also need to copy any definition files or tables of what databases are available?

@RichardRegal
1- You can get more specific answers if you provide following informations : what is your system, do you build yourself, do you download builds, from where and which version.
I guess that you are on windows.

2- For windows, the build contains and installs all that is needed. The lensfuns function is liblensfun.dll and the database is located in \share\lensfun. Nothing has to be done.

3- For Linux and Apple I cannot answer if the above lensfun elements are bundled in the build. If not, in Linux, I suppose Lensfun has to be installed.

4- To activate the correction function, go to the transform tab then lens/geometry then profiled lens correction

I hope this can help you.

I am on Windows 10 Pro version 1803 and download RT builds from your kindly provided site of nightly builds. I am currently on build 5.4.1462.

I cannot find the file liblensfun.dll in C:\Program Files\RawTherapee\share\lensfun or anywhere else on the system. I can find the profiles there though.

That leaves one remaining question which is whether the database.dtd and database.xsd files are needed if I delete all the files other than the one(s) needed for my camera and lenses?

@RichardRegal

I was not that clear.
the database is located in \share\lensfun.
liblensfun.dll is located in

I think you can safely delete the xml files you don’t use. Regarding the dtd and xsd (version_2), only the lensfun guys know for sure. The safest is to keep them.

And verify that the last section of your option file contains either
[Lensfun]
DBDirectory=

or
[Lensfun]
DBDirectory= ./share/lensfun

Edit : with this variable you can link to an external directory .

Edit : I tried deleting the .dtd and .xsd files and it works. So the best for you is to try.

Are you shure? I only use the .xml files. I build my own which contains only the lenses and cameras I have.
Here’s a copy my xml-file: MyLenses.zip (2,4 KB)

@TooWaBoo
I correct the sentance:
I think you can safely delete the xml files you don't use.

Is there a page in rawpedia in which it is documented?

I don’t know. I figured this out myself.

Thanks.