Disclaimer: None of this is my original idea, it is all pieced together from this forum, kodi forum and kodinerds. Still, I had to piece it together so I decided to make a mini tutorial for those that want to do this as well.
Also, names, directories, etc are up to you, but make sure to be consistent.
Finally, of course I assume no liability, shit can always break.
Motivation: On S905x devices connected to non-HDR TVs, the GUI is too dark (HDMI output starts in HDR mode?) up until you play a video for the first time. So making Kodi play a video before you even see the GUI is the perfect workaround. Also, startup animations are cool if they're not too long.
Step 0: SSH to your box
Step 1 - copy the default skin and make the copy usable
This step is only necessary if you use the default skin. As we have the useful "reboot from internal" command baked into that I will assume that you use this, just as I did. I will further assume that we are on Krypton betas. Of course you can name the copy anything you like, just make sure to be consistent. If you use Jarvis substitute "estuary" for "confluence". If you use another skin, skip Step 1 and substitute its name in the later steps. you get the idea.
Copy the skin's folder to writable space:
Edit the addon.xml of the copy to distinguish it from the original:
Right in the first line you find " addon id="skin.estuary" ", change this to addon id="skin.estuary.withintro". further on you find " name="Estuary" " change this to "Estuary with Intro" (or anything you like).
Press CTRL-X, Press Y followed by Enter to save the changes.
Bonus: copy your previous skin settings:
cp -r ~/.kodi/userdata/addon_data/skin.estuary/ ~/.kodi/userdata/addon_data/skin.estuary.withintro/
Now we have to enable the copied skin. First we have to restart Kodi to make it aware of the copy:
Then in the GUI enter addons - my addons - look and feel - skins. There you should find the copy with the name entered above with an X next to it. select it and choose "enable", select use and confirm. you are now using the copy.
Step 2 - copy your intro video
Choose a file you want as your intro. You can use one of the awesome contributions in this thread on the kodi forum or anything you like. Bonus points for making your own.
Rename it to "intro.mp4", depending on the file the extension should be something else.
Connect to your box via SMB. open the "videos" folder. copy the "intro.mp4" file to this folder.
Step 3 - tell your skin to show the intro when starting up
Edit Home.xml:
right at the beginning of the file you will find " <defaultcontrol>9000</defaultcontrol> "
directly after it (advisably in a new line for readablity) add:
Adapt according to your filetype (or differently named file more generally).
Press CTRL-X, Press Y followed by Enter to save the changes.
Step 4 - see it work
Kodi should startup with your video before presenting the home screen.
Step 5 - bonus
If you like you can disable Kodi's splash screen, as you now have a cool video anyway.
type out
If you already had an advancedsettings.xml, of course you only have to add the splash line.
Press CTRL-X, Press Y followed by Enter to save the changes.
Of course you can use other methods to copy the files and edit the xmls. I just think that ssh/nano is fastest.
Alternative: install Partymode Autostart addon, select a video, edit the settings to your liking. This is definitely easier and I also did this at first, but it doesn't start quite at startup so it's not as elegant as I'd like.
Have fun and maybe rate me if this helped you.