I've been trying to figure out why I rule half the world and despite almost all of it having been wrong-religion at some point and being ruled by my culture rulers, a grand total of 3 provinces have actually culture flipped. So I believe I've pretty much nailed it down.
Code:
# Bring in some settlers
province_event = {
id = 55001
desc = "EVTDESC55001"
picture = GFX_evt_carriage
only_rulers = yes
trigger = {
owner = {
top_liege = {
culture = PREV
}
NOT = { culture = ROOT }
capital_scope = {
province_id = ROOT
}
OR = {
primary_title = {
conquest_culture = PREV
}
ROOT = {
county = {
conquest_culture = PREVPREV
}
}
ROOT = {
duchy = {
holder_scope = {
character = PREVPREVPREV
}
conquest_culture = PREVPREV
}
}
ROOT = {
kingdom = {
holder_scope = {
character = PREVPREVPREV
}
conquest_culture = PREVPREV
}
}
ROOT = {
empire = {
holder_scope = {
character = PREVPREVPREV
}
conquest_culture = PREVPREV
}
}
}
}
NOT = { # Exclude provinces covered by event 55000
any_neighbor_province = {
ROOT = {
owner = {
culture = PREVPREV
}
}
owner = {
ROOT = {
owner = {
culture = PREVPREV
}
}
}
}
}
}
The key aspect is the 'conquest_culture' tag. It would seem that every single title has its own culture tag. I've only seen it work out with kingdom-level titles, but I believe that it works with empire-level titles. This would mean that changing de jure borders would result in changing culture.. However it's very unclear as to what culture each title is.
Personally, I can understand if the Byzantine Empire is Greek culture, and the Persian Empire is Persian culture, and the HRE is German culture (which explains why Italy was removed from its de jure borders).
The Arabian Empire is, however, completely ambiguous. Arabian is a culture group. As an Egyptian of the Arabian Empire, I've expanded its de jure borders quite successfully, however I just delved through the files to find that e_arabia is bedouin arabic culture so it's secretly useless in terms of culture shifting. I ask that the devs address this in the next patch as it completely screws over the non-bedouin arabic player or rather gives a massive advantage to bedouin arabs. I suggest that either the empire isn't allowed to be formed by all arabs or it's considered part of the arabic culture group as I assume that titles can't have multiple culture tags.
Looking through the files (landed_titles.txt) I see a few cases where this feature is hindered by the lack of support for multiple culture tags. The empire of scandinavia is swedish culture, which means that if Danes form it they won't see culture spread in the same way they will if Swedes form it. The kingdom of england entirely lacks a culture tag presumably to not favour the English over the Saxons or vice versa, but in practice would just screw over both of them instead of just one by making it impossible for English/Saxon culture to spread.
Simply by making either multiple culture tags possible for a title or having the culture tag of a title able to change in mid-game would address all of these issues. Ideally both of those solutions would be implemented, as I see that the Empire of Britannia is without culture tag, and presumably if it had all of the british cultures tagged then culture would spread absurdly easily from the start. Perhaps Empires having no culture tag by default and simply being made in the culture of whoever formed them would be best, as it would allow for Andalusian Spain, Welsh Britannia, Norwegian Scandinavia, etc. At the moment cultures who are able to form empire titles yet aren't actually able to culturally utilize those titles are really screwed over.
notreallyanedit: Looking at the other thread cwth linked I'm going to assume that the titles changing culture due to war thing is really badly implemented, as multiple cases of Jihads, invasions, etc. would have presumably changed the culture of the kingdoms I took and subsequently gave to vassals, yet there's no apparent effect.
I do like the idea of de jure realm influencing culture, as it gives reason to destroy titles among other things, but it really needs to be either fixed or the old culture change event added back in as it's not working right in general.