Some butterflies are either extremely rare or impossible to find on default world settings

[Ackee]

The African clouded yellow has the following spawn conditions: average yearly temperature of 28.8-38.8, precipitation of 0.2-0.4, forest% of 0-0.5, and Y level of 1.9-2 on a scale of 0 to 2 where 2 is the world height. The Y level and average yearly temperature values is where the problem lies. In a default world where the height limit is 256, the butterfly can only spawn at a Y level or 244 or higher. However, the temperature drops off the higher you go, and mountains tend to have a much lower average yearly temperature than the surrounding areas. The temperature noise map only goes up to 40 degrees when altitude is not accounted for, so even when you go somewhere with the absolute hottest possible average temperature at sea level (40 degrees), the corresponding average temperature at Y = 244 ends up in the low 20s. If I'm not mistaken, this makes it impossible for the African clouded yellow impossible to spawn naturally.

Also, the butterflies with 0.0003 and 3.0E-5 spawn chances almost never spawn in practice. Because of the way butterflies spawn, the most common ones tend to spawn in multiples. This, combined with the butterfly spawn cap, means that the spawn cap of 20 tends to get clogged by a few instances of the same common butterflies spawning in massive groups. You'll be in an area capable of spawning rare butterflies, and the 20 butterfly cap will immediately get clogged by 4 large white females, 3 large white males, 3 meadow brown males, and 5 small white males, 3 small tortoiseshells, and 2 common brimstone females while leaving no room for the elusive Comma ab. Dilutus to spawn unless you load a new area and reroll the odds again. This means that even though you have 20 rolls of the dice in theory, you only get about 4 to 6 in practice. Even if one does spawn, it is very unlikely for a player to happen upon it without mods, due to the fact that only 1 can exist at a time. 

Side note, since this is less significant than the other two points, but the African mapwing has strangely particular spawning condition, and it seems to be extremely rare. It requires temperatures from 35-45, and even if you find a hot enough zone, you need the precipitation and forest% to be as low as possible. Even getting just the temperature and precipitation to match is hard enough, but every time that happened, the forest% was very high. I ended up giving up after searching for hours in creative, even with multiple mods installed to detect butterflies and climate conditions. I was only able to get them to spawn when I made a new world with different settings.

To make it so that collecting all 169 butterflies is feasable, I suggest changing the spawning requirements of the African Clouded Yellow, African Mapwing, and changing some of the numbers to make rarer butterflies actually spawn. The only butterflies I've seen that had a 0.003 spawn chance or lower are the Freyers Purple Emperor, Blue Admiral, Ornithoptera Priamus Miokensis, Queen Victorias Birdwing, and Arctic Butterflies. These also happen to be the only butterflies that share their spawn conditions with few enough other butterflies to make it physically impossible for the spawn cap to be clogged (in most cases), thus giving them the ability to spawn eventually. I can't even conceive of how rare the Wallace's Golden Birdwing must be, since it shares its spawn conditions with 30-44 other butterflies (depending on the climate), all of whom are at least 10x likelier to spawn, but many of whom are up to 1000x likelier to spawn. It must be astronomically unlikely for it spawn before the spawn cap gets immediately filled by the dozens of other candidates.

Also please correct me if I get any small details wrong. Also I lied I've also seen a Nessus Leafwing but there's like 17 butterflies in that area with 0.003 spawn chance or rarer so one was bound to show up eventually.

[MKMoose]

9 hours ago, Ackee said:

The African clouded yellow has the following spawn conditions: average yearly temperature of 28.8-38.8, precipitation of 0.2-0.4, forest% of 0-0.5, and Y level of 1.9-2 on a scale of 0 to 2 where 2 is the world height. The Y level and average yearly temperature values is where the problem lies. In a default world where the height limit is 256, the butterfly can only spawn at a Y level or 244 or higher. However, the temperature drops off the higher you go, and mountains tend to have a much lower average yearly temperature than the surrounding areas. The temperature noise map only goes up to 40 degrees when altitude is not accounted for, so even when you go somewhere with the absolute hottest possible average temperature at sea level (40 degrees), the corresponding average temperature at Y = 244 ends up in the low 20s. If I'm not mistaken, this makes it impossible for the African clouded yellow impossible to spawn naturally.

Pretty sure that since 1.22 the temperature ranges for spawning use the temperature at a fixed elevation of (if I remember correctly) 1.09, where 1 is sea level (Y = 110 on the default configuration), so it would end up being something like Y = 123. So this one seems like it should spawn fine, though minY: 1.9 is still quite questionable and might make it extraordinarily rare (and 1.9 should be Y = 241, I think?).

Other than that the details seem fine at a glance, though I haven't verified them exactly. I can absolutely get behind making a full butterfly collection more reasonably achievable.

Edited 19 hours ago by MKMoose