Problem

Scatter/Blob ores currently determine the number of clusters to generate in a chunk by doing an integer division of the eligible volume (chunk volume limited by the ore y range) by clust_scarcity ignoring the remainder. This gives unexpected results if eligible volume and scarcity values are of similar magnitude.

This effect is easy to see using the Skyblock:zero game (I'm not affiliated with that project), because it has a convenient ore that is placed in air using clust_scarcity = 80*80*80:

1. start a new game, notice the blue thingies - the emitters - around you
2. stop the game, delete the map
3. change to clust_scarcity = 80*80*80+1 in mods/sbz_planets/biomes.lua
4. restart the game, notice that no emitters are generated at all
5. stop the game, delete the map
6. change to clust_scarcity = 80*80*40+1
7. restart the game, notice that emitters are exactly the same as with clust_scarcity = 80*80*80 (-> half the available clust_scarcity range gives the same distribution)

Other games (e.g. Mineclonia - which I'm affiliated with) are affected right now, because some ores are very rare and/or ore y range is very small (either because the game is trying to replicate a height dependent probability or because of an unfortunate intersection of the ore y range and the chunk borders, e.g. having an ore with y_max = -200). A one node high layer ore only has a usable clust_scarcity range of 1 to 6400.

Solutions

Don't ignore the remainder of the volume by scarcity division and randomly generate enough additional clusters to make up for these lost partial ore clusters.

Alternatives

The useful clust_scarcity range could be extended right now by games using an increased chunksize.

Supporting non uniform y dependent ore distributions in the engine could reduce the need for thin layer ore definitions.

EDIT: noise_params (in addition to clust_scarcity) can be used to tweak the ore distribution