Pre-Request Checklist

• I have checked that I am on the latest version of Terra.
• I have searched github for similar features requests, including closed
  ones, and found none.
• I believe this is within the scope of Terra.
• This feature request is for all of Terra, and isn't something that
  should be implemented by a pack or addon.

Feature Description

A generator that generalizes terrain, carving, and ocean, referred to as 'layers'. These layers and their generation behaviour would be config defined rather than hardcoded by the generator.

What Problem Does This Solve?

Adding new aspects of generation to a pack such as aquifers is difficult to do properly without code changes to facilitate it. Data driven generation layers should provide the functionality required for such aspects, and is also in line with the Terra design philosophy, such that concepts like 'ocean' and 'carving' are not hard coded in.

A Solution You'd Like

• Pack defined generation 'layers'

• A layer consists of block palette(s) distributed in some way, e.g via sampler:

  • These block palettes and distribution may be determined within the layer definition.

  • In addition individual biomes may define their own specific palettes and or samplers for a layer.

  • Whether a biome or the layer itself determines these parameters could be dependent on what the layer specifies:

    Default palettes and or distribution would optionally be determined within the layer definition, otherwise if not
    specified then biomes would be required to define palettes and distribution for the relevant layers.

    Biome specific palettes and or distribution config keys would dynamically be registered similar to keys under features.

    A layer palette would be analogous to the palettes key, and distribution would be analogous to the parameters under
    terrain.

• Layers could also determine sampling and interpolation behaviour: see #126.

• The way each layer generates would be determined by some kind of pack defined logic, e.g a binary decision diagram based on each layer's distribution.

  For the layers terrain, ocean and carving, the gen logic would be something like the following:

  if terrain
    if carving
      place carving palette (e.g air)
    else
      place terrain palette
  else
    if ocean
      place ocean palette
  else
    some other behaviour, or no op
  

Alternative Solutions

The best alternative available right now would be using features to generate things like aquifers, and underground lava oceans, however doing it that way lacks advantages of this generator such as control over interpolation and sampling, more control over initial chunk generation, etc..

This would probably be implemented in a new chunk generator addon, that way we can keep the noise 3d generator addon around for beginners/simple packs.

Mockup config for how the layered generator could work

# Layer predicates define a predicate that takes world coordinates
# Not sure how exactly these should be defined
# The functionality of terrain.sampler, terrain.sampler-2d, carving.sampler, and ocean.level would be utilized here
layer-predicates:
  is-terrain:
  is-carving:
  is-aquifer-border:
  is-aquifer-water:
  is-aquifer-lava:
  is-under-lava-level:
  is-under-ocean-level:

# Layer palettes define sets of block placements
layer-palettes:
  terrain:
    type: BIOME_DEFINED # Allows biomes to specify a palette, this would be dynamically registered as a config key
    default: # What the default palette is if a biome doesn't specify a palette
      type: BLOCK # A single block palette
      block: minecraft:stone
  air:
    type: BLOCK
    block: minecraft:air
  ocean:
    type: BIOME_DEFINED
    default:
      type: BLOCK
      block: minecraft:water
  underground-lava:
    type: BLOCK
    block: minecraft:lava
  aquifer-water:
    type: BIOME_DEFINED
    default:
      type: BLOCK
      block: minecraft:water
  aquifer-lava:
    type: BLOCK
    block: minecraft:lava

# Converts to a binary decision diagram
# The 'if' key specifies a layer predicate
# The 'place' key specifies a layer palette
layer-placement: 
  if: is-terrain
  then:
    if: is-carving
    then:
      if: is-aquifer-border
      then:
        place: terrain
      else:
        if: is-aquifer-water
        then:
          place: aquifer-water
        else:
          if: is-aquifer-lava
          then:
            place: aquifer-lava
          else:
            if: is-under-lava-level
            then:
              place: underground-lava
            else:
              place: air
    else:
      place: terrain
  else:
    if: is-under-ocean-level
    then:
      place: ocean
    else:
      place: air

The following layer predicates could share the same sampler:

layer-predicates:
  is-aquifer-border:
  is-aquifer-water:
  is-aquifer-lava:

So perhaps there would be another component that is used to construct the predicates, i.e a layer
distributor.

Layer distributors should supersede the capabilities of the biome keys terrain, carving,
and ocean.level, such that you can do things like specify interpolation, combining 2D & 3D samplers, etc