Fallout transform

After a nuclear blast, fallout rain sweeps outward from ground zero and converts blocks: logs become dead logs, leaves fall, ores near the centre turn into sellafite ores, and so on. The conversion rules are 1.7’s hbmFallout.json, read from a datapack folder:

data/<ns>/fallout_transform/<name>.json

HBM’s defaults are data/hbm/fallout_transform/default.json (148 rules, priority 0).

File format

{
  "priority": 0,
  "entries": [
    {
      "tag": "minecraft:logs",
      "primary": [{ "state": "hbm:waste_log[axis=y]" }],
      "max": 65.0
    },
    {
      "block": "minecraft:coal_ore",
      "primary": [
        { "state": "hbm:ore_sellafield_diamond[shade=9]", "weight": 3 },
        { "state": "hbm:ore_sellafield_emerald[shade=9]", "weight": 2 }
      ],
      "primary_chance": 0.5,
      "max": 5.0,
      "solid": true
    }
  ]
}

priority (integer, default 0) orders the files. entries is the list of rules.

Rule fields

A rule matches a block, and then picks the block it becomes.

Field	Type	Default	Meaning
`state`	block state string	none	Matches this exact state, e.g. `"minecraft:oak_log[axis=y]"`.
`block`	block id	none	Matches any state of this block.
`tag`	block tag id, without `#`	none	Matches blocks in this tag.
`preset`	string	none	Matches a named predicate (see below).
`opaque`	boolean	`false`	Also requires the block to be a full opaque block.
`primary`	list of outcomes	empty	Weighted outcomes, used with probability `primary_chance`.
`secondary`	list of outcomes	empty	Weighted outcomes, used otherwise.
`primary_chance`	double	`1.0`	Chance (0 to 1) to pick from `primary` instead of `secondary`.
`min`	double	`0.0`	The rule applies from this distance on, in percent of the fallout radius.
`max`	double	`100.0`	The rule applies up to this distance, in percent of the fallout radius.
`falloff_start`	double	`0.9`	Past `max × falloff_start` the rule is skipped more and more often, so the edge is ragged.
`solid`	boolean	`false`	A conversion by this rule counts toward the column’s depth limit.
`preserve_axis`	boolean	`false`	Keeps the `axis` of the original block (for logs and pillars).

Every matcher a rule gives must match. A rule with no matcher matches every block in its distance range.

An outcome is {"state": "<block state>", "weight": <int>}; weight defaults to 1. "state": "minecraft:air" removes the block. A block removed while it was waterlogged (seagrass, for example) leaves its water instead of air. If the chosen list (primary or secondary) is empty, the rule does nothing for that block and the next rule is tried.

Presets

Preset	Matches
`hbm:ignited_by_lava`	Blocks lava can set on fire (1.7’s wood material).
`hbm:vegetation`	Plants, vines and sugar cane.
`hbm:solid_terrain`	Opaque blocks mined with a pickaxe or shovel, except ice, snow, the redstone lamp, grass blocks and mycelium (1.7’s rock, iron, sand and ground materials).
`hbm:grass_like`	Grass blocks and mycelium.

Addons can register more presets.

How rules are evaluated

The sweep walks each column of blocks from the surface down. For each block it tries the rules in order, and the first rule that returns a block wins. A column stops after three conversions by solid rules.

The order is: files sorted by priority, highest first, then by file id; inside a file, the order of entries.

Some limits are fixed in code, as in 1.7:

Sellafite ores and slaked sellafite only ever convert to a higher shade, never back.
Sellafite bedrock never converts into anything else.
The lowest block of the world only converts to sellafite bedrock.

Adding rules

Put your rules in your own namespace with a priority above 0, so they are tried before HBM’s:

{
  "priority": 10,
  "entries": [
    {
      "tag": "minecraft:impermeable",
      "primary": [{ "state": "minecraft:air" }],
      "max": 40.0
    }
  ]
}

Only the highest-priority pack’s copy of a given file id is read. A file at data/hbm/fallout_transform/default.json replaces HBM’s whole table, so only do that when you mean to rewrite every rule.

Fallout rules are reloaded with /reload. A file that fails to parse is skipped with Fallout transform file <id> is broken, skipping it in the log.