Processors allow for modifying structures. This is used to use generic builds, which are then customized through the processors for whatever purpose they have.

Block matching processors

The most common approaches used for processors is to match blocks and then change them to other blocks. This could be to simulate decay, or switch a build from using oak_planks to spruce_planks.

Some of the more obscure supported processors are somewhat harder to find information about online though.

Changing NBT

It is possible to change the NBT of a block. This can be used to change the loot table of a chest:

{
  "processors": [
    {
      "processor_type": "minecraft:rule",
      "rules": [
        {
          "input_predicate": {
            "predicate_type": "block_match",
            "block": "minecraft:chest"
          },
          "location_predicate": {
            "predicate_type": "always_true"
          },
          "output_state": {
            "Name": "minecraft:chest"
          },
          "output_nbt": "{LootTable:\"polystruct:chests/fishing_ruin\"}"
        }
      ]
    }
  ]
}

In the above example, the LootTable is escaped. There are example datapacks online where no escaping is done, so this might be supported nowadays.

By modifying the NBT, you can add in hidden treasures in blocks that users don’t immediately associate with it. For instance, the NBT for a blast furnace could be set as containing a bundle, which contains 3 diamonds and 15 charcoal:

{Items:[
  {Slot:1, id:"minecraft:bundle", tag:{Items:[
    {id:"minecraft:diamond", Count:3b},
    {id:"minecraft:charcoal", Count:15b}
   ]},Count:1}
]}

Common errors

Not a json array

The following error can occur when the expected fields are not defined:

Caused by: java.lang.RuntimeException: Not a json array: {"processors":[{"processor_type":"minecraft:rule","rules":[{"input_predicate":{"block":"minecraft:red_sandstone","predicate_type":"minecraft:block_match"},"output_state":{"Name":"minecraft:structure_void"}}]}]}

This does not mean the JSON is incorrect. You can verify this with jq.

Instead, in the above error, the processor is missing the location_predicate field. This field always needs to be defined, so add the following to the processor:

"location_predicate": { "predicate_type": "minecraft:always_true" },