Minetest-WorldEditAdditions/.docs/specs/WorldEditAdditionsSchematic_v1.md

WorldEditAdditions Schematic File Format

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• Current version: v1-DRAFT

This file format specification describes the format of WorldEditAdditions schematic files. The words MUST, MAY, SHALL, MUST NOT, etc that are used in this document are defined as in RFC 2119.

Implementers MUST clearly specify which version(s) of this specific they support.

Explanations and descriptions have both a technical description and a formal BNF description. Where the two differ, the formal BNF will always take precedence.

Purpose

The WorldEditAdditions Schematic file format is designed to store schematics of structures in the Minetest world in an efficient text-based format. It is also designed to optionally store just the changes from one state of the world to another (deltas).

In an example situation, a structure in a Minetest world is copied into a WorldEditAdditions Schematic file and saved to disk. At some later time, the resulting schematic file may be parsed and the structure written to a given Minetest world (not necessarily the same world) once more.

The rationale behind this format are as follows:

1. No other Minetest schematic format is capable of optionally storing changesets.
2. The WorldEdit format (alternative A) is space-inefficient.
3. No simple text-based format exists for storing Minetest schematics at this time, so the best of the authors' knowledge.

File extension

The file extension for WorldEditAdditions Schematic files MUST be .weaschem. Implementers SHOULD NOT choose to also parse files without this file extension.

Compression

WorldEditAdditions Schematics MAY be gzip-compressed. In such cases, the file extension MUST be .weaschem.gz. Implementers MUST transparently decompress them and parse them as normal.

Terms

Minetest

The voxel-based sandbox building game Minetest.

Schematic file

A WorldEdit Additions schematic file.

Full schematic file(s)

A WorldEdit Additions schematic file with a header value type equal to full. See types of schematic.

Delta schematic file(s)

A WorldEdit Additions schematic file with a header value type equal to delta. See types of schematic and delta schematics.

Deltas

The differences between a given region of the world in some previous state and the current state.

Schematic origin

The origin of the schematic itself. This is always (0, 0, 0). Any schematic generated from the main Minetest world MUST be translated such that the negative X, Y, and Z corner of the defined region to be converted to a schematic is (0, 0, 0) in the generated schematic file.

Offset

An offset, expressed as a Vector3, that MUST be applied to a schematic upon loading it back into the world, though implementers MAY offer an option to disable this (but applying the offset MUST be enabled by default).

BNF

Backus-Naur form. Used to formally describe the file format.

JSON Schema

The description of the format of a JSON object. See also: https://json-schema.org/.

\n

Stands for the ASCII character new line U+0A. Wherever this symbol appears, substitute with this character.

Overview

The format can be divided into 4 sequential parts:

1. Magic bytes: The string WEASCHEM is always the beginning of the file, followed by exactly 1 space ( U+20) and an integer version number
2. Header: Contains metadata about the schematic.
3. ID map: The map of node ids to their respective node names.
4. Data tables: The actual data itself.

As an example:

WEASCHEM 1
<json_header>
<id_map>
<data_table_data>
<data_table_param1>

....where text in the form <abc> are placeholders. More formally:

<weaschem> ::= <magic_bytes> <header> <id_map> <data_table_list>

<nl> ::= "\n"

<magic_bytes> ::= WEASCHEM " " <version> <nl>

<version> ::= <number>
<number> ::= <digit> | <digit> <number>
<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

<data_table_list> ::= <data_table>
	| <data_table> <nl> <data_table_list>

The non-terminal tokens <header>, <id_map>, and <data_table> are defined below.

A complete (trivial) example full schematic file is given below:

WEASCHEM 1
{"name":"Test schematic","description": "Some description","size":{"x":5,"y":3,"z":4},"offset":{"x":1,"y":0,"z":2},"type":"full","generator": "WorldEditAdditions v1.14"}
{"0":"default:air","5":"default:stone","14":"default:dirt"}
10x5,40x14,0,5,14,5,14,5x0

Magic bytes

The string WEASCHEM is always present at the beginning of a WorldEditAdditions schematic file, followed a a single space U+20, then an integer version number.

If the string WEASCHEM is not present at the very beginning of a file, then it MUST be considered invalid and a clear error message returned.

This specification, as noted above, described WorldEditAdditions schematic files version 1. It is RECOMMENDED that implementers reject schematic files with a version number that they do not support parsing with a useful error message.

For example, WEASCHEM 1 defines a schematic file of version 1.

Header

The header of the file contains all the metadata about the schematic required to restore it into the world. The header is defined as a JSON object on a single line, followed by a new line (\n U+0A):

<header> ::= <the_actual_header_json_obj> <nl>

{"foo":"bar"}\n

This JSON object follows the following JSON schema:

{
	"$schema": "https://json-schema.org/draft/2020-12/schema",
	"title": "Header",
	"description": "The header that contains the schematic's metadata.",
	"type": "object",
	"properties": {
		"name": {
			"description": "The human-readable display name of the schematic.",
			"type": "string"
		},
		"description": {
			"description": "A short description of the schematic.",
			"type": "string"
		},
		"offset": {
			"description": "The offset to place the schematic at in the world, relative to the schematic origin, which is always (0, 0, 0).",
			"$ref": "#/$defs/Vector3"
		},
		"size": {
			"description": "The size of the schematic, INDEXED from 0.",
			"$ref": "#/$defs/Vector3"
		},
		"type": {
			"description": "The type of schematic this is. Valid values: full, delta.",
			"type": "string",
			"pattern": "^(?:full|delta)$"
		},
		"generator": {
			"description": "The name and version of the software that generated this schematic.",
			"type": "string"
		}
	},
	"required": [ "name", "size", "offset", "type", "generator" ],
	"$defs": {
		"Vector3": {
			"properties": {
				"x": {
					"description": "The x co-ordinate of the Vector3.",
					"type": "integer",
				},
				"y": {
					"description": "The y co-ordinate of the Vector3.",
					"type": "integer",
				},
				"z": {
					"description": "The z co-ordinate of the Vector3.",
					"type": "integer",
				}
			}
		}
	}
}

A specific example of a header JSON object is noted below. This example is pretty-printed for convenience, but in the real file format it is stored compacted - i.e. all on one line with no pretty-printed whitespace.

{
	"name": "A castle",
	"description": "A grand fairy-tale style castle with multiple towers.",
	"size": { "x": 50, "y": 25, "z": 75 },
	"offset": { "x": 3, "y": 0, "z": 5 },
	"type": "full",
	"generator": "WorldEditAdditions v1.14"
}

Note: Implementers MUST ignore any additional unexpected properties in the header JSON object.

Size and positioning

Schematic size and positioning are controlled by 2 properties in the header JSON object:

1. size: The size of the schematic stored in the schematic
2. offset: The offset to apply to the schematic when placing it back in the world.

Sizing is measured from the negative (-x, -y, -z) corner of the region.

Writing: In Minetest VoxelManipulators do not always load the exact area you ask them for, and load additional area around them. Therefore, it is necessary to first extract the exact area you want to save before encoding the data array in a WorldEditAdditions Schematic file.

Reading: When reading a schematic file from disk, is mentioned above the (-x, -y, -z) corner of the schematic SHOULD be translated to match the (-x, -y, -z) corner, or position 1 if position 2 is defined. Then, the offset in the offset field of the header MUST be applied to translate the schematic into the final position it will be written to the target Minetest world.

Implementers MAY choose to add a disabled-by-default option to disable the application of offset.

Types of schematic

WorldEditAdditions Schematic files support two types of schematic:

1. full: Full schematic files that contain a static snapshot of the world.
2. delta: A schematic that contains only the changes between some previous state of the world and the current state of the world.

From here on, these differing types of WorldEditAdditions schematic files will be referred to as follows:

• full: Full schematic file(s)
• delta: Delta schematic file(s)

Specifically, the node id value of -2 represents the fact that no change was made between the previous state and the current state of the world.

Delta schematic files

Delta schematic files, as described above, store only the changes made to a given Minetest world between 2 different states. The format of delta schematic files differs slightly in that they store both the node id of the previous state and the node id of the current state.

Writing: When writing out a delta schematic file, the node ID in the previous state and the node ID of the current state must be written as described in the section on data tables.

Reading: When reading and parsing a delta schematic file, nodes with a node id of -2 MUST NOT be changed in the target world. Depending on the operation desired, the implementer may choose to discard either the node id of the previous or the current state:

• Undo operations: Node ids of the previous state should be kept and used, and node ids of the current state discarded.
• Redo operations: Node ids of the current state should be kept and used, with node ids of the previous state discarded.

See also the specific notes in the sections below.

ID map

Like the header, the ID map is also defined as a JSON object on a single line, followed by a new line character (\n U+0A):

<id_map> ::= <the_actual_id_map_json_obj> <nl>

{"5": "foo"}\n

The ID map's purpose is to map node IDs in the schematic file to canonical node names, since Minetest does not guarantee that node names will remain the same across different worlds.

Node names in a schematic file MUST be the full canonical node name, and not an abbreviation. For example, default:stone is valid, but stone is invalid (does not specify the mod).

Node IDs in the id map MUST be an integer, and MUST NOT be less than 0. Specifically, node ids in the id map that are less than 0 are considered invalid, and node ids greater than -1 are considered valid. See also special node ids.

Implementers MAY perform arbitrary transformations on node names when reading or writing a WorldEditAdditions schematic file, but any such transformations are beyond the scope of this specification.

When loading a schematic, it is RECOMMENDED that implementers use a cache when translating schematic node ids (from this ID map) to node ids from the target world.

Formally, the ID map can be described by the following JSON schema:

{
	"$schema": "https://json-schema.org/draft/2020-12/schema",
	"type": "object",
	"title": "ID Map",
	"description": "The JSON schema for the ID Map of a WorldEditAdditions Schematic file.",
	
	"patternProperties": {
		"^[0-9]+$": {
			"type": "string",
			"pattern": "^[^\s]+:[^\s]+$"
		}
	},
	"additionalProperties": false
}

In other words, an example ID map might look like this (pretty-printed for convenience, but in an actual file this would compacted and all on one line):

{
	"0": "default:air",
	"5": "default:stone",
	"14": "default:dirt"
}

It is not required that Node IDs in the schematic file be sequential.

Data tables

The data tables store the actual data in the schematic. This data is stored in flat arrays with optional run-length encoding. Using the size given in the header, it is possible to read the data and understand where in the array each block is located. Each data table occupies a single line (delimited with \n U+0A as with everything else above).

<data_table> ::= <data_table_item>
	| <data_table_item> "," <data_table>

<data_table_item> ::= <number>
	| <number> x <number>

<number> ::= <digit>
	| <digit> <number>

<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

There are currently the following data tables that are stored in schematics, that MUST be present in the following orders, depending on the type of schematic file (see types of schematic).

For full schematic files:

1. data: The list of node IDs
2. param2: The list of param2 values

For delta schematic files:

1. data (previous state): The list of node IDs for the previous state
2. param2 (previous state): The list of param2 values for the previous state
3. data (current state): The list of node IDs for the current state
4. param2 (current state): The list of param2 values for the current state

Note: Either type of schematic file MAY contain additional data tables beyond those described here. In such cases, implementers MUST ignore them, and any such tables MUST be present only after the data tables described above.

Here's a specific example of the data tables for a sample full schematic file:

10x5,40x14,0,5,14,5,14,5x0
51x0,255,8x0

The first line is the node IDs table (data), and the second line are the param2 values.

Implementers MAY choose to optimise filesize by compacting the node IDs used (e.g. mapping the node id 456 to an id of 5).

Note: Node IDs in a schematic file need to be mapped to the node IDs of the target world when writing a schematic to a Minetest world, as they are unlikely to match.

Special node ids

The following node id values have specific meaning:

Node ID	Purpose
-1	Represents a value of null/nil. This node id value indicates that no node is stored in this cell. When reading a schematic and writing it to the world, implementers MUST NOT alter the pre-existing node in the world that corresponds with this cell.
-2	In a delta schematic only, This node id value indicates that no change was made between the time the schematic was written and the previous time it was compared against. Using this value as a node in a schematic with a type of full is invalid, and in such a case implementers MUST raise an error message and abort reading of the schematic.