Overmap Generation


The overmap is what you see in the game when you use the view map (m) command.

It looks something like this:


In the context of overmap generation, the overmap is the collection of data and features that define the locations in the game world at a more abstract level than the local map that the player directly interacts with, providing the necessary context for local map generation to occur.

By example, the overmap tells the game:

  • where the cities are
  • where the roads are
  • where the buildings are
  • what types the buildings are

but it does not tell the game:

  • what the actual road terrain looks like
  • what the actual building layout looks like
  • what items are in the building

It can sometimes be useful to think of the overmap as the outline for the game world which will then be filled in as the player explores. The rest of this document is a discussion of how we can create that outline.

Overmap generation

Generating an overmap happens in the following sequence of functions ( see generate::overmap in overmap.cpp ):

populate_connections_out_from_neighbors( north, east, south, west );
place_rivers( north, east, south, west );
place_roads( north, east, south, west );
place_specials( enabled_specials );

This has some consequences on overmap special spawning, as discussed in the relevant section.

Terminology and Types

First we need to briefly discuss some of the data types used by the overmap.


The fundamental unit of the overmap is the overmap_terrain, which defines the id, name, symbol, color (and more, but we’ll get to that) used to represent a single location on the overmap. The overmap is 180 overmap terrains wide, 180 overmap terrains tall, and 21 overmap terrains deep (these are the z-levels).

In this example snippet of an overmap, each character corresponds one entry in the overmap which references a given overmap terrain:


So for example, the F is a forest which has a definition like this:

    "type": "overmap_terrain",
    "id": "forest",
    "name": "forest",
    "sym": "F",
    "color": "green"

and the ^ is a house which has a definition like this:

    "type": "overmap_terrain",
    "id": "house",
    "name": "house",
    "sym": "^",
    "color": "light_green"

It’s important to note that a single overmap terrain definition is referenced for every usage of that overmap terrain in the overmap–if we were to change the color of our forest from green to red, every forest in the overmap would be red.

overmap_special / city_building

The next important concept for the overmap is that of the overmap_special and city_building. These types, which are similar in structure, are the mechanism by which multiple overmap terrains can be collected together into one conceptual entity for placement on the overmap. If you wanted to have a sprawling mansion, a two story house, a large pond, or a secret lair placed under an unassuming bookstore, you would need to use an overmap_special or a city_building.

These types are effectively a list of the overmap terrains that compose them, the placement of those overmap terrains relative to each other, and some data used to drive the placement of the overmap special / city building (e.g. how far from a city, should be it connected to a road, can it be placed in a forest/field/river, etc).

When retiring or migrating an overmap special, the id should be defined in a new overmap_special_migration, usually defined in data/json/overmap/overmap_special/overmap_special_migration.json as follows:

  "type": "overmap_special_migration",
  "id": "Military Bunker",
  "new_id": "military_bunker"

Specials that have been removed (not just renamed) should have a blank new_id, or just omit the new_id field altogether.


Speaking of roads, the concept of linear features like roads, sewers, subways, railroads, and forest trails are managed through a combination of overmap terrain attributes and another type called an overmap_connection.

An overmap connection effectively defines the types of overmap terrain on which a given connection can be made, the “cost” to make that connection, and the type of terrain to be placed when making the connection. This, for example, is what allows us to say that:

  • a road may be placed on fields, forests, swamps and rivers
  • fields are preferred over forests, which are preferred over swamps, which are preferred over rivers
  • a road crossing a river will be a bridge

The overmap_connection data will then be used to create a linear road feature connecting two points, applying the previously defined rules.


We’ve previously mentioned defining valid overmap terrain types for the placement of overmap specials, city buildings, and overmap connections, but one thing to clarify is these actually leverage another type called an overmap_location rather than referencing overmap_terrain values directly.

Simply put, an overmap_location is just a named collection of overmap_terrain values.

For example, here are two simple definitions.

    "type": "overmap_location",
    "id": "forest",
    "terrains": [ "forest" ]
    "type": "overmap_location",
    "id": "wilderness",
    "terrains": [ "forest", "field" ]

The value of these is that they allow for a given overmap terrain to belong to several different locations and provide a level of indirection so that when new overmap terrains are added (or existing ones removed), only the relevant overmap_location entries need to be changed rather than every overmap_connection, overmap_special, and city_building that needs those groups.

For example, with the addition of a new forest overmap terrain forest_thick, we only have to update these definitions as follows:

    "type": "overmap_location",
    "id": "forest",
    "terrains": [ "forest", "forest_thick" ]
    "type": "overmap_location",
    "id": "wilderness",
    "terrains": [ "forest", "forest_thick", "field" ]

Overmap Terrain


If an overmap terrain can be rotated (i.e. it does not have the NO_ROTATE flag), then when the game loads the definition from JSON, it will create the rotated definitions automatically, suffixing the id defined here with _north, _east, _south or _west. This will be particularly relevant if the overmap terrains are used in overmap_special or city_building definitions, because if those are allowed to rotate, it’s desirable to specify a particular rotation for the referenced overmap terrains (e.g. the _north version for all).


Identifier Description
type Must be "overmap_terrain".
id Unique id.
name Name for the location shown in game.
sym Symbol used when drawing the location, like "F" (or you may use an ASCII value like 70).
color Color to draw the symbol in. See COLOR.md.
looks_like Id of another overmap terrain to be used for the graphical tile, if this doesn’t have one.
connect_group Specify that this overmap terrain might be graphically connected to its neighbours, should a tileset wish to. It will connect to any other overmap_terrain with the same connect_group.
see_cost Affects player vision on overmap. Higher values obstruct vision more.
travel_cost_type How to treat this location when planning a route using autotravel on the overmap. Valid values are road,field,dirt_road,trail,forest,shore,swamp,water,air,impassable,other. Some types are harder to travel through with different types of vehicles, or on foot.
extras Reference to a named map_extras in region_settings, defines which map extras can be applied.
mondensity Summed with values for adjacent overmap terrains to influence density of monsters spawned here.
spawns Spawns added once at mapgen. Monster group, % chance, population range (min/max).
flags See Overmap terrains in JSON_FLAGS.md.
mapgen Specify a C++ mapgen function. Don’t do this–use JSON.
mapgen_straight Specify a C++ mapgen function for a LINEAR feature variation. Prefer JSON instead.
mapgen_curved Specify a C++ mapgen function for a LINEAR feature variation. Prefer JSON instead.
mapgen_end Specify a C++ mapgen function for a LINEAR feature variation. Prefer JSON instead.
mapgen_tee Specify a C++ mapgen function for a LINEAR feature variation. Prefer JSON instead.
mapgen_four_way Specify a C++ mapgen function for a LINEAR feature variation. Prefer JSON instead.
eoc Supply an effect_on_condition id or an inline effect_on_condition. The condition of the eoc will be tested to see if the special can be placed. The effect of the eoc will be run when the special is placed. See effect_on_condition.md.
entry_eoc An effect on condition ID that will run when you enter this location.
exit_eoc An effect on condition ID that will run when you exit this location.


A real overmap_terrain wouldn’t have all these defined at the same time, but in the interest of an exhaustive example…

    "type": "overmap_terrain",
    "id": "field",
    "name": "field",
    "sym": ".",
    "color": "brown",
    "looks_like": "forest",
    "see_cost": 2,
    "extras": "field",
    "mondensity": 2,
    "spawns": { "group": "GROUP_FOREST", "population": [ 0, 1 ], "chance": 13 },
    "flags": [ "NO_ROTATE" ],
    "mapgen": [ { "method": "builtin", "name": "bridge" } ],
    "mapgen_straight": [ { "method": "builtin", "name": "road_straight" } ],
    "mapgen_curved": [ { "method": "builtin", "name": "road_curved" } ],
    "mapgen_end": [ { "method": "builtin", "name": "road_end" } ],
    "mapgen_tee": [ { "method": "builtin", "name": "road_tee" } ],
    "mapgen_four_way": [ { "method": "builtin", "name": "road_four_way" } ],
    "travel_cost_type": "field",
    "eoc": {
      "condition": { "compare_num": [ { "global_val": "var", "var_name": "refugee_centers", "default": 0 }, "<", { "const": 1 } ] },
      "effect": [ { "arithmetic": [ { "global_val": "var", "var_name": "refugee_centers" }, "++" ] } ]

Overmap Special

An overmap special is an entity that is placed in the overmap after the city generation process has completed–they are the “non-city” counterpart to the city_building type. They commonly are made up of multiple overmap terrains (though not always), may have overmap connections (e.g. roads, sewers, subways), and have JSON-defined rules guiding their placement.

Special placement

There are a finite number of sectors in which overmap specials can be placed during overmap generation, each being a square with side length equal to OMSPEC_FREQ (defined in omdata.h; at the time of writing OMSPEC_FREQ= 15 meaning each overmap has 144 sectors for placing specials). At the beginning of overmap generation, a list of eligible map specials is built (overmap_special_batch). Next, a free sector is chosen where a special will be placed. A random point in that sector is checked against a random rotation of a random special from the special batch to see if it can be placed there. If not, a new random point in the sector is checked against a new special, and so on until a valid spawn is rolled.

Fixed vs mutable specials

There are two subtypes of overmap special: fixed and mutable. Fixed overmap specials have a fixed defined layout which can span many OMTs, and can rotate (see below) but will always look essentially the same.

Mutable overmap specials have a more flexible layout. They are defined in terms of a collection of overmap terrains and the ways they can fit together, like a jigsaw puzzle where pieces can fit together in multiple ways. This can be used to form more organic shapes, such as tunnels dug underground by giant ants, or larger sprawling buildings with mismatched wings and extensions.

Mutable specials require a lot more care to design such that they can be reliably placed without error.


In general, it is desirable to define your overmap special as allowing rotation–this will provide variety in the game world and allow for more possible valid locations when attempting to place the overmap special. A consequence of the relationship between rotating an overmap special and rotating the underlying overmap terrains that make up the special is that the overmap special should reference a specific rotated version of the associated overmap terrain–generally, this is the _north rotation as it corresponds to the way in which the JSON for mapgen is defined.


The overmap special can be connected to the road, subway or sewer networks. Specifying a connection point causes the appropriate connection to be automatically generated from the nearest matching terrain , unless existing is set to true.

Connections with existing set to true are used to test the validity of an overmap special’s placement. Unlike normal connection points these do not have to reference road/tunnel terrain types. They will not generate new terrain, and may even be overwritten by the overmap special’s terrain. However, since the overmap special algorithm considers a limited number of random locations per overmap, the use of existing connections that target a rare terrain lowers the chances of the special spawning considerably.

Occurrences (default)

Occurrences is the way to set the baseline rarity of a special. The field can behave in two ways: by default, it sets the minimum and maximum occurrences of the special per overmap. Currently all overmap specials have a minimum occurrence of 0, to keep the overmaps from being too similar to each other. In addition, there are no specials with a maximum occurrence of 1. This is important because each normal special has a very high chance of being placed at least once per overmap, owing to some quirks of the code (most notably, the number of specials is only slightly more than the number of slots per overmap, specials that failed placement don’t get disqualified and can be rolled for again, and placement iterates until all sectors are occupied). For specials that are not common enough to warrant appearing more than once per overmap please use the “UNIQUE” flag. For specials that should only have one instance per world use “GLOBALLY_UNIQUE”.


When the special has the “UNIQUE” or “GLOBALLY_UNIQUE” flag, instead of defining the minimum and maximum number placed. the occurrences field defines the chance of the special to be included in any one given overmap. Before any placement rolls, all specials with this flag have to succeed in an x_in_y (first value, second value) roll to be included in the overmap_special_batch for the currently generated overmap; any special that failed this roll will never be considered for placement. Currently all UNIQUE specials use [x, 100] occurrences - percentages - for ease of understanding, but this is not required.


The overmap special has two mechanisms for specifying the valid locations (overmap_location) that it may be placed on. Each individual entry in overmaps may have the valid locations specified, which is useful if different parts of a special are allowed on different types of terrain (e.g. a dock that should have one part on the shore and one part in the water). If all values are the same, locations may instead be specified for the entire special using the top level locations key, The value for an individual entry takes precedence over the top level value, so you may define the top level value and then only specify it for individual entries that differ.

City distance and size

During generation of a new overmap, cities and their connecting roads will be generated before specials are placed. Each city gets assigned a size at generation and will begin its life as a single intersection. The city distance field specifies the minimum and maximum distance the special can be placed from the edge of the urban radius of a city, and not from the center of the city. Both city size and city distance requirements are only checked for the “nearest” city, measured from the original intersection.


Identifier Description
type Must be "overmap_special".
id Unique id.
subtype Either "fixed" or "mutable". Defaults to "fixed" if not specified.
locations List of overmap_location ids that the special may be placed on.
city_distance Min/max distance from a city edge that the special may be placed. Use -1 for unbounded.
city_sizes Min/max city size for a city that the special may be placed near. Use -1 for unbounded.
occurrences Min/max number of occurrences when placing the special. If UNIQUE flag is set, becomes X of Y chance.
priority Warning: Do not use this unnecessarily. The generation process is executed in the order of specials with the highest value. Can be used when maps are difficult to generate. (large maps, maps that are or require dependencies etc) It is strongly recommended to set it to 1 (HIGH priority) or -1 (LOW priority) if used. (default = 0)
flags See Overmap specials in JSON_FLAGS.md.
rotate Whether the special can rotate. True if not specified.

Depending on the subtype, there are further relevant fields:

Further fields for fixed overmap specials

Identifier Description
overmaps List of overmap terrains and their relative [ x, y, z ] location within the special.
connections List of overmap connections and their relative [ x, y, z ] location within the special.

Further fields for mutable overmap specials

Identifier Description
check_for_locations List of pairs [ [ x, y, z ], [ locations, ... ] ] defining the locations that must exist for initial placement.
check_for_locations_area List of check_for_locations area objects to be considered in addition to the explicit check_for_locations pairs.
overmaps Definitions of the various overmaps and how they join to one another.
root The initial overmap from which the mutable overmap will be grown.
phases A specification of how to grow the overmap special from the root OMT.

Example fixed special

    "type": "overmap_special",
    "id": "campground",
    "overmaps": [
      { "point": [ 0, 0, 0 ], "overmap": "campground_1a_north", "locations": [ "forest_edge" ] },
      { "point": [ 1, 0, 0 ], "overmap": "campground_1b_north" },
      { "point": [ 0, 1, 0 ], "overmap": "campground_2a_north" },
      { "point": [ 1, 1, 0 ], "overmap": "campground_2b_north" }
    "connections": [ { "point": [ 1, -1, 0 ], "terrain": "road", "connection": "local_road", "from": [ 1, 0, 0 ] } ],
    "locations": [ "forest" ],
    "city_distance": [ 10, -1 ],
    "city_sizes": [ 3, 12 ],
    "occurrences": [ 0, 5 ],
    "flags": [ "CLASSIC" ],
    "rotate" : true

Fixed special overmaps

Identifier Description
point [ x, y, z] of the overmap terrain within the special.
overmap Id of the overmap_terrain to place at the location.
locations List of overmap_location ids that this overmap terrain may be placed on.


Identifier Description
point [ x, y, z] of the connection end point. Cannot overlap an overmap terrain entry for the special.
terrain Will go away in favor of connection eventually. Use road, subway, sewer, etc.
connection Id of the overmap_connection to build. Optional for now, but you should specify it explicitly.
from Optional point [ x, y, z] within the special to treat as the origin of the connection.
existing Boolean, default false. If the special requires a preexisting terrain to spawn.

Example mutable special

    "type": "overmap_special",
    "id": "anthill",
    "subtype": "mutable",
    "locations": [ "subterranean_empty" ],
    "city_distance": [ 25, -1 ],
    "city_sizes": [ 0, 20 ],
    "occurrences": [ 0, 1 ],
    "flags": [ "CLASSIC", "WILDERNESS" ],
    "//example": "The following check_for_locations_area field is superfluous in this example, and serves as a proof of concept",
    "check_for_locations_area": [
      { "type": [ "subterranean_empty" ], "from": [ -1, -1, -1 ], "to": [ 1, 1, -1 ] }
    "check_for_locations": [
      [ [ 0, 0, 0 ], [ "land" ] ],
      [ [ 0, 0, -1 ], [ "subterranean_empty" ] ],
      [ [ 1, 0, -1 ], [ "subterranean_empty" ] ],
      [ [ 0, 1, -1 ], [ "subterranean_empty" ] ],
      [ [ -1, 0, -1 ], [ "subterranean_empty" ] ],
      [ [ 0, -1, -1 ], [ "subterranean_empty" ] ]
    "joins": [ "surface_to_tunnel", "tunnel_to_tunnel" ],
    "overmaps": {
      "surface": { "overmap": "anthill", "below": "surface_to_tunnel", "locations": [ "land" ] },
      "below_entrance": {
        "overmap": "ants_nesw",
        "above": "surface_to_tunnel",
        "north": "tunnel_to_tunnel",
        "east": "tunnel_to_tunnel",
        "south": "tunnel_to_tunnel",
        "west": "tunnel_to_tunnel"
      "crossroads": {
        "overmap": "ants_nesw",
        "north": "tunnel_to_tunnel",
        "east": "tunnel_to_tunnel",
        "south": "tunnel_to_tunnel",
        "west": "tunnel_to_tunnel"
      "tee": { "overmap": "ants_nes", "north": "tunnel_to_tunnel", "east": "tunnel_to_tunnel", "south": "tunnel_to_tunnel" },
      "straight_tunnel": { "overmap": "ants_ns", "north": "tunnel_to_tunnel", "south": "tunnel_to_tunnel" },
      "corner": { "overmap": "ants_ne", "north": "tunnel_to_tunnel", "east": "tunnel_to_tunnel" },
      "dead_end": { "overmap": "ants_end_south", "north": "tunnel_to_tunnel" },
      "queen": { "overmap": "ants_queen", "north": "tunnel_to_tunnel" },
      "larvae": { "overmap": "ants_larvae", "north": "tunnel_to_tunnel" },
      "food": { "overmap": "ants_food", "north": "tunnel_to_tunnel" }
    "root": "surface",
    "phases": [
      [ { "overmap": "below_entrance", "max": 1 } ],
        { "overmap": "straight_tunnel", "max": 20 },
        { "overmap": "corner", "max": { "poisson": 5 } },
        { "overmap": "tee", "max": 10 }
      [ { "overmap": "queen", "max": 1 } ],
      [ { "overmap": "food", "max": 5 }, { "overmap": "larvae", "max": 5 } ],
        { "overmap": "dead_end", "weight": 2000 },
        { "overmap": "straight_tunnel", "weight": 100 },
        { "overmap": "corner", "weight": 100 },
        { "overmap": "tee", "weight": 10 },
        { "overmap": "crossroads", "weight": 1 }

How mutable specials are placed

Overmaps and joins

A mutable special has a collection of overmaps which define the OMTs used to build it and joins which define the way in which they are permitted to connect with one another. Each overmap may specify a join for each of its edges (four cardinal directions, above, and below). These joins must match the opposite join for the adjacent overmap in that direction.

In the above example, we see that the surface overmap specifies "below": "surface_to_tunnel", meaning that the join below it must be surface_to_tunnel. So, the overmap below must specify "above": "surface_to_tunnel". Only the below_entrance overmap does that, so we know that overmap must be placed beneath the surface overmap.

Overmaps can always be rotated, so a north constraint can correspond to other directions. So, the above dead_end overmap can represent a dead end tunnel in any direction, but it’s important that the chosen OMT ants_end_south is consistent with the north join for the generated map to make sense.

Overmaps can also specify connections. For example, an overmap might be defined as:

"where_road_connects": {
  "overmap": "road_end_north",
  "west": "parking_lot_to_road",
  "connections": { "north": { "connection": "local_road" } }

Once the mutable special placement is complete, a local_road connection will be built from the north edge of this overmap (again, ‘north’ is a relative term, that will rotate as the overmap is rotated) in the same way as connections are built for fixed specials. ‘Existing’ connections are not supported for mutable specials.

Layout phases

After all the joins and overmaps are defined, the manner in which the special is laid out is given by root and phases.

root specifies the overmap which is placed first, at the origin point for this special.

Then phases gives a list of growth phases used to place more overmaps. These phases are processed strictly in order.

Each phase is a list of rules. Each rule specifies an overmap and an integer max and/or weight.

Weight must always be a simple integer, but max may also be an object or array defining a probability distribution over integers. Each time the special is spawned, a value is sampled from that distribution. Currently uniform, binomial and Poisson distributions are supported. Uniform is specified by using an array such as "max": [ 1, 5 ] resulting in an evenly distributed 20% chance for each number from 1 to 5 being picked as the max. Poisson and binomial are specified via an object such as "max": { "poisson": 5 } where 5 will be the mean of the poisson distribution (λ) or "max": { "binomial": [ 5, 0.3 ] } where 5 will be the number of trials and 0.3 the success probability of the binomial distribution ( n, p ). Both have a higher chance for values to fall closer to the mean (λ in the case of Poisson and n x p in the case of binomial) with Poisson being more symetrical while binomial can be skewed by altering p to result in most values being on the lower end of the range 0-n or vice versa useful to produce more consistent results with rarer chances of large or small values or just one kind respectively. Hard bounds may be specified in addition to poisson or binomial to limit the range of possibilities, useful for guarenteeing a max of at least 1 or to prevent large values making overall size management difficult. To do this add an array bounds such as "max": { "poisson": 5, "bounds": [ 1, -1 ] } in this case guarenteeing at least a max of 1 without bounding upper values. Any value that would fall outside of these bounds becomes the relevant bound (as opposed to being rerolled).

Within each phase, the game looks for unsatisfied joins from the existing overmaps and attempts to find an overmap from amongst those available in its rules to satisfy that join. Priority is given to whichever joins are listed first in the list which defines the joins for this special, but if multiple joins of the same (highest priority) id are present then one is chosen at random.

First the rules are filtered to contain only those which can satisfy the joins for a particular location, and then a weighted selection from the filtered list is made. The weight is given by the minimum of max and weight specified in the rule. The difference between max and weight is that each time a rule is used, max is decremented by one. Therefore, it limits the number of times that rule can be chosen. Rules which only specify weight can be chosen an arbitrary number of times.

If no rule in the current phase is able to satisfy the joins for a particular location, that location is set aside to be tried again in later phases.

Once all joins are either satisfied or set aside, the phase ends and generation proceeds to the next phase.

If all phases complete and unsatisfied joins remain, this is considered an error and a debugmsg will be displayed with more details.


A placement rule in the phases can specify multiple overmaps to be placed in a particular configuration. This is useful if you want to place some feature that’s larger than a single OMT. Here is an example from the microlab:

  "name": "subway_chunk_at_-2",
  "chunk": [
    { "overmap": "microlab_sub_entry", "pos": [ 0, 0, 0 ], "rot": "north" },
    { "overmap": "microlab_sub_station", "pos": [ 0, -1, 0 ] },
    { "overmap": "microlab_subway", "pos": [ 0, -2, 0 ] }
  "max": 1

The "name" of a chunk is only for debugging messages when something goes wrong. "max" and "weight" are handled as above.

The new feature is "chunks" which specifies a list of overmaps and their relative positions and rotations. The overmaps are taken from the ones defined for this special. Rotation of "north" is the default, so specifying that has no effect, but it’s included here to demonstrate the syntax.

The positions and rotations are relative. The chunk can be placed at any offset and rotation, so long as all the overmaps are shifted and rotated together like a rigid body.

Techniques to avoid placement errors

To help avoid these errors, some additional features of the mutable special placement can help you.


check_for_locations defines a list of extra constraints that are checked before the special is attempted to be placed. Each constraint is a pair of a position (relative to the root) and a set of locations. The existing OMT in each position must fall into one of the given locations, else the attempted placement is aborted.

The check_for_locations constraints ensure that the below_entrance overmap can be placed below the root and that all four cardinal-adjacent OMTs are subterranean_empty, which is needed to add any further overmaps satisfying the four other joins of below_entrance.


check_for_locations_area defines a list of extra, rectangular regions of constraints to be considered in addition to the check_for_locations constraints. It is an array of objects that define a singular location, as well as a from and to tripoint that define the square region of constraints, like this:

"check_for_locations_area": [
  { "type": [ "subterranean_empty" ], "from": [ -1, -1, -1 ], "to": [ 1, 1, -1 ] }

For the above, the check_for_locations_area field is equivalent to manually adding the following constraints to the check_for_locations array:

[ [ 0, 0, -1 ], [ "subterranean_empty" ] ],
[ [ 1, 0, -1 ], [ "subterranean_empty" ] ],
[ [ 0, 1, -1 ], [ "subterranean_empty" ] ],
[ [ -1, 0, -1 ], [ "subterranean_empty" ] ],
[ [ 0, -1, -1 ], [ "subterranean_empty" ] ],
[ [ 1, 1, -1 ], [ "subterranean_empty" ] ],
[ [ 1, -1, -1 ], [ "subterranean_empty" ] ],
[ [ -1, 1, -1 ], [ "subterranean_empty" ] ],
[ [ -1, -1, -1 ], [ "subterranean_empty" ] ]

The check_for_locations_area field in the example mutable special is superfluous and serves only to illustrate the syntax of the field.

Look at /json/overmap/overmap_mutable/nether_monster_corpse.json for an application of this field in a real mutable special tile.


Each join also has an associated list of locations. This defaults to the locations for the special, but it can be overridden for a particular join like this:

"joins": [
  { "id": "surface_to_surface", "into_locations": [ "land" ] },

For an overmap to be placed when satisfying an unresolved join, it is not sufficient for it to satisfy the existing joins adjacent to a particular location. Any residual joins it possesses beyond those which already match up must point to OMTs with terrains consistent with that join’s locations.

For the particular case of the anthill example above, we can see how these two additions ensure that placement is always successful and no unsatisfied joins remain.

The next few phases of placement will attempt to place various tunnels. The join constraints will ensure that the unsatisfied joins (the open ends of tunnels) will always point into subterranean_empty OMTs.

Ensuring complete coverage in the final phase

In the final phase, we have five different rules intended to cap off any unsatisfied joins without growing the anthill further. It is important that the rules whose overmaps have fewer joins get higher weights. In the normal case, every unsatisfied join will be simply closed off using dead_end. However, we also have to cater for the possibility that two unsatisfied joins point to the same OMT, in which case dead_end will not fit (and will be filtered out), but straight_tunnel or corner will, and one of those will likely be chosen since they have the highest weights. We don’t want tee to be chosen (even though it might fit) because that would lead to a new unsatisfied join and further grow the tunnels. But it’s not a big problem if tee is chosen occasionally in this situation; the new join will likely simply be satisfied using dead_end.

When designing your own mutable overmap specials, you will have to think through these permutations to ensure that all joins will be satisfied by the end of the last phase.

Optional joins

Rather than having lots of rules designed to satisfy all possible situations in the final phase, in some situations you can make this easier using optional joins. This feature can also be used in other phases.

When specifying the joins associated with an overmap in a mutable special, you can elaborate with a type, like this example from the homeless_camp_mutable overmap special:

"overmaps": {
      "camp_core": {
        "overmap": "homelesscamp_north",
        "north": "camp_to_camp",
        "east": "camp_to_camp",
        "south": "camp_to_camp",
        "west": "camp_to_camp"
      "camp_edge": {
        "overmap": "homelesscamp_north",
        "north": "camp_to_camp",
        "east": { "id": "camp_to_camp", "type": "available" },
        "south": { "id": "camp_to_camp", "type": "available" },
        "west": { "id": "camp_to_camp", "type": "available" }

The definition of camp_edge has one mandatory join to the north, and three ‘available’ joins to the other cardinal directions. The semantics of an ‘available’ join are that it will not be considered an unresolved join, and therefore will never cause more overmaps to be placed, but it can satisfy other joins into a particular tile when necessary to allow an existing unresolved join to be satisfied.

The overmap will always be rotated in such a way that as many of its mandatory joins as possible are satisfied and available joins are left to point in other directions that don’t currently need joins.

As such, this camp_edge overmap can satisfy any unresolved joins for the homeless_camp_mutable special without generating any new unresolved joins of its own. This makes it great to finish off the special in the final phase.

Asymmetric joins

Sometimes you want two different OMTs to connect, but wouldn’t want either to connect with themselves. In this case you wouldn’t want to use the same join on both. Instead, you can define two joins which form a pair, by specifying one as the opposite of the other.

Another situation where this can arise is when the two sides of a join need different location constraints. For example, in the anthill, the surface and subterranean components need different locations. We could improve the definition of its joins by making the join between surface and tunnels asymmetric, like this:

"joins": [
  { "id": "surface_to_tunnel", "opposite": "tunnel_to_surface" },
  { "id": "tunnel_to_surface", "opposite": "surface_to_tunnel", "into_locations": [ "land" ] },

As you can see, the tunnel_to_surface part of the pair needs to override the default value of into_locations because it points towards the surface.

Alternative joins

Sometimes you want the next phase(s) of a mutable special to be able to link to existing unresolved joins without themselves generating any unresolved joins of that type. This helps to create a clean break between the old and the new.

For example, this happens in the microlab_mutable special. This special has some structured hallway OMTs surrounded by a clump of microlab OMTs. The hallways have hallway_to_microlab joins pointing out to their sides, so we need microlab OMTs to have microlab_to_hallway joins (the opposite of hallway_to_microlab) in order to match them.

However, we don’t want the unresolved edges of a microlab OMT to require more hallways all around, so we mostly want them to use microlab_to_microlab joins. How can we satisfy these apparently conflicting requirements without making many different variants of microlab with different numbers of each type of join? Alternative joins can help us here.

The definition of the microlab overmap might look like this:

"microlab": {
  "overmap": "microlab_generic",
  "north": { "id": "microlab_to_microlab", "alternatives": [ "microlab_to_hallway" ] },
  "east": { "id": "microlab_to_microlab", "alternatives": [ "microlab_to_hallway" ] },
  "south": { "id": "microlab_to_microlab", "alternatives": [ "microlab_to_hallway" ] },
  "west": { "id": "microlab_to_microlab", "alternatives": [ "microlab_to_hallway" ] }

This allows it to join with hallways which are already placed on the overmap, but new unresolved joins will only match more microlabs.

Testing your new mutable special

If you want to exhaustively test your mutable special for placement errors, and you are in a position to compile the game, then an easy way to do so is to use the existing test in tests/overmap_test.cpp.

In that file, look for TEST_CASE( "mutable_overmap_placement". At the start of that function there is a list of mutable special ids that tests tries spawning. Replace one of them with your new special’s id, recompile and run the test.

The test will attempt to place your special a few thousand times, and should find most ways in which placement might fail.


A join definition can be a simple string, which will be its id. Alternatively, it can be a dictionary with some of these keys:

Identifier Description
id Id of the join being defined.
opposite Id of the join which must match this one from the adjacent terrain.
into_locations List of overmap_location ids that this join may point towards.

Mutable special overmaps

The overmaps are a JSON dictionary. Each overmap must have an id (local to this special) which is the JSON dictionary key, and then the fields within the value may be:

Identifier Description
overmap Id of the overmap_terrain to place at the location.
locations List of overmap_location ids that this overmap terrain may be placed on. If not specified, defaults to the locations value from the special definition.
north Join which must align with the north edge of this OMT
east Join which must align with the east edge of this OMT
south Join which must align with the south edge of this OMT
west Join which must align with the west edge of this OMT
above Join which must link this to the OMT above
below Join which must link this to the OMT below

Each join associated with a direction can be a simple string, interpreted as a join id. Alternatively it can be a JSON object with the following keys:

Identifier Description
id Id of the join used here.
type Either "mandatory" or "available". Default: "mandatory".
alternatives List of join ids that may be used instead of the one listed under id, but only when placing this overmap. Unresolved joins created by its placement will only be the primary join id.

Generation rules

Identifier Description
overmap Id of the overmap to place.
max Maximum number of times this rule should be used.
weight Weight with which to select this rule.

One of max and weight must be specified. max will be used as the weight when weight is not specified.

City Building

A city building is an entity that is placed in the overmap during the city generation process–they are the “city” counterpart to the overmap_special type. The definition for a city building is a subset of that for an overmap special, and consequently will not be repeated in detail here.

Mandatory Overmap Specials / Region Settings

City buildings are not subject to the same quantity limitations as overmap specials, and in fact the occurrences attribute does not apply at all. Instead, the placement of city buildings is driven by the frequency assigned to the city building within the region_settings. Consult REGION_SETTINGS.md for more details.


Identifier Description
type Must be “city_building”.
id Unique id.
overmaps As in overmap_special, with one caveat: all point x and y values must be >= 0.
locations As in overmap_special.
flags As in overmap_special.
rotate As in overmap_special.


    "type": "city_building",
    "id": "zoo",
    "locations": [ "land" ],
    "overmaps": [
      { "point": [ 0, 0, 0 ], "overmap": "zoo_0_0_north" },
      { "point": [ 1, 0, 0 ], "overmap": "zoo_1_0_north" },
      { "point": [ 2, 0, 0 ], "overmap": "zoo_2_0_north" },
      { "point": [ 0, 1, 0 ], "overmap": "zoo_0_1_north" },
      { "point": [ 1, 1, 0 ], "overmap": "zoo_1_1_north" },
      { "point": [ 2, 1, 0 ], "overmap": "zoo_2_1_north" },
      { "point": [ 0, 2, 0 ], "overmap": "zoo_0_2_north" },
      { "point": [ 1, 2, 0 ], "overmap": "zoo_1_2_north" },
      { "point": [ 2, 2, 0 ], "overmap": "zoo_2_2_north" }
    "flags": [ "CLASSIC" ],
    "rotate" : true

Overmap Connection


Identifier Description
type Must be “overmap_connection”.
id Unique id.
subtypes List of entries used to determine valid locations, terrain cost, and resulting overmap terrain.


    "type": "overmap_connection",
    "id": "local_road",
    "subtypes": [
      { "terrain": "road", "locations": [ "field", "road" ] },
      { "terrain": "road", "locations": [ "forest_without_trail" ], "basic_cost": 20 },
      { "terrain": "road", "locations": [ "forest_trail" ], "basic_cost": 25 },
      { "terrain": "road", "locations": [ "swamp" ], "basic_cost": 40 },
      { "terrain": "road_nesw_manhole", "locations": [  ] },
      { "terrain": "bridge", "locations": [ "water" ], "basic_cost": 120 }
    "type": "overmap_connection",
    "id": "subway_tunnel",
    "subtypes": [ { "terrain": "subway", "locations": [ "subterranean_subway" ], "flags": [ "ORTHOGONAL" ] } ]


Identifier Description
terrain overmap_terrain to be placed when the placement location matches locations.
locations List of overmap_location that this subtype applies to. Can be empty; signifies terrain is valid as is.
basic_cost Cost of this subtype when pathfinding a route. Default 0.
flags See Overmap connections in JSON_FLAGS.md.

Overmap Location


Identifier Description
type Must be “overmap_location”.
id Unique id.
terrains List of overmap_terrain that can be considered part of this location.


    "type": "overmap_location",
    "id": "wilderness",
    "terrains": [ "forest", "forest_thick", "field", "forest_trail" ]


Object of city type define cities that would appear on overmap in specific location. Note that no random cities would be spawned if there is at least one city defined.


Identifier Description
id Id of the city.
database_id Id of the city in MA database.
name Name of the city.
population Original population of the city.
size Size of the city.
pos_om location of the city (in overmap coordinates).
pos location of the city (in overmap terrain coordinates).


    "type": "city",
    "id": "35",
    "database_id": 35,
    "name": "BOSTON",
    "population": 617594,
    "size": 64,
    "pos_om": [ 46, 15 ],
    "pos": [ 57, 94 ]