lua_api.txt

Minetest Lua Modding API Reference 0.4.6
========================================
More information at http://www.minetest.net/
Developer Wiki: http://dev.minetest.net/

Introduction
-------------
Content and functionality can be added to Minetest 0.4 by using Lua
scripting in run-time loaded mods.

A mod is a self-contained bunch of scripts, textures and other related
things that is loaded by and interfaces with Minetest.

Mods are contained and ran solely on the server side. Definitions and media
files are automatically transferred to the client.

If you see a deficiency in the API, feel free to attempt to add the
functionality in the engine and API. You can send such improvements as
source code patches to <celeron55@gmail.com>.

Programming in Lua
-------------------
If you have any difficulty in understanding this, please read:
  http://www.lua.org/pil/

Startup
--------
Mods are loaded during server startup from the mod load paths by running
the init.lua scripts in a shared environment.

Paths
-----
RUN_IN_PLACE=1: (Windows release, local build)
 $path_user:  Linux:    <build directory>
              Windows:  <build directory>
 $path_share: Linux:    <build directory>
              Windows:  <build directory>

RUN_IN_PLACE=0: (Linux release)
 $path_share: Linux:    /usr/share/minetest
              Windows:  <install directory>/minetest-0.4.x
 $path_user:  Linux:    ~/.minetest
              Windows:  C:/users/<user>/AppData/minetest (maybe)

Games
-----
Games are looked up from:
  $path_share/games/gameid/
  $path_user/games/gameid/
where gameid is unique to each game.

The game directory contains the file game.conf, which contains these fields:
  name = <Human-readable full name of the game>
eg.
  name = Minetest

The game directory can contain the file minetest.conf, which will be used
to set default settings when running the particular game.

Mod load path
-------------
Generic:
  $path_share/games/gameid/mods/
  $path_share/mods/
  $path_user/games/gameid/mods/
  $path_user/mods/ <-- User-installed mods
  $worldpath/worldmods/

In a run-in-place version (eg. the distributed windows version):
  minetest-0.4.x/games/gameid/mods/
  minetest-0.4.x/mods/gameid/ <-- User-installed mods
  minetest-0.4.x/worlds/worldname/worldmods/

On an installed version on linux:
  /usr/share/minetest/games/gameid/mods/
  ~/.minetest/mods/gameid/ <-- User-installed mods
  ~/.minetest/worlds/worldname/worldmods

Mod load path for world-specific games
--------------------------------------
It is possible to include a game in a world; in this case, no mods or
games are loaded or checked from anywhere else.

This is useful for eg. adventure worlds.

This happens if the following directory exists:
  $world/game/

Mods should be then be placed in:
  $world/game/mods/

Modpack support
----------------
Mods can be put in a subdirectory, if the parent directory, which otherwise
should be a mod, contains a file named modpack.txt. This file shall be
empty, except for lines starting with #, which are comments.

Mod directory structure
------------------------
mods
|-- modname
|   |-- depends.txt
|   |-- init.lua
|   |-- textures
|   |   |-- modname_stuff.png
|   |   `-- modname_something_else.png
|   |-- sounds
|   |-- media
|   `-- <custom data>
`-- another

modname:
  The location of this directory can be fetched by using
  minetest.get_modpath(modname)

depends.txt:
  List of mods that have to be loaded before loading this mod.
  A single line contains a single modname.

  Optional dependencies can be defined by appending a question mark
  to a single modname. Their meaning is that if the specified mod
  is missing, that does not prevent this mod from being loaded.

optdepends.txt:
  An alternative way of specifying optional dependencies.
  Like depends.txt, a single line contains a single modname.

  NOTE: This file exists for compatibility purposes only and
  support for it will be removed from the engine by the end of 2013.

init.lua:
  The main Lua script. Running this script should register everything it
  wants to register. Subsequent execution depends on minetest calling the
  registered callbacks.

  minetest.setting_get(name) and minetest.setting_getbool(name) can be used
  to read custom or existing settings at load time, if necessary.

textures, sounds, media:
  Media files (textures, sounds, whatever) that will be transferred to the
  client and will be available for use by the mod.

Naming convention for registered textual names
----------------------------------------------
Registered names should generally be in this format:
  "modname:<whatever>" (<whatever> can have characters a-zA-Z0-9_)

This is to prevent conflicting names from corrupting maps and is
enforced by the mod loader.

Example: mod "experimental", ideal item/node/entity name "tnt":
         -> the name should be "experimental:tnt".

Enforcement can be overridden by prefixing the name with ":". This can
be used for overriding the registrations of some other mod.

Example: Any mod can redefine experimental:tnt by using the name
         ":experimental:tnt" when registering it.
(also that mod is required to have "experimental" as a dependency)

The ":" prefix can also be used for maintaining backwards compatibility.

Aliases
-------
Aliases can be added by using minetest.register_alias(name, convert_to)

This will make Minetest to convert things called name to things called
convert_to.

This can be used for maintaining backwards compatibility.

This can be also used for setting quick access names for things, eg. if
you have an item called epiclylongmodname:stuff, you could do
  minetest.register_alias("stuff", "epiclylongmodname:stuff")
and be able to use "/giveme stuff".

Textures
--------
Mods should generally prefix their textures with modname_, eg. given
the mod name "foomod", a texture could be called
  "foomod_foothing.png"

Textures are referred to by their complete name, or alternatively by
stripping out the file extension:
  eg. foomod_foothing.png
  eg. foomod_foothing

Sounds
-------
Only OGG files are supported.

For positional playing of sounds, only single-channel (mono) files are
supported. Otherwise OpenAL will play them non-positionally.

Mods should generally prefix their sounds with modname_, eg. given
the mod name "foomod", a sound could be called
  "foomod_foosound.ogg"

Sounds are referred to by their name with a dot, a single digit and the
file extension stripped out.  When a sound is played, the actual sound file
is chosen randomly from the matching sounds.

When playing the sound "foomod_foosound", the sound is chosen randomly
from the available ones of the following files:
  foomod_foosound.ogg
  foomod_foosound.0.ogg
  foomod_foosound.1.ogg
  ...
  foomod_foosound.9.ogg

Examples of sound parameter tables:
-- Play locationless on all clients
{
    gain = 1.0, -- default
}
-- Play locationless to a player
{
    to_player = name,
    gain = 1.0, -- default
}
-- Play in a location
{
    pos = {x=1,y=2,z=3},
    gain = 1.0, -- default
    max_hear_distance = 32, -- default
}
-- Play connected to an object, looped
{
    object = <an ObjectRef>,
    gain = 1.0, -- default
    max_hear_distance = 32, -- default
    loop = true, -- only sounds connected to objects can be looped
}

SimpleSoundSpec:
eg. ""
eg. "default_place_node"
eg. {}
eg. {name="default_place_node"}
eg. {name="default_place_node", gain=1.0}

Registered definitions of stuff
--------------------------------
Anything added using certain minetest.register_* functions get added to
the global minetest.registered_* tables.

minetest.register_entity(name, prototype table)
 -> minetest.registered_entities[name]

minetest.register_node(name, node definition)
 -> minetest.registered_items[name]
 -> minetest.registered_nodes[name]

minetest.register_tool(name, item definition)
 -> minetest.registered_items[name]

minetest.register_craftitem(name, item definition)
 -> minetest.registered_items[name]

Note that in some cases you will stumble upon things that are not contained
in these tables (eg. when a mod has been removed). Always check for
existence before trying to access the fields.

Example: If you want to check the drawtype of a node, you could do:

local function get_nodedef_field(nodename, fieldname)
    if not minetest.registered_nodes[nodename] then
        return nil
    end
    return minetest.registered_nodes[nodename][fieldname]
end
local drawtype = get_nodedef_field(nodename, "drawtype")

Example: minetest.get_item_group(name, group) has been implemented as:

function minetest.get_item_group(name, group)
    if not minetest.registered_items[name] or not
            minetest.registered_items[name].groups[group] then
        return 0
    end
    return minetest.registered_items[name].groups[group]
end

Nodes
------
Nodes are the bulk data of the world: cubes and other things that take the
space of a cube. Huge amounts of them are handled efficiently, but they
are quite static.

The definition of a node is stored and can be accessed by name in
  minetest.registered_nodes[node.name]
See "Registered definitions of stuff".

Nodes are passed by value between Lua and the engine.
They are represented by a table:
  {name="name", param1=num, param2=num}

param1 and param2 are 8 bit integers. The engine uses them for certain
automated functions. If you don't use these functions, you can use them to
store arbitrary values.

The functions of param1 and param2 are determined by certain fields in the
node definition:
param1 is reserved for the engine when paramtype != "none":
  paramtype = "light"
  ^ The value stores light with and without sun in it's
    upper and lower 4 bits.
param2 is reserved for the engine when any of these are used:
  liquidtype == "flowing"
  ^ The level and some flags of the liquid is stored in param2
  drawtype == "flowingliquid"
  ^ The drawn liquid level is read from param2
  drawtype == "torchlike"
  drawtype == "signlike"
  paramtype2 == "wallmounted"
  ^ The rotation of the node is stored in param2. You can make this value
    by using minetest.dir_to_wallmounted().
  paramtype2 == "facedir"
  ^ The rotation of the node is stored in param2. Furnaces and chests are
    rotated this way. Can be made by using minetest.dir_to_facedir().
    Values range 0 - 23
    facedir modulo 4 = axisdir
    0 = y+    1 = z+    2 = z-    3 = x+    4 = x-    5 = y-
    facedir's two less significant bits are rotation around the axis

Nodes can also contain extra data. See "Node Metadata".

Node drawtypes
---------------
There are a bunch of different looking node types. These are mostly just
copied from Minetest 0.3; more may be made in the future.

Look for examples in games/minimal or games/minetest_game.

- normal
- airlike
- liquid
- flowingliquid
- glasslike
- glasslike_framed
- allfaces
- allfaces_optional
- torchlike
- signlike
- plantlike
- fencelike
- raillike
- nodebox -- See below. EXPERIMENTAL

Node boxes
-----------
Node selection boxes are defined using "node boxes"

The "nodebox" node drawtype allows defining visual of nodes consisting of
arbitrary number of boxes. It allows defining stuff like stairs. Only the
"fixed" box type is supported for these.
^ Please note that this is still experimental, and may be incompatibly
  changed in the future.

A nodebox is defined as any of:
{
    -- A normal cube; the default in most things
    type = "regular"
}
{
    -- A fixed box (facedir param2 is used, if applicable)
    type = "fixed",
    fixed = box OR {box1, box2, ...}
}
{
    -- A box like the selection box for torches
    -- (wallmounted param2 is used, if applicable)
    type = "wallmounted",
    wall_top = box,
    wall_bottom = box,
    wall_side = box
}

A box is defined as:
  {x1, y1, z1, x2, y2, z2}
A box of a regular node would look like:
  {-0.5, -0.5, -0.5, 0.5, 0.5, 0.5},

Ore types
---------------
These tell in what manner the ore is generated.
All default ores are of the uniformly-distributed scatter type.

- scatter
    Randomly chooses a location and generates a cluster of ore.
    If noise_params is specified, the ore will be placed if the 3d perlin noise at 
    that point is greater than the noise_threshhold, giving the ability to create a non-equal
    distribution of ore.
- sheet
    Creates a sheet of ore in a blob shape according to the 2d perlin noise described by noise_params.
    The relative height of the sheet can be controlled by the same perlin noise as well, by specifying
    a non-zero 'scale' parameter in noise_params.  IMPORTANT: The noise is not transformed by offset or
    scale when comparing against the noise threshhold, but scale is used to determine relative height.
    The height of the blob is randomly scattered, with a maximum height of clust_size.
    clust_scarcity and clust_num_ores are ignored.
    This is essentially an improved version of the so-called "stratus" ore seen in some unofficial mods.
- claylike - NOT YET IMPLEMENTED
    Places ore if there are no more than clust_scarcity number of specified nodes within a Von Neumann
    neighborhood of clust_size radius.

Ore attributes
-------------------
Currently supported flags:  absheight
 - absheight
    Also produce this same ore between the height range of -height_max and -height_min.
    Useful for having ore in sky realms without having to duplicate ore entries.

HUD element types
-------------------
The position field is used for all element types.
To account for differing resolutions, the position coordinates are the percentage of the screen,
ranging in value from 0 to 1.
The name field is not yet used, but should contain a description of what the HUD element represents.
The direction field is the direction in which something is drawn.
0 draws from left to right, 1 draws from right to left, 2 draws from top to bottom, and 3 draws from bottom to top.
The alignment field specifies how the item will be aligned. It ranges from -1 to 1,
with 0 being the center, -1 is moved to the left/up, and 1 is to the right/down. Fractional
values can be used.
The offset field specifies a pixel offset from the position. Contrary to position,
the offset is not scaled to screen size. This allows for some precisely-positioned
items in the HUD.
Below are the specific uses for fields in each type; fields not listed for that type are ignored.

Note: Future revisions to the HUD API may be incompatible; the HUD API is still in the experimental stages.

- image
    Displays an image on the HUD.
	- scale: The scale of the image, with 1 being the original texture size.
             Only the X coordinate scale is used.
    - text: The name of the texture that is displayed.
    - alignment: The alignment of the image.
    - offset: offset in pixels from position.
- text
    Displays text on the HUD.
    - scale: Defines the bounding rectangle of the text.
             A value such as {x=100, y=100} should work.
    - text: The text to be displayed in the HUD element.
    - number: An integer containing the RGB value of the color used to draw the text.
              Specify 0xFFFFFF for white text, 0xFF0000 for red, and so on.
    - alignment: The alignment of the text.
    - offset: offset in pixels from position.
- statbar
    Displays a horizontal bar made up of half-images.
    - text: The name of the texture that is used.
    - number: The number of half-textures that are displayed.
              If odd, will end with a vertically center-split texture.
    - direction
    - offset: offset in pixels from position.
- inventory
    - text: The name of the inventory list to be displayed.
    - number: Number of items in the inventory to be displayed.
    - item: Position of item that is selected.
    - direction

Representations of simple things
--------------------------------
Position/vector:
  {x=num, y=num, z=num}
Currently the API does not provide any helper functions for addition,
subtraction and whatever; you can define those that you need yourself.

pointed_thing:
  {type="nothing"}
  {type="node", under=pos, above=pos}
  {type="object", ref=ObjectRef}

Items
------
Node (register_node):
  A node from the world
Tool (register_tool):
  A tool/weapon that can dig and damage things according to tool_capabilities
Craftitem (register_craftitem):
  A miscellaneous item

Items and item stacks can exist in three formats:

Serialized; This is called stackstring or itemstring:
eg. 'default:dirt 5'
eg. 'default:pick_wood 21323'
eg. 'default:apple'

Table format:
eg. {name="default:dirt", count=5, wear=0, metadata=""} 
    ^ 5 dirt nodes
eg. {name="default:pick_wood", count=1, wear=21323, metadata=""}
    ^ a wooden pick about 1/3 weared out
eg. {name="default:apple", count=1, wear=0, metadata=""}
    ^ an apple.

ItemStack:
C++ native format with many helper methods. Useful for converting between
formats. See the Class reference section for details.

When an item must be passed to a function, it can usually be in any of
these formats.

Groups
-------
In a number of places, there is a group table. Groups define the
properties of a thing (item, node, armor of entity, capabilities of
tool) in such a way that the engine and other mods can can interact with
the thing without actually knowing what the thing is.

Usage:
- Groups are stored in a table, having the group names with keys and the
  group ratings as values. For example:
    groups = {crumbly=3, soil=1}
    ^ Default dirt
    groups = {crumbly=2, soil=1, level=2, outerspace=1}
    ^ A more special dirt-kind of thing
- Groups always have a rating associated with them. If there is no
  useful meaning for a rating for an enabled group, it shall be 1.
- When not defined, the rating of a group defaults to 0. Thus when you
  read groups, you must interpret nil and 0 as the same value, 0.

You can read the rating of a group for an item or a node by using
  minetest.get_item_group(itemname, groupname)

Groups of items
----------------
Groups of items can define what kind of an item it is (eg. wool).

Groups of nodes
----------------
In addition to the general item things, groups are used to define whether
a node is destroyable and how long it takes to destroy by a tool.

Groups of entities
-------------------
For entities, groups are, as of now, used only for calculating damage.
The rating is the percentage of damage caused by tools with this damage group.
See "Entity damage mechanism".

object.get_armor_groups() -> a group-rating table (eg. {fleshy=100})
object.set_armor_groups({fleshy=30, cracky=80})

Groups of tools
----------------
Groups in tools define which groups of nodes and entities they are
effective towards.

Groups in crafting recipes
---------------------------
An example: Make meat soup from any meat, any water and any bowl
{
    output = 'food:meat_soup_raw',
    recipe = {
        {'group:meat'},
        {'group:water'},
        {'group:bowl'},
    },
    -- preserve = {'group:bowl'}, -- Not implemented yet (TODO)
}
An another example: Make red wool from white wool and red dye
{
    type = 'shapeless',
    output = 'wool:red',
    recipe = {'wool:white', 'group:dye,basecolor_red'},
}

Special groups
---------------
- immortal: Disables the group damage system for an entity
- level: Can be used to give an additional sense of progression in the game.
  - A larger level will cause eg. a weapon of a lower level make much less
    damage, and get weared out much faster, or not be able to get drops
    from destroyed nodes.
  - 0 is something that is directly accessible at the start of gameplay
  - There is no upper limit
- dig_immediate: (player can always pick up node without tool wear)
  - 2: node is removed without tool wear after 0.5 seconds or so
       (rail, sign)
  - 3: node is removed without tool wear immediately (torch)
- disable_jump: Player (and possibly other things) cannot jump from node
- fall_damage_add_percent: damage speed = speed * (1 + value/100)
- bouncy: value is bounce speed in percent
- falling_node: if there is no walkable block under the node it will fall
- attached_node: if the node under it is not a walkable block the node will be
                  dropped as an item. If the node is wallmounted the
                  wallmounted direction is checked.
- soil: saplings will grow on nodes in this group
- connect_to_raillike: makes nodes of raillike drawtype connect to
                       other group members with same drawtype

Known damage and digging time defining groups
----------------------------------------------
- crumbly: dirt, sand
- cracky: tough but crackable stuff like stone.
- snappy: something that can be cut using fine tools; eg. leaves, small
          plants, wire, sheets of metal
- choppy: something that can be cut using force; eg. trees, wooden planks
- fleshy: Living things like animals and the player. This could imply
          some blood effects when hitting.
- explody: Especially prone to explosions
- oddly_breakable_by_hand:
   Can be added to nodes that shouldn't logically be breakable by the
   hand but are. Somewhat similar to dig_immediate, but times are more
   like {[1]=3.50,[2]=2.00,[3]=0.70} and this does not override the
   speed of a tool if the tool can dig at a faster speed than this
   suggests for the hand.

Examples of custom groups
--------------------------
Item groups are often used for defining, well, //groups of items//.
- meat: any meat-kind of a thing (rating might define the size or healing
  ability or be irrelevant - it is not defined as of yet)
- eatable: anything that can be eaten. Rating might define HP gain in half
  hearts.
- flammable: can be set on fire. Rating might define the intensity of the
  fire, affecting eg. the speed of the spreading of an open fire.
- wool: any wool (any origin, any color)
- metal: any metal
- weapon: any weapon
- heavy: anything considerably heavy

Digging time calculation specifics
-----------------------------------
Groups such as **crumbly**, **cracky** and **snappy** are used for this
purpose. Rating is 1, 2 or 3. A higher rating for such a group implies
faster digging time.

The **level** group is used to limit the toughness of nodes a tool can dig
and to scale the digging times / damage to a greater extent.

^ PLEASE DO UNDERSTAND THIS, otherwise you cannot use the system to it's
  full potential.

Tools define their properties by a list of parameters for groups. They
cannot dig other groups; thus it is important to use a standard bunch of
groups to enable interaction with tools.

**Tools define:**
  * Full punch interval
  * Maximum drop level
  * For an arbitrary list of groups:
    * Uses (until the tool breaks)
    * Maximum level (usually 0, 1, 2 or 3)
    * Digging times
    * Damage groups

**Full punch interval**:
When used as a weapon, the tool will do full damage if this time is spent
between punches. If eg. half the time is spent, the tool will do half
damage.

**Maximum drop level**
Suggests the maximum level of node, when dug with the tool, that will drop
it's useful item. (eg. iron ore to drop a lump of iron).
- This is not automated; it is the responsibility of the node definition
  to implement this

**Uses**
Determines how many uses the tool has when it is used for digging a node,
of this group, of the maximum level. For lower leveled nodes, the use count
is multiplied by 3^leveldiff.
- uses=10, leveldiff=0 -> actual uses: 10
- uses=10, leveldiff=1 -> actual uses: 30
- uses=10, leveldiff=2 -> actual uses: 90

**Maximum level**
Tells what is the maximum level of a node of this group that the tool will
be able to dig.

**Digging times**
List of digging times for different ratings of the group, for nodes of the
maximum level.
  * For example, as a lua table, ''times={2=2.00, 3=0.70}''. This would
    result in the tool to be able to dig nodes that have a rating of 2 or 3
    for this group, and unable to dig the rating 1, which is the toughest.
    Unless there is a matching group that enables digging otherwise.

**Damage groups**
List of damage for groups of entities. See "Entity damage mechanism".

Example definition of the capabilities of a tool
-------------------------------------------------
tool_capabilities = {
    full_punch_interval=1.5,
    max_drop_level=1,
    groupcaps={
        crumbly={maxlevel=2, uses=20, times={[1]=1.60, [2]=1.20, [3]=0.80}}
    }
    damage_groups = {fleshy=2},
}

This makes the tool be able to dig nodes that fullfill both of these:
- Have the **crumbly** group
- Have a **level** group less or equal to 2

Table of resulting digging times:
crumbly        0     1     2     3     4  <- level
     ->  0     -     -     -     -     -
         1  0.80  1.60  1.60     -     -
         2  0.60  1.20  1.20     -     -
         3  0.40  0.80  0.80     -     -

level diff:    2     1     0    -1    -2

Table of resulting tool uses:
     ->  0     -     -     -     -     -
         1   180    60    20     -     -
         2   180    60    20     -     -
         3   180    60    20     -     -

Notes:
- At crumbly=0, the node is not diggable.
- At crumbly=3, the level difference digging time divider kicks in and makes
  easy nodes to be quickly breakable.
- At level > 2, the node is not diggable, because it's level > maxlevel

Entity damage mechanism
------------------------
Damage calculation:
damage = 0
foreach group in cap.damage_groups:
    damage += cap.damage_groups[group] * limit(actual_interval / cap.full_punch_interval, 0.0, 1.0)
        * (object.armor_groups[group] / 100.0)
        -- Where object.armor_groups[group] is 0 for inexisting values
return damage

Client predicts damage based on damage groups. Because of this, it is able to
give an immediate response when an entity is damaged or dies; the response is
pre-defined somehow (eg. by defining a sprite animation) (not implemented;
TODO).
- Currently a smoke puff will appear when an entity dies.

The group **immortal** completely disables normal damage.

Entities can define a special armor group, which is **punch_operable**. This
group disables the regular damage mechanism for players punching it by hand or
a non-tool item, so that it can do something else than take damage.

On the Lua side, every punch calls ''entity:on_punch(puncher,
time_from_last_punch, tool_capabilities, direction)''. This should never be
called directly, because damage is usually not handled by the entity itself.
  * ''puncher'' is the object performing the punch. Can be nil. Should never be
    accessed unless absolutely required, to encourage interoperability.
  * ''time_from_last_punch'' is time from last punch (by puncher) or nil.
  * ''tool_capabilities'' can be nil.
  * ''direction'' is a unit vector, pointing from the source of the punch to
    the punched object.

To punch an entity/object in Lua, call ''object:punch(puncher,
time_from_last_punch, tool_capabilities, direction)''.
  * Return value is tool wear.
  * Parameters are equal to the above callback.
  * If ''direction'' is nil and ''puncher'' is not nil, ''direction'' will be
    automatically filled in based on the location of ''puncher''.

Node Metadata
-------------
The instance of a node in the world normally only contains the three values
mentioned in "Nodes". However, it is possible to insert extra data into a
node. It is called "node metadata"; See "NodeMetaRef".

Metadata contains two things:
- A key-value store
- An inventory

Some of the values in the key-value store are handled specially:
- formspec: Defines a right-click inventory menu. See "Formspec".
- infotext: Text shown on the screen when the node is pointed at

Example stuff:

local meta = minetest.get_meta(pos)
meta:set_string("formspec",
        "invsize[8,9;]"..
        "list[context;main;0,0;8,4;]"..
        "list[current_player;main;0,5;8,4;]")
meta:set_string("infotext", "Chest");
local inv = meta:get_inventory()
inv:set_size("main", 8*4)
print(dump(meta:to_table()))
meta:from_table({
    inventory = {
        main = {[1] = "default:dirt", [2] = "", [3] = "", [4] = "", [5] = "", [6] = "", [7] = "", [8] = "", [9] = "", [10] = "", [11] = "", [12] = "", [13] = "", [14] = "default:cobble", [15] = "", [16] = "", [17] = "", [18] = "", [19] = "", [20] = "default:cobble", [21] = "", [22] = "", [23] = "", [24] = "", [25] = "", [26] = "", [27] = "", [28] = "", [29] = "", [30] = "", [31] = "", [32] = ""}
    },
    fields = {
        formspec = "invsize[8,9;]list[context;main;0,0;8,4;]list[current_player;main;0,5;8,4;]",
        infotext = "Chest"
    }
})

Formspec
--------
Formspec defines a menu. Currently not much else than inventories are
supported. It is a string, with a somewhat strange format.

Spaces and newlines can be inserted between the blocks, as is used in the
examples.

Examples:
- Chest:
    invsize[8,9;]
    list[context;main;0,0;8,4;]
    list[current_player;main;0,5;8,4;]
- Furnace:
    invsize[8,9;]
    list[context;fuel;2,3;1,1;]
    list[context;src;2,1;1,1;]
    list[context;dst;5,1;2,2;]
    list[current_player;main;0,5;8,4;]
- Minecraft-like player inventory
    invsize[8,7.5;]
    image[1,0.6;1,2;player.png]
    list[current_player;main;0,3.5;8,4;]
    list[current_player;craft;3,0;3,3;]
    list[current_player;craftpreview;7,1;1,1;]

Elements:

size[<W>,<H>]
^ Define the size of the menu in inventory slots
^ deprecated: invsize[<W>,<H>;]

list[<inventory location>;<list name>;<X>,<Y>;<W>,<H>;]
list[<inventory location>;<list name>;<X>,<Y>;<W>,<H>;<starting item index>]
^ Show an inventory list

image[<X>,<Y>;<W>,<H>;<texture name>]
^ Show an image
^ Position and size units are inventory slots

item_image[<X>,<Y>;<W>,<H>;<item name>]
^ Show an inventory image of registered item/node
^ Position and size units are inventory slots

background[<X>,<Y>;<W>,<H>;<texture name>]
^ Use a background. Inventory rectangles are not drawn then.
^ Position and size units are inventory slots
^ Example for formspec 8x4 in 16x resolution: image shall be sized 8*16px x 4*16px

field[<X>,<Y>;<W>,<H>;<name>;<label>;<default>]
^ Textual field; will be sent to server when a button is clicked
^ x and y position the field relative to the top left of the menu
^ w and h are the size of the field
^ fields are a set height, but will be vertically centred on h
^ Position and size units are inventory slots
^ name is the name of the field as returned in fields to on_receive_fields
^ label, if not blank, will be text printed on the top left above the field
^ default is the default value of the field
  ^ default may contain variable references such as '${text}' which
    will fill the value from the metadata value 'text'
    ^ Note: no extra text or more than a single variable is supported ATM.

field[<name>;<label>;<default>]
^ as above but without position/size units
^ special field for creating simple forms, such as sign text input
^ must be used without a size[] element
^ a 'Proceed' button will be added automatically

textarea[<X>,<Y>;<W>,<H>;<name>;<label>;<default>]
^ same as fields above, but with multi-line input

label[<X>,<Y>;<label>]
^ x and y work as per field
^ label is the text on the label
^ Position and size units are inventory slots

button[<X>,<Y>;<W>,<H>;<name>;<label>]
^ Clickable button. When clicked, fields will be sent.
^ x, y and name work as per field
^ w and h are the size of the button
^ label is the text on the button
^ Position and size units are inventory slots

image_button[<X>,<Y>;<W>,<H>;<texture name>;<name>;<label>]
^ x, y, w, h, and name work as per button
^ image is the filename of an image
^ Position and size units are inventory slots

item_image_button[<X>,<Y>;<W>,<H>;<item name>;<name>;<label>]
^ x, y, w, h, name and label work as per button
^ item name is the registered name of an item/node,
  tooltip will be made out of its descritption
^ Position and size units are inventory slots

button_exit[<X>,<Y>;<W>,<H>;<name>;<label>]
^ When clicked, fields will be sent and the form will quit.

image_button_exit[<X>,<Y>;<W>,<H>;<texture name>;<name>;<label>]
^ When clicked, fields will be sent and the form will quit.

Inventory location:

- "context": Selected node metadata (deprecated: "current_name")
- "current_player": Player to whom the menu is shown
- "player:<name>": Any player
- "nodemeta:<X>,<Y>,<Z>": Any node metadata
- "detached:<name>": A detached inventory

Helper functions
-----------------
dump2(obj, name="_", dumped={})
^ Return object serialized as a string, handles reference loops
dump(obj, dumped={})
^ Return object serialized as a string
string:split(separator)
^ eg. string:split("a,b", ",") == {"a","b"}
string:trim()
^ eg. string.trim("\n \t\tfoo bar\t ") == "foo bar"
minetest.pos_to_string({x=X,y=Y,z=Z}) -> "(X,Y,Z)"
^ Convert position to a printable string
minetest.string_to_pos(string) -> position
^ Same but in reverse
minetest.formspec_escape(string) -> string
^ escapes characters like [, ], and \ that can not be used in formspecs

minetest namespace reference
-----------------------------
Utilities:
minetest.get_current_modname() -> string
minetest.get_modpath(modname) -> eg. "/home/user/.minetest/usermods/modname"
^ Useful for loading additional .lua modules or static data from mod
minetest.get_modnames() -> list of installed mods
^ Return a list of installed mods, sorted alphabetically
minetest.get_worldpath() -> eg. "/home/user/.minetest/world"
^ Useful for storing custom data
minetest.is_singleplayer()
minetest.features
^ table containing API feature flags: {foo=true, bar=true}
minetest.has_feature(arg) -> bool, missing_features
^ arg: string or table in format {foo=true, bar=true}
^ missing_features: {foo=true, bar=true}

Logging:
minetest.debug(line)
^ Always printed to stderr and logfile (print() is redirected here)
minetest.log(line)
minetest.log(loglevel, line)
^ loglevel one of "error", "action", "info", "verbose"

Registration functions: (Call these only at load time)
minetest.register_entity(name, prototype table)
minetest.register_abm(abm definition)
minetest.register_node(name, node definition)
minetest.register_tool(name, item definition)
minetest.register_craftitem(name, item definition)
minetest.register_alias(name, convert_to)
minetest.register_craft(recipe)
minetest.register_ore(ore definition)

Global callback registration functions: (Call these only at load time)
minetest.register_globalstep(func(dtime))
^ Called every server step, usually interval of 0.05s
minetest.register_on_shutdown(func())
^ Called before server shutdown
^ WARNING: If the server terminates abnormally (i.e. crashes), the registered
           callbacks WILL LIKELY NOT BE RUN.  Data should be saved at
           semi-frequent intervals as well as on server shutdown.
minetest.register_on_placenode(func(pos, newnode, placer, oldnode, itemstack))
^ Called when a node has been placed
^ If return true no item is taken from itemstack
^ Not recommended; use on_construct or after_place_node in node definition
^                  whenever possible
minetest.register_on_dignode(func(pos, oldnode, digger))
^ Called when a node has been dug.
^ Not recommended: Use on_destruct or after_dig_node in node definition
^                  whenever possible
minetest.register_on_punchnode(func(pos, node, puncher))
^ Called when a node is punched
minetest.register_on_generated(func(minp, maxp, blockseed))
^ Called after generating a piece of world. Modifying nodes inside the area
  is a bit faster than usually.
minetest.register_on_newplayer(func(ObjectRef))
^ Called after a new player has been created
minetest.register_on_dieplayer(func(ObjectRef))
^ Called when a player dies
minetest.register_on_respawnplayer(func(ObjectRef))
^ Called when player is to be respawned
^ Called _before_ repositioning of player occurs
^ return true in func to disable regular player placement
minetest.register_on_joinplayer(func(ObjectRef))
^ Called when a player joins the game
minetest.register_on_leaveplayer(func(ObjectRef))
^ Called when a player leaves the game
minetest.register_on_chat_message(func(name, message))
^ Called always when a player says something
minetest.register_on_player_receive_fields(func(player, formname, fields))
^ Called when a button is pressed in player's inventory form
^ Newest functions are called first
^ If function returns true, remaining functions are not called

Other registration functions:
minetest.register_chatcommand(cmd, chatcommand definition)
minetest.register_privilege(name, definition)
^ definition: "description text"
^ definition: {
      description = "description text",
      give_to_singleplayer = boolean, -- default: true
  }
minetest.register_authentication_handler(handler)
^ See minetest.builtin_auth_handler in builtin.lua for reference