minetest_modding_book/_en/map/objects.md

title	layout	root	idx	description	degrad
Objects, Players, and Entities	default	../..	3.4	Using an ObjectRef	level title message warning Degrees and Radians Attachment rotation is set in degrees, whereas object rotation is in radians. Make sure to convert to the correct angle measurement.

Introduction

In this chapter, you will learn how to manipulate objects and how to define your own.

• What are Objects, Players, and Entities?
• Position and Velocity
• Object Properties
• Entities
• Health and Damage
  • Health Points (HP)
  • Punch, Damage Groups, and Armor Groups
  • Example Damage Calculation
• Attachments
• Your Turn

What are Objects, Players, and Entities?

Players and Entities are both types of Objects. An object is something that can move independently of the node grid and has properties such as velocity and scale. Objects aren't items, and they have their own separate registration system.

There are a few differences between Players and Entities. The biggest one is that Players are player-controlled, whereas Entities are mod-controlled. This means that the velocity of a player cannot be set by mods - players are client-side, and entities are server-side. Another difference is that Players will cause map blocks to be loaded, whereas Entities will just be saved and become inactive.

This distinction is muddied by the fact that Entities are controlled using a table which is referred to as a Lua entity, as discussed later.

Position and Velocity

get_pos and set_pos exist to allow you to get and set an entity's position.

local object = core.get_player_by_name("bob")
local pos    = object:get_pos()
object:set_pos({ x = pos.x, y = pos.y + 1, z = pos.z })

set_pos immediately sets the position, with no animation. If you'd like to smoothly animate an object to the new position, you should use move_to. This, unfortunately, only works for entities.

object:move_to({ x = pos.x, y = pos.y + 1, z = pos.z })

An important thing to think about when dealing with entities is network latency. In an ideal world, messages about entity movements would arrive immediately, in the correct order, and with a similar interval as to how you sent them. However, unless you're in singleplayer, this isn't an ideal world. Messages will take a while to arrive. Position messages may arrive out of order, resulting in some set_pos calls being skipped as there's no point going to a position older than the current known position. Moves may not be similarly spaced, which makes it difficult to use them for animation. All this results in the client seeing different things to the server, which is something you need to be aware of.

Object Properties

Object properties are used to tell the client how to render and deal with an object. It's not possible to define custom properties, because the properties are for the engine to use, by definition.

Unlike nodes, objects have a dynamic rather than set appearance. You can change how an object looks, among other things, at any time by updating its properties.

object:set_properties({
    visual      = "mesh",
    mesh        = "character.b3d",
    textures    = {"character_texture.png"},
    visual_size = {x=1, y=1},
})

The updated properties will be sent to all players in range. This is very useful to get a large amount of variety very cheaply, such as having different skins per-player.

As shown in the next section, entities can have initial properties provided in their definition. The default Player properties are defined in the engine, however, so you'll need to use set_properties() in on_joinplayer to set the properties for newly joined players.

Entities

An Entity has a definition table that resembles an item definition table. This table can contain callback methods, initial object properties, and custom members.

local MyEntity = {
    initial_properties = {
        hp_max = 1,
        physical = true,
        collide_with_objects = false,
        collisionbox = {-0.3, -0.3, -0.3, 0.3, 0.3, 0.3},
        visual = "wielditem",
        visual_size = {x = 0.4, y = 0.4},
        textures = {""},
        spritediv = {x = 1, y = 1},
        initial_sprite_basepos = {x = 0, y = 0},
    },

    message = "Default message",
}

function MyEntity:set_message(msg)
    self.message = msg
end

Entity definitions differ in one very important way from Item definitions. When an entity is emerged (ie: loaded or created), a new table is created for that entity that inherits from the definition table.

Both an ObjectRef and an entity table provide ways to get the counterpart:

local entity = object:get_luaentity()
local object = entity.object
print("entity is at " .. core.pos_to_string(object:get_pos()))

There are a number of available callbacks for use with entities. A complete list can be found in lua_api.md.

function MyEntity:on_step(dtime)
    local pos      = self.object:get_pos()
    local pos_down = vector.subtract(pos, vector.new(0, 1, 0))

    local delta
    if core.get_node(pos_down).name == "air" then
        delta = vector.new(0, -1, 0)
    elseif core.get_node(pos).name == "air" then
        delta = vector.new(0, 0, 1)
    else
        delta = vector.new(0, 1, 0)
    end

    delta = vector.multiply(delta, dtime)

    self.object:move_to(vector.add(pos, delta))
end

function MyEntity:on_punch(hitter)
    core.chat_send_player(hitter:get_player_name(), self.message)
end

Now, if you were to spawn and use this entity, you'd notice that the message would be forgotten when the entity becomes inactive then active again. This is because the message isn't saved. Rather than saving everything in the entity table, Minetest gives you control over how to save things. Staticdata is a string which contains all the custom information that needs to stored.

function MyEntity:get_staticdata()
    return core.write_json({
        message = self.message,
    })
end

function MyEntity:on_activate(staticdata, dtime_s)
    if staticdata ~= "" and staticdata ~= nil then
        local data = core.parse_json(staticdata) or {}
        self:set_message(data.message)
    end
end

Minetest may call get_staticdata() as many times as it wants and at any time. This is because Minetest doesn't wait for a MapBlock to become inactive to save it, as this would result in data loss. MapBlocks are saved roughly every 18 seconds, so you should notice a similar interval for get_staticdata() being called.

on_activate(), on the other hand, will only be called when an entity becomes active either from the MapBlock becoming active or from the entity spawning. This means that staticdata could be empty.

Finally, you need to register the type table using the aptly named register_entity.

core.register_entity("mymod:entity", MyEntity)

The entity can be spawned by a mod like so:

local pos = { x = 1, y = 2, z = 3 }
local obj = core.add_entity(pos, "mymod:entity", nil)

The third parameter is the initial staticdata. To set the message, you can use the entity table method:

obj:get_luaentity():set_message("hello!")

Players with the give privilege can use a chat command to spawn entities:

/spawnentity mymod:entity

Health and Damage

Health Points (HP)

Each object has a Health Points (HP) number, which represents the current health. Players have a maximum hp set using the hp_max object property. An object will die if its hp reaches 0.

local hp = object:get_hp()
object:set_hp(hp + 3)

Punch, Damage Groups, and Armor Groups

Damage is the reduction of an object's HP. An object can punch another object to inflict damage. A punch isn't necessarily an actual punch - it can be an explosion, a sword slash, or something else.

The total damage is calculated by multiplying the punch's damage groups with the target's vulnerabilities. This is then limited depending on how recent the last punch was. We will go over an example of this calculation in a bit.

Just like node dig groups, these groups can take any name and do not need to be registered. However, it's common to use the same group names as with node digging.

How vulnerable an object is to particular types of damage depends on its armor_groups. Despite its misleading name, armor_groups specify the percentage damage taken from particular damage groups, not the resistance. If a damage group is not listed in an object's armor groups, that object is completely invulnerable to it.

target:set_armor_groups({
    fleshy = 90,
    crumbly = 50,
})

In the above example, the object will take 90% of fleshy damage and 50% of crumbly damage.

When a player punches an object, the damage groups come from the item they are currently wielding. In other cases, mods decide which damage groups are used.

Example Damage Calculation

Let's punch the target object:

local tool_capabilities = {
    full_punch_interval = 0.8,
    damage_groups = { fleshy = 5, choppy = 10 },

    -- This is only used for digging nodes, but is still required
    max_drop_level=1,
    groupcaps={
        fleshy={times={[1]=2.5, [2]=1.20, [3]=0.35}, uses=30, maxlevel=2},
    },
}

local time_since_last_punch = tool_capabilities.full_punch_interval
target:punch(object, time_since_last_punch, tool_capabilities)

Now, let's work out what the damage will be. The punch's damage groups are fleshy=5 and choppy=10, and target will take 90% damage from fleshy and 0% from choppy.

First, we multiply the damage groups by the vulnerability and sum the result. We then multiply by a number between 0 or 1 depending on the time_since_last_punch.

= (5*90/100 + 10*0/100) * limit(time_since_last_punch / full_punch_interval, 0, 1)
= (5*90/100 + 10*0/100) * 1
= 4.5

As HP is an integer, the damage is rounded to 5 points.

Attachments

Attached objects will move when the parent - the object they are attached to - is moved. An attached object is said to be a child of the parent. An object can have an unlimited number of children, but at most one parent.

child:set_attach(parent, bone, position, rotation)

An object's get_pos() will always return the global position of the object, no matter whether it is attached or not. set_attach takes a relative position, but not as you'd expect. The attachment position is relative to the parent's origin as scaled up by 10 times. So, 0,5,0 would be half a node above the parent's origin.

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For 3D models with animations, the bone argument is used to attach the entity to a bone. 3D animations are based on skeletons - a network of bones in the model where each bone can be given a position and rotation to change the model, for example, to move the arm. Attaching to a bone is useful if you want to make a character hold something:

obj:set_attach(player,
    "Arm_Right",           -- default bone
    {x=0.2, y=6.5, z=3},   -- default position
    {x=-100, y=225, z=90}) -- default rotation

Your Turn

• Make a windmill by combining nodes and an entity.
• Make a mob of your choice (using just the entity API, and without using any other mods).