minetest_modding_book

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title	layout	root	idx	description
Objects, Players, and Entities	default	../..	3.4	Using an ObjectRef

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
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.

Entities are sometimes known as Lua Entities. Don't be fooled though, all entities are Lua entities.

Position and Velocity

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

local object = minetest.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

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 type table much like an item does. This table can contain callback methods, default object properties, and custom elements.

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

When an entity has emerged, a table is created for it by copying everything from its type table. This table can be used to store variables for that particular entity.

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 " .. minetest.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.txt]({{ page.root }}/lua_api.html#registered-entities).

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 minetest.get_node(pos_down).name == "air" then
        delta = vector.new(0, -1, 0)
    elseif minetest.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)
    minetest.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 minetest.write_json({
        message = self.message,
    })
end

function MyEntity:on_activate(staticdata, dtime_s)
    if staticdata ~= "" and staticdata ~= nil then
        local data = minetest.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.

minetest.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 = minetest.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

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.

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.

Your Turn

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

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