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Basic Map Operations

Introduction

In this chapter, you will learn how to perform basic actions on the map, such as adding, removing, and finding nodes.

• Map Structure
• Reading
  • Reading Nodes
  • Finding Nodes
• Writing
  • Writing Nodes
  • Removing Nodes
• Loading Blocks
• Deleting Blocks

Map Structure

The Minetest map is split into MapBlocks, each MapBlocks being a cube of size 16. As players travel around the map, MapBlocks are created, loaded, activated, and unloaded. Areas of the map which are not yet loaded are full of ignore nodes, an impassable unselectable placeholder node. Empty space is full of air nodes, an invisible node you can walk through.

An active MapBlock is one which is loaded and has updates running on it.

Loaded map blocks are often referred to as active blocks. Active Blocks can be read from or written to by mods or players, and have active entities. The Engine also performs operations on the map, such as performing liquid physics.

MapBlocks can either be loaded from the world database or generated. MapBlocks will be generated up to the map generation limit (mapgen_limit) which is set to its maximum value, 31000, by default. Existing MapBlocks can, however, be loaded from the world database outside of the generation limit.

Reading

Reading Nodes

You can read from the map once you have a position:

local node = core.get_node({ x = 1, y = 3, z = 4 })
print(dump(node)) --> { name=.., param1=.., param2=.. }

If the position is a decimal, it will be rounded to the containing node. The function will always return a table containing the node information:

• name - The node name, which will be ignore when the area is unloaded.
• param1 - See the node definition. This will commonly be light.
• param2 - See the node definition.

It’s worth noting that the function won’t load the containing block if the block is inactive, but will instead return a table with name being ignore.

You can use core.get_node_or_nil instead, which will return nil rather than a table with a name of ignore. It still won’t load the block, however. This may still return ignore if a block actually contains ignore. This will happen near the edge of the map as defined by the map generation limit (mapgen_limit).

Finding Nodes

Minetest offers a number of helper functions to speed up common map actions. The most commonly used of these are for finding nodes.

For example, say we wanted to make a certain type of plant that grows better near mese; you would need to search for any nearby mese nodes, and adapt the growth rate accordingly.

core.find_node_near will return the first found node in a certain radius which matches the node names or groups given. In the following example, we look for a mese node within 5 nodes of the position:

local grow_speed = 1
local node_pos   = core.find_node_near(pos, 5, { "default:mese" })
if node_pos then
    core.chat_send_all("Node found at: " .. dump(node_pos))
    grow_speed = 2
end

Let’s say, for example, that the growth rate increases the more mese there is nearby. You should then use a function that can find multiple nodes in the area:

local pos1       = vector.subtract(pos, { x = 5, y = 5, z = 5 })
local pos2       = vector.add(pos, { x = 5, y = 5, z = 5 })
local pos_list   =
        core.find_nodes_in_area(pos1, pos2, { "default:mese" })
local grow_speed = 1 + #pos_list

The above code finds the number of nodes in a cuboid volume. This is different to find_node_near, which uses the distance to the position (ie: a sphere). In order to fix this, we will need to manually check the range ourselves:

local pos1       = vector.subtract(pos, { x = 5, y = 5, z = 5 })
local pos2       = vector.add(pos, { x = 5, y = 5, z = 5 })
local pos_list   =
        core.find_nodes_in_area(pos1, pos2, { "default:mese" })
local grow_speed = 1
for i=1, #pos_list do
    local delta = vector.subtract(pos_list[i], pos)
    if delta.x*delta.x + delta.y*delta.y + delta.z*delta.z <= 5*5 then
        grow_speed = grow_speed + 1
    end
end

Now the code will correctly increase grow_speed based on mese nodes in range.

Note how we compared the squared distance from the position, rather than square rooting it to obtain the actual distance. This is because computers find square roots computationally expensive, so they should avoided as much as possible.

There are more variations of the above two functions, such as find_nodes_with_meta and find_nodes_in_area_under_air, which work similarly and are useful in other circumstances.

Writing

Writing Nodes

You can use set_node to write to the map. Each call to set_node will cause lighting to be recalculated and node callbacks to run, which means that set_node is fairly slow for large numbers of nodes.

core.set_node({ x = 1, y = 3, z = 4 }, { name = "default:mese" })

local node = core.get_node({ x = 1, y = 3, z = 4 })
print(node.name) --> default:mese

set_node will remove any associated metadata or inventory from that position. This isn’t desirable in all circumstances, especially if you’re using multiple node definitions to represent one conceptual node. An example of this is the furnace node - whilst you conceptually think of it as one node, it’s actually two.

You can set a node without deleting metadata or the inventory like so:

core.swap_node({ x = 1, y = 3, z = 4 }, { name = "default:mese" })

Removing Nodes

A node must always be present. To remove a node, you set the position to air.

The following two lines will both remove a node, and are both identical:

core.remove_node(pos)
core.set_node(pos, { name = "air" })

In fact, remove_node is just a helper function that calls set_node with "air".

Loading Blocks

You can use core.emerge_area to load map blocks. Emerge area is asynchronous, meaning the blocks won’t be loaded instantly. Instead, they will be loaded soon in the future, and the callback will be called each time.

-- Load a 20x20x20 area
local halfsize = { x = 10, y = 10, z = 10 }
local pos1 = vector.subtract(pos, halfsize)
local pos2 = vector.add     (pos, halfsize)

local context = {} -- persist data between callback calls
core.emerge_area(pos1, pos2, emerge_callback, context)

Minetest will call emerge_callback whenever it loads a block, with some progress information.

local function emerge_callback(pos, action,
        num_calls_remaining, context)
    -- On first call, record number of blocks
    if not context.total_blocks then
        context.total_blocks  = num_calls_remaining + 1
        context.loaded_blocks = 0
    end

    -- Increment number of blocks loaded
    context.loaded_blocks = context.loaded_blocks + 1

    -- Send progress message
    if context.total_blocks == context.loaded_blocks then
        core.chat_send_all("Finished loading blocks!")
    else
        local perc = 100 * context.loaded_blocks / context.total_blocks
        local msg  = string.format("Loading blocks %d/%d (%.2f%%)",
                context.loaded_blocks, context.total_blocks, perc)
        core.chat_send_all(msg)
    end
end

This is not the only way of loading blocks; using an Lua Voxel Manipulator (LVM) will also cause the encompassed blocks to be loaded synchronously.

Deleting Blocks

You can use delete_blocks to delete a range of map blocks:

-- Delete a 20x20x20 area
local halfsize = { x = 10, y = 10, z = 10 }
local pos1 = vector.subtract(pos, halfsize)
local pos2 = vector.add     (pos, halfsize)

core.delete_area(pos1, pos2)

This will delete all map blocks in that area, inclusive. This means that some nodes will be deleted outside the area as they will be on a mapblock which overlaps the area bounds.