ABMs: created

_includes/header.html

@@ -11,7 +11,8 @@
 			<li><a href="{{ page.root }}index.html" class="title">Minetest Tutorials</a></li>
 			<li><a href="{{ page.root }}chapters/folders.html">1 - Folder Structure</a></li>
 			<li><a href="{{ page.root }}chapters/nodes_items_crafting.html">2 - Nodes, Items and Crafting</a></li>
-			<li><a href="{{ page.root }}chapters/formspecs.html">3 - Formspecs</a></li>
+			<li><a href="{{ page.root }}chapters/abms.html">3 - Active Block Modifiers</a></li>
+			<li><a href="{{ page.root }}chapters/formspecs.html">4 - Formspecs</a></li>
 			<li><hr></li>
 			<li><a href="{{ page.root }}book_index.html">Index</a></li>
 		</ul>

chapters/abms.md

@@ -0,0 +1,58 @@
+---
+title: Active Block Modifiers
+layout: default
+root: ../
+---
+
+Introduction
+------------
+
+In this chapter we will learn how to create an **A**ctive **B**lock **M**odifier (**ABM**).
+An active block modifier allows you to run code on certain nodes at certain
+intervals.
+Please be warned, ABMs which are too frequent or act on too many nodes cause
+massive amounts of lag. Use them lightly.
+
+* Special Growing Grass
+
+Special Growing Grass
+---------------------
+
+We are now going to make a mod (Yay!).
+It will add a type of grass called alien grass - it grows near water on grassy
+blocks.
+
+{% highlight lua %}
+minetest.register_node("aliens:grass", {
+	description = "Alien Grass",
+	light_source = 3, -- The node radiates light. Values can be from 1 to 15
+	tiles = {"aliens_grass.png"},
+	groups = {choppy=1},
+	on_use = minetest.item_eat(20)
+})
+
+minetest.register_abm({
+	nodenames = {"default:dirt_with_grass"},
+	neighbors = {"default:water_source", "default:water_flowing"},
+	interval = 10.0, -- Run every 10 seconds
+	chance = 50, -- Select every 1 in 50 nodes
+	action = function(pos, node, active_object_count, active_object_count_wider)
+		minetest.set_node({x = pos.x, y = pos.y + 1, z = pos.z}, {name = "aliens:grass"})
+	end
+})
+{% endhighlight %}
+
+Every ten seconds the ABM is run. Each node which has the correct nodename and
+the correct neighbors then has a 1 in 5 chance of being run. If  a node is run on,
+an alien grass block is placed above it. Please be warned, that will delete any
+blocks above grass blocks - you should check there is space by doing minetest.get_node.
+
+That's really all there is to ABMs. Specifying a neighbor is optional, so is chance.
+
+Tasks
+-----
+
+* **Midas touch**: Make water turn to gold blocks with a 1 in 100 chance, every 5 seconds.
+* **Decay**: Make wood turn into dirt when water is a neighbor.
+* **Burnin'**: Make every air node catch on fire. (Tip: "air" and "fire:basic_flame").
+  Warning: expect the game to crash.