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<title>Generating terrain in MCServer</title>
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<h1>Generating terrain in MCServer</h1>
<p>This article explains the principles behind the terrain generator in MCServer. It is not strictly
specific to MCServer, though, it can be viewed as a generic guide to various terrain-generating algorithms,
with specific implementation notes regarding MCServer.</p>
<h2>Preface: How it's done in real life</h2>
<p>The nature has many complicated geological, physical and biological processes working on all scales from
microscopic to planet-wide scale, that have shaped the terrain into what we see today. The tectonic plates
collide, push mountain ranges up and ocean trenches down. Erosion dulls the sharp shapes. Plantlife takes
over to further change the overall look of the world.</p>
<p>Generally speaking, the processes take what's there and change it. Unlike computer generating, which
usually creates a finished terrain from scratch, or maybe with only a few iterations. It would be unfeasible
for software to emulate all the natural processes in enough detail to provide world generation for a game,
mainly because in the nature everything interacts with everything. If a mountain range rises, it changes the
way that the precipitation is carried by the wind to the lands beyond the mountains, thus changing the
erosion rate there and the vegetation type. </p>
<h2>Expected properties</h2>
<p>For a MineCraft-like game terrain generator we need the generator to have several properties:
<ul>
<li>The generator must be able to generate terrain in small chunks. This means it must be possible to
generate each of the chunks separately, without dependencies on the neighboring chunks. Note that this
doesn't mean chunks cannot coordinate together, it means that "a tree in one chunk cannot ask if there's
a building in the neighbor chunk", simply because the neighbor chunk may not be generated yet.</li>
<li>The generated chunk needs to be the same if re-generated. This property is not exactly required, but it
makes available several techniques that wouldn't be possible otherwise.</li>
<li>The generator needs to be reasonably fast. For a server application this means at least some 20 chunks
per second for chunks close to each other, and 5 chunks per second for distant chunks. The reason for this
distinction will be discussed later.</li>
</ul>
</p>
<h2>Reversing the flow</h2>
<p>As already mentioned, the nature works basically by generating raw terrain composition, then "applying"
erosion, vegetation and finally this leads to biomes being formed. Let's now try a somewhat inverse
approach: First generate biomes, then fit them with appropriate terrain, and finally cover in vegetation
and all the other stuff.</p>
<p>Splitting the parts like this suddenly makes it possible to create a generator with the required
properties. We can generate a reasonable biome map chunk-wise, independently of all the other data. Once we
have the biomes, we can compose the terrain for the chunk by using the biome data for the chunk, and
possibly even for neighboring chunks. Note that we're not breaking the first property, the biomes can be
generated separately so a neighboring chunk's biome map can be generated without the need for the entire
neighboring chunk to be present. Similarly, once we have the terrain composition for a chunk, we can
generate all the vegetation and structures in it, and those can again use the terrain composition in
neighboring chunks.</p>
<h2>The ComposableGenerator pipeline</h2>
<p>This leads us directly to the main pipeline that is used for generating terrain in MCServer. For
technical reasons, the terrain composition step is further subdivided into Height generation and Composition
generation, and the structures are really called Finishers. For each chunk the generator generates, in this
sequence:
<ul>
<li>Biomes</li>
<li>Terrain height</li>
<li>Terrain composition</li>
<li>Finishers</li>
</ul>
</p>
<img src="img/biomes.jpg" />
<img src="img/terrainheight.jpg" />
<img src="img/terraincomposition.jpg" />
<img src="img/finishers.jpg" />
<p>The beautiful thing about this is that the individual components can be changed independently. You can
have 5 biome generators and 3 height generators and you can let the users mix'n'match.
</p>
<h2>Using coherent noise for the generation</h2>
<p>For a great tutorial on coherent noise, see the <a href="http://libnoise.sourceforge.net/">LibNoise
documentation</a>.</p>
<p>Coherent noise is a type of noise that has three important properties that we can use to our advantage:
<ul>
<li>The noise is smooth</li>
<li>The noise is algorithmically generated, which means that the same data is generated when the same
parameters are given to the noise functions.</li>
<li>The noise can be seamlessly extended in any direction</li>
</ul></p>
<p>We'll be mostly using Perlin noise in this article. It is the easiest one to visualise and use and is one
of the most useful kinds of coherent noises. Here's an example of a Perlin noise generated in 2 dimensions:</p>
<img src="img/perlin.png" />
<p>It comes only naturally that such a 2D noise can be used as a terrain height map directly:</p>
<img src="img/perlinheightmap.png" />
<p>However, this is not the only use for this noise, and 2 dimensions is not the limit - this noise can be
generated for any number of dimensions.</p>
<h2>Generating biomes</h2>
<p>The easiest way to generate biomes is to not generate at all - simply assign a single constant biome to
everywhere. And indeed there are times when this kind of "generator" is useful - for the MineCraft's Flat
world type, or for testing purposes, or for tematic maps. In MCServer, this is exactly what the Constant
biome generator does.</p>
<p>Of course, there are more interesting test scenarios for which multiple biomes must be generated as easy
as possible. For these special needs, there's a CheckerBoard biome generator. As the name suggests, it
generates a grid of biomes.</p>
<h3>Voronoi diagram</h3>
<p>These two generators are more of a technicality, we need to make something more interesting if we're
going for a natural look. The Voronoi generator is the first step towards such a change. Recall that a
<a href="http://en.wikipedia.org/wiki/Voronoi_diagram">Voronoi diagram</a> is a construct that creates a
set of areas where each point in an area is closer to the appropriate seed of the area than the seeds of any
other area:</p>
<img src="http://upload.wikimedia.org/wikipedia/commons/8/80/Euclidean_Voronoi_Diagram.png" />
<p>The overall shape of a Voronoi diagram is governed by the placement of the seeds. In extreme cases, a
seed could affect the entire diagram, which is what we don't want - we need our locality, so that we can
generate a chunk's worth of biome data. We also don't want the too much irregular diagrams that are produced
when the seeds are in small clusters. We need our seeds to come in random, yet somewhat uniform fashion.</p>
<p>Luckily, we have just the tool: Grid with jitter. Originally used in antialiasing techniques, they can be
successfully applied as a source of the seeds for a Voronoi diagram. Simply take a regular 2D grid of seeds
with the grid distance being N, and move each seed along the X and Y axis by a random distance, usually in
the range [-N / 2, +N / 2]:</p>
<img src="img/jittergrid.png" />
<p>Such a grid is the ideal seed source for a Voronoi biome generator, because not
only are the Voronoi cells "reasonable", but the seed placement's effect on the diagram is localized - each
pixel in the diagram depends on at most 5 x 5 seeds around it:</p>
<img src="img/jittergriddependency.jpg" />
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