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| author | Mattes D <github@xoft.cz> | 2014-05-30 22:31:51 +0200 |
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| committer | Mattes D <github@xoft.cz> | 2014-05-30 22:31:51 +0200 |
| commit | 376d5e7ff0d2daab13aee0bf184a3ef5375bd1e6 (patch) | |
| tree | b0b077c20bcfa537936a4b6666033c967c6b55a9 /docs | |
| parent | Merge pull request #993 from mc-server/GridStructGen (diff) | |
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diff --git a/docs/Generator.html b/docs/Generator.html new file mode 100644 index 000000000..f71ce1c71 --- /dev/null +++ b/docs/Generator.html @@ -0,0 +1,119 @@ +<html> +<head> +<title>Generating terrain in MCServer</title> +</head> +<body> +<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" /> +</body> +</html> |