+So far we've been discussing only the Overworld generator. But MineCraft contains more than that. The +Nether has a completely different look and feel, and quite different processes are required to generate that. +Recall that MineCraft's Nether is 128 blocks high, with bedrock both at the top and the bottom. Between these +two, the terrain looks more like a cavern than a surface. Not surprisingly, the Nether doesn't need a +complicated height generator, it can use the flat height. However, the terrain composition must take an +altogether different approach.
+ +The very first idea is to use the Perlin noise, but generate it in 3D, rather than 2D. Then, for each +block, evaluate the noise value, if below 0, make it air, if not, make it netherrack. + +
To make it so that the bedrock at the top and at the bottom is never revealed, we can add a value +increasing the more the Y coord gets towards the bottom or the top. This way the thresholding then guarantees +that there will be no air anywhere near the bedrock.
+(TODO)
(TODO)
+Finishers are a vast category of various additions to the terrain generator. They range from very easy +ones, such as generating snow on top of the terrain in cold biomes, through medium ones, such as growing +patches of flowers, complicated ones, such as placing trees and generating caves, all the way to very +complicated ones such as villages and nether fortresses. There is no formal distinction between all these +"categories", the only thing they have in common is that they take a chunk of blocks and modify it in some +way.
+Snow is probably the easiest of the finishers. It generates a block of snow on top of each block that is +on top of the terrain and is not marked as non-snowable. It checks the chunk's heightmap to determine the top +block, then checks whether the block supports snow on its top. Rails, levers and grass don't support snow, +for example.
+ +Another example of an easy finisher. This scans through the world and turn each water block on the surface +into an ice block if the biome is cold. This means that any water block that is under any kind of other +block, such as under a tree's leaves, will still stay water. Thus an additional improvement could be made by +scanning down from the surface block through blocks that we deem as non-surface, such as leaves, torches, +ladders, fences etc. Note that MCServer currently implements only the easy solution.
+ +Most worlds in MineCraft have lava lakes at their bottom. Generating these is pretty straightforward: Use +the user-configured depth and replace all the air blocks below this depth with lava blocks. Note however, +that this makes this generator dependent on the order in which the finishers are applied. If the mineshafts +generate before bottom lava, the mineshafts that are below the lava level will get filled with lava. On the +other hand, if bottom lava is generated before the mineshafts, it is possible for a mineshaft to "drill +through" a lake of lava. MCServer doesn't try to solve this and instead lets the admin choose whichever they +prefer.
+ +There are generators for specific kinds of foliage. The dead bushes in the desert biome and lilypads in +the swamp biome both share the same generating pattern. They are both specific to a single biome and they +both require a specific block underneath them in order to generate. Their implementation is simple: pick +several random columns in the chunk. If the column is of the correct biome and has the correct top block, +add the foliage block on top.
+ +In order to generate the same set of coordinates when the chunk is re-generated, we use the Perlin noise's +basis functions (the ones providing the random values for Perlin cell vertices). These basically work as a +hash function for the coorinates - the same input coordinates generate the same output value. We use the +chunk's coordinates as two of the coords, and the iteration number as the third coordinate, to generate a +random number. We then check the biome and the top block at those coordinates, if they allow, we generate the +foliage block on top.
+ +Another example of specific foliage is the tall grass in the plains biome. There are quite a lot of these +tall grass blocks, it would be inefficient to generate them using the random-coords approach described above. +Instead, we will use a 2D Perlin noise again, with a threshold defining where to put the grass and where +not.
+ +For the flowers, grass, mushrooms in caves etc. we want to use a slightly different algorithm. These +foliage blocks are customarily generated in small "clumps" - there are several blocks of the same type near +together. To generate these, we first select random coords, using the coord hash functions, for a center of a +clump. Then we select the type of block to generate. Finally, we loop over adding a random (coord hash) +number to the clump center coords to get the block where to generate the foliage block:
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+In order to make the clump more "round" and "centered", we want the offsets to be closer to the clump +center more often. This is done using a thing called Gaussian function distribution. Instead of having each +random number generate with the same probability, we want higher probability for the numbers around zero, +like this:
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+Instead of doing complicated calculations to match this shape exactly, we will use a much easier shape. +By adding together two random numbers in the same range, we get the probability distribution that has a +"roof" shape, enough for our needs:
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+(For the curious, there is a proof that adding together infinitely many uniform-distributed random number +produces random numbers with the Gaussian distribution.)
+ +This scheme can be used to produce clumps of flowers, when we select the 2D coords of the clump center on +the top surface of the terrain. We simply generate the 2D coords of the foliage blocks and use the terrain +height to find the third coord. If we want to generate clumps of mushrooms in the caves, however, we need to +generate the clump center coords in 3D and either use 3 offsets for the mushrooms, or use 2 offsets plus +searching for the closest opening Y-wise in the terrain.
+ +Water and lava springs are essential for making the underground quite a lot more interesting. They are +rather easy to generate, but a bit more difficult to get right. Generating simply means that a few random +locations (obtained by our familiar coord hashing) are checked and if the block type in there is stone. Then +we see all the horizontal neighbors of the block, plus the block underneath. If all of them except one are +stone, and the one left is air, our block is suitable for turning into a spring. If there were more air +neighbors, the spring would look somewhat unnatural; if there were no air neighbors, the spring won't flow +anywhere, so it would be rather useless.
+ +The difficult part about springs is the amount of them to generate. There should be a few springs on the +surface, perhaps a bit more in the mountaineous biomes. There should be quite a few more springs underground, +but there should definitely be more water springs than lava springs in the upper levels of the terrain, while +there should be more lava springs and almost no water springs near the bottom. To accomodate this, the +MCServer team has made a tool that scanned through MineCraft's terrain and counted the amount of both types +of springs in relation to their height. Two curves have been found for the distribution of each type of the +spring:
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+MCServer uses an approximation of the above curves to choose the height at which to generate the +spring.
+ +