I have created a custom world type extending ForgeWorldType.

It shows up in the create world screen, and it generates properly.

However, I would like to override the nether generation. How can I do that?

Edited April 1, 20214 yr by NorthWestWind
The problem was solved

Wait, do you want to create a new world type which looks similar to the Nether, or do you want to actually change the vanilla Nether generation itself?

The latter. My world type generates a Skyblock world with overworld biome provider and custom chunk generator that generates nothing except the island.

Which is your goal exactly? I mean, how you intend to transform the nether generation?

I want to make the nether only generates features like fortresses, bastions but not bones, and remove all other terrains and the bedrock roof while still keeping the biomes and lava.

The problem is that I don't even know how I can edit them.

Edited April 1, 20214 yr by NorthWestWind

So, removing some of the features from nether biomes is quite simple, you would need to handle the BiomeLoadingEvent, retrieve the features you don't want from each Nether biome and remove them from the feature list. Example on how to use the BiomeLoadingEvent to add or remove features here: https://github.com/Beethoven92/BetterEndForge/blob/master/src/main/java/mod/beethoven92/betterendforge/common/event/forge/BiomeModification.java

You should specify a bit more what do you mean by "remove all other terrains"...but for things like that and removing the bedrock ceiling there is no easy or mod-compatible way to achieve them. You would need to create a custom DimensionSettings object for the Nether and replace the vanilla one. Useful classes to look at are: NoiseChunkGenerator and DimensionSettings

Thank you for giving me examples, will definitely try it very soon.

And by "remove all other terrains", I mean I want nether biomes to generate on islands, like in the end (but smaller).

Also about the DimensionSettings thing, I would actually like to override the chunk generator of the nether instead so I can take full control about the nether generation. Is there any way to achieve this?

Edited April 1, 20214 yr by NorthWestWind

You need to look how the floating islands world preset is generated then (in the DimensionSettings class you can see the settings used by the floating islands world type)..to override the Nether chunk generator you would have to create a new NoiseChunkGenerator instance. Look at the getNetherChunkGenerator method in the DimensionType class..however instead of trying to replace the vanilla nether, unless you really want to do that, i would go for a custom world type which generates how you want (which would be far easier and would not require core modding)

After looking around a little more, I found the way to use custom chunk generator in the nether.

It turns out ForgeWorldType.IChunkGeneratorFactory has a method like this (Note: I'm using official mappings):

default DimensionGeneratorSettings createSettings(DynamicRegistries dynamicRegistries, long seed, boolean generateStructures, boolean bonusChest, String generatorSettings) {
    Registry<Biome> biomeRegistry = dynamicRegistries.registryOrThrow(Registry.BIOME_REGISTRY);
    Registry<DimensionType> dimensionTypeRegistry = dynamicRegistries.registryOrThrow(Registry.DIMENSION_TYPE_REGISTRY);
    Registry<DimensionSettings> dimensionSettingsRegistry = dynamicRegistries.registryOrThrow(Registry.NOISE_GENERATOR_SETTINGS_REGISTRY);
    return new DimensionGeneratorSettings(seed, generateStructures, bonusChest,
        DimensionGeneratorSettings.withOverworld(dimensionTypeRegistry,
            DimensionType.defaultDimensions(dimensionTypeRegistry, biomeRegistry, dimensionSettingsRegistry, seed),
            createChunkGenerator(biomeRegistry, dimensionSettingsRegistry, seed, generatorSettings)));
}

DimensionType.defaultDimensions actually handles the nether and end chunk generators. So, what I did was copy their entire method, which originally looks like this:

private static ChunkGenerator defaultEndGenerator(Registry < Biome > p_242717_0_, Registry < DimensionSettings > p_242717_1_, long p_242717_2_) {
    return new NoiseChunkGenerator(new EndBiomeProvider(p_242717_0_, p_242717_2_), p_242717_2_, () - > {
        return p_242717_1_.getOrThrow(DimensionSettings.END);
    });
}

private static ChunkGenerator defaultNetherGenerator(Registry < Biome > p_242720_0_, Registry < DimensionSettings > p_242720_1_, long p_242720_2_) {
    return new NoiseChunkGenerator(NetherBiomeProvider.Preset.NETHER.biomeSource(p_242720_0_, p_242720_2_), p_242720_2_, () - > {
        return p_242720_1_.getOrThrow(DimensionSettings.NETHER);
    });
}

public static SimpleRegistry < Dimension > defaultDimensions(Registry < DimensionType > p_242718_0_, Registry < Biome > p_242718_1_, Registry < DimensionSettings > p_242718_2_, long p_242718_3_) {
    SimpleRegistry < Dimension > simpleregistry = new SimpleRegistry < > (Registry.LEVEL_STEM_REGISTRY, Lifecycle.experimental());
    simpleregistry.register(Dimension.NETHER, new Dimension(() - > {
        return p_242718_0_.getOrThrow(NETHER_LOCATION);
    }, defaultNetherGenerator(p_242718_1_, p_242718_2_, p_242718_3_)), Lifecycle.stable());
    simpleregistry.register(Dimension.END, new Dimension(() - > {
        return p_242718_0_.getOrThrow(END_LOCATION);
    }, defaultEndGenerator(p_242718_1_, p_242718_2_, p_242718_3_)), Lifecycle.stable());
    return simpleregistry;
}

And changed 

simpleregistry.register(Dimension.NETHER, new Dimension(() - > {
    return p_242718_0_.getOrThrow(NETHER_LOCATION);
}, defaultNetherGenerator(p_242718_1_, p_242718_2_, p_242718_3_)), Lifecycle.stable());

into 

simpleregistry.register(Dimension.NETHER, new Dimension(() -> dimensionTypes.getOrThrow(DimensionType.NETHER_LOCATION), new SkyblockNetherChunkGenerator(NetherBiomeProvider.Preset.NETHER.biomeSource(biomeRegistry, seed), seed, () -> settings.getOrThrow(DimensionSettings.NETHER))), Lifecycle.stable());

Now I have full control about how the nether generates.