I became a developer for Minetest
http://www.minetest.net/ a year ago and have added a C++ fractal world generator to the engine.
Minetest is a free C++ multi-platform open-source game engine similar to Minecraft, it is possibly the most popular voxel sandbox after Minecraft and was recently considered the best open-source game of 2015 by opensource.com.
Advantages over Minecraft are a huge world height of 62km instead of 256m, and very easy modding and subgame creation using the scripting language Lua. These make it ideal for those interested in experimenting and generating large 3D structures.
We are a little understaffed and would welcome help from C++ programmers, contact us at the forum or at the IRC #minetest-dev channel.
Here are the details of fractals supported and parameters, the fractals can be translated, scaled and stretched and there is control over number of iterations, W-slice and Julia co-ordinate.
By default the fractal is a horizontally stretched island 10km x 8km x 1km height, rising from a shallow sea. The fractal worlds are fully playable with 10 biomes, tunnels and caves, resources for mining and craftable materials for building.
All fractals were created from my study of
http://www.bugman123.com/Hypercomplex/index.html thanks to bugman123 for this excellent page and the formulas.
#### Mapgen fractal
# Choice of 18 fractals from 9 formulas.
# 1 = 4D "Roundy" mandelbrot set.
# 2 = 4D "Roundy" julia set.
# 3 = 4D "Squarry" mandelbrot set.
# 4 = 4D "Squarry" julia set.
# 5 = 4D "Mandy Cousin" mandelbrot set.
# 6 = 4D "Mandy Cousin" julia set.
# 7 = 4D "Variation" mandelbrot set.
# 8 = 4D "Variation" julia set.
# 9 = 3D "Mandelbrot/Mandelbar" mandelbrot set.
# 10 = 3D "Mandelbrot/Mandelbar" julia set.
# 11 = 3D "Christmas Tree" mandelbrot set.
# 12 = 3D "Christmas Tree" julia set.
# 13 = 3D "Mandelbulb" mandelbrot set.
# 14 = 3D "Mandelbulb" julia set.
# 15 = 3D "Cosine Mandelbulb" mandelbrot set.
# 16 = 3D "Cosine Mandelbulb" julia set.
# 17 = 4D "Mandelbulb" mandelbrot set.
# 18 = 4D "Mandelbulb" julia set.
# type: int min: 1 max: 18
# mgfractal_fractal = 1
# Iterations of the recursive function.
# Controls the amount of fine detail.
# type: int
# mgfractal_iterations = 11
# Approximate (X,Y,Z) scale of fractal in nodes.
# type: v3f
# mgfractal_scale = (4096.0, 1024.0, 4096.0)
# (X,Y,Z) offset of fractal from world centre in units of 'scale'.
# Used to move a suitable spawn area of low land close to (0, 0).
# The default is suitable for mandelbrot sets, it needs to be edited for julia sets.
# Range roughly -2 to 2. Multiply by 'scale' for offset in nodes.
# type: v3f
# mgfractal_offset = (1.79, 0.0, 0.0)
# W co-ordinate of the generated 3D slice of a 4D fractal.
# Determines which 3D slice of the 4D shape is generated.
# Has no effect on 3D fractals.
# Range roughly -2 to 2.
# type: float
# mgfractal_slice_w = 0.0
# Julia set only: X component of hypercomplex constant determining julia shape.
# Range roughly -2 to 2.
# type: float
# mgfractal_julia_x = 0.33
# Julia set only: Y component of hypercomplex constant determining julia shape.
# Range roughly -2 to 2.
# type: float
# mgfractal_julia_y = 0.33
# Julia set only: Z component of hypercomplex constant determining julia shape.
# Range roughly -2 to 2.
# type: float
# mgfractal_julia_z = 0.33
# Julia set only: W component of hypercomplex constant determining julia shape.
# Has no effect on 3D fractals.
# Range roughly -2 to 2.
# type: float
# mgfractal_julia_w = 0.33
# type: noise_params
# mgfractal_np_seabed = -14, 9, (600, 600, 600), 41900, 5, 0.6, 2.0
# type: noise_params
# mgfractal_np_filler_depth = 0, 1.2, (150, 150, 150), 261, 3, 0.7, 2.0
# type: noise_params
# mgfractal_np_cave1 = 0, 12, (128, 128, 128), 52534, 4, 0.5, 2.0
# type: noise_params
# mgfractal_np_cave2 = 0, 12, (128, 128, 128), 10325, 4, 0.5, 2.0