As a sweeping and perhaps wildly over-simplistic generalization, programs that generate 3D terrain fall into two classes: Home Page - https://www.garagegames.com/
Those that generate the entire map algorithmically, based on a few user-defined settings like fractal persistence, number of perlin octaves, etcetera, and;
Those with which the user builds the terrain by hand, using mouse-driven brush tools.
L3DT mixes some features of both approaches. You can, if you want, make a whole map by setting a few parameters and hitting the 'next' button (a la the 1st class). You can also manually 'design' the height map in L3DT a la the 2nd class. However, you don't have to do all the finely detailed manual labor yourself. Instead, you can edit a high-level 'design map' that sets out the general shape of the terrain and specifies the locations of various overlays like mountains, plateaus, volcanoes, cliffs and erosion. When L3DT generates the terrain, based on your design map, you get the features you want without having to do everything yourself.
Another significant feature of L3DT is that it can handle really big maps. Importantly, you don't need gigs of RAM for these maps - L3DT includes an automatic paging system that swaps map 'tiles' from RAM to the hard disk drive as required. It's called a mosaic map, and it usually caps the RAM usage to the 100-200Mb range (even for gigabyte- or terabyte-sized maps).
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