There's virtually no difference though, whether you store the galaxy as a file and load it in, or generate it randomly. You still have to build the galaxy, texture planet models etc etc... so the best saving is on disk space.
Imagine if you had a terrain that was generated randomly every time, instead of saving the terrain data. Really a standard for a terrain these days might be 1024x1024, which would take up 4mb of disk space. But the main thing with all of this is whether to actually do it that way or not. A human will always have trouble generating landscape after landscape, placing countless objects... it's a project killer to have a wad of work like that just waiting for someone to do. Even messing around with the files can be a drag. I personally would not consider designing terrain from scratch, I'd always generate the terrain then adjust it to suit my needs.
One other option is to use control data, rather than just a seed, which is usually a random number anyway. Imagine your terrain has 16 poles, from corner to corner laid out on a 4x4 grid. 16 heights would be stored, which is not a lot of data at all. Then once these heights are stored, the terrain could be generated, based on those initial poles, using perlin noise algo's. One benefit in that is you have some control over the makeup of the terrain with those poles - you could set the terrain to be an island, have side walls, or even use a master heightmap, where each 4x4 section of pixels is a terrain seed. That might be a good way to make an epic sized terrain. If each tile on your terrain is 1m squared, then you use a 256x256 heightmap to feed perlin 'poles' on terrains at say, 256x256, then you would have a terrain that stretches for 4million tiles, or over 3000 miles across, or almost 7 million square miles. All for the cost of a 256x256 heightmap.
We don't have big teams of level editors to do the grind work, we have to look at other options like procedural techniques, because nobody has the time to model too many terrains. I have a project that uses a big terrain, and with the automatic generation, well they look better than what I could come up with, but the project is fun to work on, because if I get sick of the terrain, I just change the seed
- I don't have to model for 6 hours.
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