xna - In a semi-infinite procedural (chunk based) map, how do you handle objects larger than a single chunk?

I'll use an example to try and be more specific. Assume this is for a 2D top-down game. Chunks in this are made up of 32x32 tiles.

If procedural buildings were to be generated based on some kind of noise algorithm and a building was typically 3x3 chunks in size, how do you ensure the seed for this map will always produce the same buildings?

Given two adjacent chunks: chunk A and chunk B, what if they both meet the noise algorithm's requirement for being a spawn point for a building?

If they are both far from the player's current view range, it could depend on how the player approaches these chunks as far as which one gets generated first.

My first thought was to only check for buildings every so many chunks, however, I'd also like to have some objects that may span tens of chunks. I don't think this is a proper solution.

Should I simply be using a different type of algorithm to identify where buildings should be able to be spawned at, an ever expanding Voronoi diagram as an example, or is there something else I'm just not considering with how I'm already trying to attempt this? Perhaps I should be handling cross chunk objects on a grander scale (like a collection of 32x32 chunks)?

Thanks!

Answer

First, your generator(s) should definitely be working at the global, world scale and then applied at the chunk scale. You saw that already and it was a great observation. Here are some notes on working with procedurally generated content that will hopefully make sense. I think that you're confusing "Random World" with "Procedural World," and they're related but not the same. What you want is a Procedural World that can be always be the same if you want, or different each time you run.

Noise is not random. Noise output is always the same from the same input parameters. The great thing about noise is that it is 100% repeatable.

You control the input parameters, this is how you come up with different noise fields and the only place randomness occurs!

It's very common for one, single seed to change the base set of numbers and then everything else is based on those. In an example case, the input parameters to the noise would be based on chunk location in global space modified by that global base set. It doesn't matter which chunk gets created first since the chunk is always in the same place and parameters are always the same for the same seed.

Noise does not have to stand alone. You can have one noise controlling your chunks, another noise controlling your building placement, and so on. This allows you to tweak buildings without messing with your terrain. Each of these noise fields can be at different scales, etc...

You can use more than one random number generator. You can seed multiple generators and use each one independently. Random numbers come in a variety of styles depending on what you want from them, it's a deep subject with lots of statistics involved, but something to consider in the future.

In general the idea is that everything your algorithms based on noise create should be 100% repeatable and independent of anything else. Your choice of seed guarantees not that each run will be different, but that each run will be the same until a different seed is chosen.

It is this repeatable behavior combined with working globally and applying locally that will solve your buildings problem.