On Friday, 3 January 2014 at 01:43:09 UTC, Chris Cain wrote:
So, it sounds like the OP is using the x and y coords for a
seed to generate a single number and he was curious to whether
it costs too much to reseed like this for every point.
FWIW, I'm under the impression that this is a fairly common
strategy, but usually when I've seen this used more than one
number is generated at a time.
I *thought* he wanted to do something like that, but was
surprised by the fact he wanted to reseed *per* element...
You can still do this, in this case. For example, divide x by
10 and generate 10 elements (column wise) in the noise map each
time and it reduces the number of reseeds by a factor of 10.
Some effort might be wasted, but as long as you need a decent
chunk of the noise map at any one point in time, this should
work out pretty well in practice.
My biggest concern with this approach is that you must take
care that your usage of seeding with x and y coordinates does
not cause repeated elements to occur. For instance, using
`Random rng = Random(x + y);` will _not_ work properly (unless
you want strange mirroring down the diagonal in your noise
map). There's numerous landmines to avoid when doing something
like this. Some approaches may not be perfect, but depending on
what you're doing they may be sufficient, however.
The approach I'd take here is to eagerly generate a "uint[X /
1000][Y / 1000]" grid, that holds randomly generated numbers, to
be used as seeds for individual 1000*1000 blocks of the noise
map. I don't know how good that is though in terms of
correlation...?
Or, even better, to create a single generator of type "Gen", and
store a "Gen[X / 1000][Y / 1000]": EG: the generator, with a
"checkpoint" every 1_000_000 elements. This should reduce
correlation down to 0.
AFAIK, all generators above the "linear congruential generator"
have a period above 10^12, so this approach should be fine. The
only issue with such an approach might be the size of the PRNG's
themselves: XOrShift, for example, is only a few bytes big, so
that's fine. But MT is about 700B, which is a hefty amount to
chug along.
