2D Random Numbers

I am trying to create a seeded random number generator that generates a number based on coordinates.

Code:

---

int rand(int x, int y);

---

Basically, given the same seed, every time this function is called, it will give the same random number for the same coordinates.

What I am really aiming for is some sort of seeded Noise function that will accept any X and Y values.

Some requirements.

Ideally, resulting values would be either between 0 and RAND_MAX, then I can manipulate as required.

(-1,1) and (1,-1) must return different values.

I had thought to do something like the following:

Code:

---

int rand(int x, int y)
{
  srand(x + seed);// Seed based on X
  srand(y + rand());// Seed based on Y and the results of X
  return rand();// Return Random value
}

---

Now this does work, however I noticed some weirdness. The resulting values actually seem to be clustered. So if I use (1, 1), (-1, -1), (-1, 1), (1, -1), each returns a different value, however all values are within a small range (eg. 5000, 5010, 4996, 4985). I would expect to see each value being significantly different.

Does anyone have another suggestion for how this could be accomplished? Something simpler? Is this clustering effect a flaw in the C++ rand/srand? I did notice that if I simply call srand(seed), and rand(), the resulting random number seems to get higher as the seed gets higher. That doesn't seem very random to me.

Boost has a powerful set of random number generation functions that you could check out.

http://www.boost.org/libs/random/index.html

Thanks, but I am looking for how to create my own method. I would prefer to avoid using anything outside the standard libs (for portability).

Also, couldn't see anything in there for 2D like I am wanting.

Quote:

---

Originally Posted by Eggman002

Does anyone have another suggestion for how this could be accomplished? Something simpler? Is this clustering effect a flaw in the C++ rand/srand? I did notice that if I simply call srand(seed), and rand(), the resulting random number seems to get higher as the seed gets higher. That doesn't seem very random to me.

---

It doesn't make much sense to constantly re-seed the random generator. All you are using in your app is a *FIRST* member of pseudo-random sequence.
My understanding is that each sequence consists of pseudo-random numbers, but no claims are made about the first member and its relation to the seed.
In a typical application, you seed the generator once on a start-up, usually with time(NULL).
If you want any correlation with the input pair – what’s random about that?

Further thought on this matter has led me to realize that what I actually want is a way to take a set of coordinates, and create a unique integer that corresponds to those coordinates. That integer would then be used to seed the random number generator, which would then be used for a variety of purposes.

So I need a way to take X,Y and convert it to a unique integer using a function we will name getseed.

Rules:
getseed(X1,Y1) != getseed(X2, Y2)
getseed(-X1,-Y1) != getseed(X1,Y1)

So the idea is that each set of coordinates would have a different random number generator. That random number generator would be the same every time you use the same coordinates, but it would be different at each other set of coordinates.