Perlin Noise to Percentage

Question

I'm coding a map generator based on a perlin noise and ran into a problem:

Lets say I would want 30% water and 70% dirt tiles. With a usual random generator there is no problem:

tile = rnd.nextFloat() < 0.7f ? DIRT : WATER;

But a perlin noise is normal distributed (ranges from -1 to 1, mean at 0) so it's not that easy.

Does anyone know a way to transform a normal to an uniform distribution or a different way I could get a percentage from a noise value?

EDIT: The 70% are just an example, I'd want to be able to use any value dynamically, at best with 0.1% precision.

EDIT2: I want to transformate perlin noise to a uniform distribution, not to normal (which it already is alike).

Answer 1

A solution I figured out: Firstly, I generate 100,000,000 perlin noises and store them in an array. I sort it, and afterwards I can take every 10,000 value as a threshold for one per mille. Now I can hardcode these thresholds, so I've just an array with 1,000 floats for lookup at runtime.

Advantages:

It's really fast, as it's just one array access at runtime.

Drawbacks:

If you change the algorithm, you have to regenerate your threshold array. Secondly, the mean scales to about 10 per mille, making a 50% threshold either 49.5% or 50.5% (depending on whether you use < or <= comperator). Thirdly, the increased memory footprint (4kb with per mill precision). You can reduce it by using percent precision or a logarithmic precision scale.

Generation code:

final PerlinNoiseGenerator perlin = new PerlinNoiseGenerator(new Random().nextInt());

final int size = 10000; //Size gets sqared, so it's actually 100,000,000

final float[] values = new float[size * size];
for (int x = 0; x < size; x++)
    for (int y = 0; y < size; y++) {
        final float value = perlin.noise2(x / 10f, y / 10f);
        values[x * size + y] = value;
    }
System.out.println("Calculated");

Arrays.sort(values);
System.out.println("Sorted");

final float[] steps = new float[1000];
steps[999] = 1;
for (int i = 0; i < 999; i++)
    steps[i] = values[size * size / 1000 * (i + 1)];
System.out.println("Calculated steps");

for (int i = 0; i < 10; i++) {
    System.out.println();
    for (int j = 0; j < 100; j++)
        System.out.print(steps[i * 100 + j] + "f, "); //Output usuable for array initialization
    System.out.println();
    System.out.println();
}

Lookup code:

public final static float[] perlinThresholds = new float[]{}; //Initialize it with the generated thresholds.

public static float getThreshold(float percent) {
    return perlinThresholds[(int)(percent * 1000)];
}

public static float getThreshold(int promill) {
    return perlinThresholds[promill];
}

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