MathUtils.seededRandom() returned value is linearly correlated to the parameter

Hey,

According to the doc, MathUtils.seededRandom() should return a (pseudo random) float in the interval [0, 1]. However the returned value is linearly correlated to the provided integer parameter: the bigger the seed, the bigger the result.

For instance:

MathUtils.seededRandom(1); // returns 0.000007825903601782307
MathUtils.seededRandom(10); // returns 0.00007826322696941274
MathUtils.seededRandom(100); // returns 0.0007826364606457171
MathUtils.seededRandom(1000); // returns 0.00782636879740876

As far as I know, the results of a prng function shouldn’t be so predictable, but maybe I’m misunderstanding something.

Three.js version: 0.138.0

Cheers

[Mugen87]

Okay, let’s give this a try then 👍 .

[thibka]

Does the current behavior introduce any issues on app level?

Well the current behavior just doesn’t work as it should. So I’m not sure the current seededRandom function could be used in a scenario where a PRNG is needed.

The result of seededRandom() should be a number and not a function in order to keep the usage simple.

That’s true. In that case you could always rewrite the MathUtils.seededRandom function with the mulberry32 algorithm this way:

// Deterministic pseudo-random float in the interval [ 0, 1 ]
function seededRandom( s ) {

	if ( s !== undefined ) _seed = s;

	// Mulberry32 generator

	var t = _seed += 0x6D2B79F5;

	t = Math.imul(t ^ t >>> 15, t | 1);

	t ^= t + Math.imul(t ^ t >>> 7, t | 61);

	return ((t ^ t >>> 14) >>> 0) / 4294967296;

}

The behavior would then be fixed:

THREE.MathUtils.seededRandom(); // 0.6074679309967905 (depends on the previous value of the private _seed variable)
THREE.MathUtils.seededRandom(); // 0.19144689152017236 (depends on the previous value of the private _seed variable)
THREE.MathUtils.seededRandom(1); // 0.6270739405881613 (always)
THREE.MathUtils.seededRandom(10); // 0.5019920116756111 (always)
THREE.MathUtils.seededRandom(100); // 0.2043598669115454 (always)
THREE.MathUtils.seededRandom(); // 0.3148212195374071 (depends on the previous value of the private _seed variable)