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Make Sobol seeding more robust.

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This way incremental seeds can be passed (e.g. 0, 1, 2, etc.) and
still get statistically independent Sobol sequences.
This commit is contained in:
Nathan Vegdahl 2020-04-22 14:19:57 +09:00
parent 085d1d655e
commit 660576ec2b
1 changed files with 3 additions and 4 deletions
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7
sub_crates/sobol/src/lib.rs
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@ -9,15 +9,14 @@ include!(concat!(env!("OUT_DIR"), "/vectors.inc"));
/// within the sequence. /// within the sequence.
/// ///
/// A different `seed` parameter results in a statistically independent Sobol /// A different `seed` parameter results in a statistically independent Sobol
/// sequence, uncorrelated to others with different seeds. However, seed /// sequence, uncorrelated to others with different seeds.
/// itself needs to be sufficiently random: you can't just pass 1, 2, 3, etc.
/// ///
/// Note: generates a maximum of 2^16 samples per dimension. If the `index` /// Note: generates a maximum of 2^16 samples per dimension. If the `index`
/// parameter exceeds 2^16-1, the sample set will start repeating. /// parameter exceeds 2^16-1, the sample set will start repeating.
#[inline] #[inline]
pub fn sample(dimension: u32, index: u32, seed: u32) -> f32 { pub fn sample(dimension: u32, index: u32, seed: u32) -> f32 {
let shuffled_index = owen_scramble(index, hash(seed));
let scramble = hash(dimension ^ seed); let scramble = hash(dimension ^ seed);
let shuffled_index = owen_scramble(index, seed);
u32_to_0_1_f32(owen_scramble( u32_to_0_1_f32(owen_scramble(
sobol_u32(dimension, shuffled_index), sobol_u32(dimension, shuffled_index),
scramble, scramble,
@ -104,7 +103,7 @@ fn owen_scramble_slow(mut n: u32, scramble: u32) -> u32 {
#[inline(always)] #[inline(always)]
fn hash(n: u32) -> u32 { fn hash(n: u32) -> u32 {
let mut hash = n; let mut hash = n ^ 0x912f69ba;
for _ in 0..3 { for _ in 0..3 {
hash = hash.wrapping_mul(0x736caf6f); hash = hash.wrapping_mul(0x736caf6f);
hash ^= hash.wrapping_shr(16); hash ^= hash.wrapping_shr(16);