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Improvments to the owen scrambling.

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This commit is contained in:
Nathan Vegdahl 2020-03-11 23:22:07 +09:00
parent db9efc6a55
commit 9ba51cd43a
1 changed files with 15 additions and 5 deletions
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20
sub_crates/sobol/src/lib.rs
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@ -52,10 +52,19 @@ pub fn sample_rd_scramble(dimension: u32, index: u32, scramble: u32) -> f32 {
/// dimension. For example, reusing the dimension parameter itself works well. /// dimension. For example, reusing the dimension parameter itself works well.
#[inline] #[inline]
pub fn sample_owen_scramble(dimension: u32, index: u32, seed: u32) -> f32 { pub fn sample_owen_scramble(dimension: u32, index: u32, seed: u32) -> f32 {
// Do a weak "hash" on the seed. This isn't meant to be a real hash,
// we're just mixing the higher bits down into the lower bits so that
// naive seeds still work.
let mut seed = seed;
seed ^= seed.wrapping_mul(0x54bbba73);
seed ^= seed.wrapping_shr(16);
seed ^= seed.wrapping_mul(0x736caf6f);
seed ^= seed.wrapping_shr(16);
// Get the sobol point. // Get the sobol point.
let mut n = sample_u32(dimension, index); let mut n = sample_u32(dimension, index);
// We first apply the seed as if doing random digit scrambling. // Apply the "hashed" seed as if doing random digit scrambling.
// This is valid because random digit scrambling is a strict subset of // This is valid because random digit scrambling is a strict subset of
// Owen scrambling, and therefore does not invalidate the Owen scrambling // Owen scrambling, and therefore does not invalidate the Owen scrambling
// below. Instead, this simply serves to seed the Owen scrambling. // below. Instead, this simply serves to seed the Owen scrambling.
@ -72,10 +81,11 @@ pub fn sample_owen_scramble(dimension: u32, index: u32, seed: u32) -> f32 {
// have an equal chance of affecting all other bits. But in this case that // have an equal chance of affecting all other bits. But in this case that
// only-upwards behavior is exactly what we want, because it ends up being // only-upwards behavior is exactly what we want, because it ends up being
// equivalent to Owen scrambling. // equivalent to Owen scrambling.
for _ in 0..4 { // The constants here are large primes.
// The constant here is a large prime * 2. n ^= n.wrapping_mul(0x54bbba73 * 2);
n ^= n * 0xa97774e6; n ^= n.wrapping_mul(0x736caf6f * 2);
} n ^= n.wrapping_mul(0x54bbba73 * 2);
n ^= n.wrapping_mul(0x736caf6f * 2);
u32_to_0_1_f32(n) u32_to_0_1_f32(n)
} }