Noticed that z-scrambling is actually just base-4 Owen scrambling.

Updated function name and comments to reflect that.

src/renderer.rs

@@ -20,7 +20,7 @@ use crate::{
     morton,
     ray::{Ray, RayBatch},
     scene::{Scene, SceneLightSample},
-    scramble::z_scramble,
+    scramble::owen4,
     surface,
     timer::Timer,
     tracer::Tracer,
@@ -249,7 +249,7 @@ impl<'a> Renderer<'a> {
                     // and golden ratio sampling, we do this up-front here rather than in
                     // the samplers themselves.
                     let si_offset =
-                        z_scramble(morton::xy2i(x, y), self.seed).wrapping_mul(self.spp as u32);
+                        owen4(morton::xy2i(x, y), self.seed).wrapping_mul(self.spp as u32);
                     for si in 0..self.spp {
                         let si = (si as u32).wrapping_add(si_offset);
 

src/scramble.rs

@@ -1,7 +1,7 @@
 use crate::hash::hash_u32;
 
 /// Performs a base-2 Owen scramble on an integer.
-pub fn owen(n: u32, seed: u32) -> u32 {
+pub fn owen2(n: u32, seed: u32) -> u32 {
     let mut result = n;
 
     for i in 0..32 {
@@ -12,11 +12,19 @@ pub fn owen(n: u32, seed: u32) -> u32 {
     result
 }
 
-/// Performs z-scrambling from the paper "Screen-Space Blue-Noise
-/// Diffusion of Monte Carlo Sampling Error via Hierarchical Ordering
-/// of Pixels" by Ahmed et al., except using a more complete and
-/// efficient algorithm based on the Owen scrambling function above.
-pub fn z_scramble(n: u32, seed: u32) -> u32 {
+/// Performs a base-4 Owen scramble on an integer.
+///
+/// This is motivated by z-scrambling from the paper "Screen-Space
+/// Blue-Noise Diffusion of Monte Carlo Sampling Error via Hierarchical
+/// Ordering of Pixels" by Ahmed et al.  Their scrambling function is
+/// actually just Owen scrambling in base 4, but their attempt at
+/// implementating that function isn't great, both in terms of memory
+/// usage and scramble quality.  (The paper itself is otherwise
+/// fantastic, though.)
+///
+/// The implementation below is a full proper base-4 Owen scramble,
+/// requiring only a 24-byte lookup table.
+pub fn owen4(n: u32, seed: u32) -> u32 {
     const PERM_TABLE: [u8; 24] = [
         0 | (1 << 2) | (2 << 4) | (3 << 6), // [0, 1, 2, 3],
         0 | (1 << 2) | (3 << 4) | (2 << 6), // [0, 1, 3, 2],