Cycles: add Sobol-Burley sampling pattern

Based on the paper "Practical Hash-based Owen Scrambling" by Brent Burley,
2020, Journal of Computer Graphics Techniques.

It is distinct from the existing Sobol sampler in two important ways:
* It is Owen scrambled, which gives it a much better convergence rate in many
  situations.
* It uses padding for higher dimensions, rather than using higher Sobol
  dimensions directly. In practice this is advantagous because high-dimensional
  Sobol sequences have holes in their sampling patterns that don't resolve
  until an unreasonable number of samples are taken. (See Burley's paper for
  details.)

The pattern reduces noise in some benchmark scenes, however it is also slower,
particularly on the CPU. So for now Progressive Multi-Jittered sampling remains
the default.

Differential Revision: https://developer.blender.org/D15679

1 files changed, 53 insertions, 0 deletions

diff --git a/intern/cycles/kernel/tables.h b/intern/cycles/kernel/tables.h
index c1fdbba3fa7..399eea1e2b1 100644
--- a/intern/cycles/kernel/tables.h
+++ b/intern/cycles/kernel/tables.h
@@ -63,4 +63,57 @@ ccl_inline_constant float cie_colour_match[][3] = {
   {0.0001f, 0.0000f, 0.0000f}, {0.0001f, 0.0000f, 0.0000f}, {0.0000f, 0.0000f, 0.0000f}
 };
 
+/*
+ * The direction vectors for the first four dimensions of the Sobol
+ * sequence, stored with reversed-order bits.
+ *
+ * This is used in the Sobol-Burley sampler implementation.  We don't
+ * need more than four dimensions because we achieve higher dimensions
+ * with padding.  They're stored with reversed bits because we need
+ * them reversed for the fast hash-based Owen scrambling anyway, and
+ * this avoids doing that at run time.
+ */
+ccl_inline_constant unsigned int sobol_burley_table[4][32] = {
+  {
+    0x00000001, 0x00000002, 0x00000004, 0x00000008,
+    0x00000010, 0x00000020, 0x00000040, 0x00000080,
+    0x00000100, 0x00000200, 0x00000400, 0x00000800,
+    0x00001000, 0x00002000, 0x00004000, 0x00008000,
+    0x00010000, 0x00020000, 0x00040000, 0x00080000,
+    0x00100000, 0x00200000, 0x00400000, 0x00800000,
+    0x01000000, 0x02000000, 0x04000000, 0x08000000,
+    0x10000000, 0x20000000, 0x40000000, 0x80000000,
+  },
+  {
+    0x00000001, 0x00000003, 0x00000005, 0x0000000f,
+    0x00000011, 0x00000033, 0x00000055, 0x000000ff,
+    0x00000101, 0x00000303, 0x00000505, 0x00000f0f,
+    0x00001111, 0x00003333, 0x00005555, 0x0000ffff,
+    0x00010001, 0x00030003, 0x00050005, 0x000f000f,
+    0x00110011, 0x00330033, 0x00550055, 0x00ff00ff,
+    0x01010101, 0x03030303, 0x05050505, 0x0f0f0f0f,
+    0x11111111, 0x33333333, 0x55555555, 0xffffffff,
+  },
+  {
+    0x00000001, 0x00000003, 0x00000006, 0x00000009,
+    0x00000017, 0x0000003a, 0x00000071, 0x000000a3,
+    0x00000116, 0x00000339, 0x00000677, 0x000009aa,
+    0x00001601, 0x00003903, 0x00007706, 0x0000aa09,
+    0x00010117, 0x0003033a, 0x00060671, 0x000909a3,
+    0x00171616, 0x003a3939, 0x00717777, 0x00a3aaaa,
+    0x01170001, 0x033a0003, 0x06710006, 0x09a30009,
+    0x16160017, 0x3939003a, 0x77770071, 0xaaaa00a3,
+  },
+  {
+    0x00000001, 0x00000003, 0x00000004, 0x0000000a,
+    0x0000001f, 0x0000002e, 0x00000045, 0x000000c9,
+    0x0000011b, 0x000002a4, 0x0000079a, 0x00000b67,
+    0x0000101e, 0x0000302d, 0x00004041, 0x0000a0c3,
+    0x0001f104, 0x0002e28a, 0x000457df, 0x000c9bae,
+    0x0011a105, 0x002a7289, 0x0079e7db, 0x00b6dba4,
+    0x0100011a, 0x030002a7, 0x0400079e, 0x0a000b6d,
+    0x1f001001, 0x2e003003, 0x45004004, 0xc900a00a,
+  },
+};
+
 /* clang-format on */