Generate sobol direction numbers from text file.

This way the numbers aren't cluttering up the source file, and
this also makes it easier to play with other direction numbers.

src/renderer.rs

@@ -707,8 +707,14 @@ fn get_sample(dimension: u32, i: u32, pixel_co: (u32, u32), seed: u32) -> f32 {
             n as f32 * (1.0 / (1u64 << 32) as f32)
         }
         n if (n - 1) < sobol::MAX_DIMENSION as u32 => {
+            let dim = n - 1;
             // Sobol sampling.
-            sobol::sample_owen(dimension - 1, i, hash_u32(dimension, scramble))
+            // We skip the first three dimensions here because it gives better
+            // results on many scenes for some reason.  This is likely worth
+            // investigating in the future, as it may indicate poor sampling
+            // elsewhere in the rendering pipeline (and skipping the first
+            // two dimensions may just mask it).
+            sobol::sample_owen(dim + 3, i, hash_u32(dim, scramble))
         }
         _ => {
             // Random sampling.

sub_crates/sobol/direction_numbers/LICENSE.txt

@@ -0,0 +1,43 @@
+The data in this folder is from the website
+http://web.maths.unsw.edu.au/~fkuo/sobol/
+
+From these papers:
+  * S. Joe and F. Y. Kuo, Remark on Algorithm 659: Implementing
+    Sobol's quasirandom sequence generator, ACM Trans. Math. Softw. 29,
+    49-57 (2003)
+  * S. Joe and F. Y. Kuo, Constructing Sobol sequences with better
+    two-dimensional projections, SIAM J. Sci. Comput. 30, 2635-2654
+    (2008)
+
+Copyright (c) 2008, Frances Y. Kuo and Stephen Joe
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+  * Redistributions of source code must retain the above copyright
+    notice, this list of conditions and the following disclaimer.
+
+  * Redistributions in binary form must reproduce the above copyright
+    notice, this list of conditions and the following disclaimer in the
+    documentation and/or other materials provided with the
+    distribution.
+
+  * Neither the names of the copyright holders nor the names of the
+    University of New South Wales and the University of Waikato
+    and its contributors may be used to endorse or promote products
+    derived from this software without specific prior written
+    permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS ``AS IS'' AND ANY
+EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
+BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
+OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
+IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.