From 05b448094c298945c9ad6113e4e75aa24c7005c9 Mon Sep 17 00:00:00 2001
From: Robert Swain <robert.swain@gmail.com>
Date: Thu, 29 Apr 2021 00:27:38 +0200
Subject: [PATCH] Add ModalDownsampler

---
 .../src/multiresolution/chunk_pyramid.rs      | 30 ++++++++++++-
 .../src/multiresolution/sampling.rs           | 44 +++++++++++++++++++
 2 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/crates/building_blocks_storage/src/multiresolution/chunk_pyramid.rs b/crates/building_blocks_storage/src/multiresolution/chunk_pyramid.rs
index ad122bcb..95505ba5 100644
--- a/crates/building_blocks_storage/src/multiresolution/chunk_pyramid.rs
+++ b/crates/building_blocks_storage/src/multiresolution/chunk_pyramid.rs
@@ -376,7 +376,7 @@ pub type CompressibleChunkPyramid3<By, T> = CompressibleChunkPyramid<[i32; 3], B
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::{Sd8, SdfMeanDownsampler, TransformMap};
+    use crate::{ModalDownsampler, Sd8, SdfMeanDownsampler, TransformMap};
 
     #[test]
     fn downsample_destination_for_one_level_up() {
@@ -463,4 +463,32 @@ mod tests {
             &lod0_extent,
         );
     }
+
+    #[test]
+    fn downsample_modal_chunks_with_index() {
+        let num_lods = 2;
+        let chunk_shape = Point3i::fill(16);
+        let superchunk_shape = Point3i::fill(32);
+
+        let lod0_extent =
+            Extent3i::from_min_and_shape(Point3i::ZERO, superchunk_shape);
+
+        // Build a chunk map for LOD0.
+        let ambient = 0;
+        let pyramid_builder = ChunkMapBuilder3x1::new(chunk_shape, ambient);
+        let mut pyramid = ChunkPyramid3::new(pyramid_builder, || SmallKeyHashMap::new(), num_lods);
+        let lod0 = pyramid.level_mut(0);
+        lod0.fill_extent(&Extent3i::from_min_and_shape(Point3i::ZERO, chunk_shape + PointN([chunk_shape.x(), chunk_shape.y(), 0])), 1);
+        *lod0.get_mut(chunk_shape) = 1;
+
+        let index = OctreeChunkIndex::index_chunk_map(superchunk_shape, &lod0);
+
+        pyramid.downsample_chunks_with_index(
+            &index,
+            &ModalDownsampler,
+            &lod0_extent,
+        );
+
+        assert_eq!(pyramid.level(1).get(PointN([0, 0, 0])), 1);
+    }
 }
diff --git a/crates/building_blocks_storage/src/multiresolution/sampling.rs b/crates/building_blocks_storage/src/multiresolution/sampling.rs
index edbe7e2d..ac9da455 100644
--- a/crates/building_blocks_storage/src/multiresolution/sampling.rs
+++ b/crates/building_blocks_storage/src/multiresolution/sampling.rs
@@ -1,3 +1,5 @@
+use std::{collections::HashMap, hash::Hash};
+
 use crate::{prelude::*, ArrayIndexer, ArrayNx1};
 
 use building_blocks_core::prelude::*;
@@ -44,6 +46,48 @@ where
     }
 }
 
+/// A `ChunkDownsampler` that selects the most frequent (i.e. modal) voxel type from each `2x2x2` region, discarding the rest.
+pub struct ModalDownsampler;
+
+impl<N, Src, T> ChunkDownsampler<N, T, Src> for ModalDownsampler
+where
+    N: ArrayIndexer<N>,
+    PointN<N>: IntegerPoint<N>,
+    T: 'static + Copy + Eq + Hash + std::fmt::Debug,
+    Src: Get<Local<N>, Item = T> + IndexedArray<N>,
+{
+    fn downsample(
+        &self,
+        src_chunk: &Src,
+        dst_chunk: &mut ArrayNx1<N, T>,
+        dst_min: Local<N>,
+        lod_delta: u8,
+    ) {
+        // PERF: the access pattern here might not be very cache friendly
+
+        debug_assert!(lod_delta > 0);
+        let lod_delta = lod_delta as i32;
+
+        let lod_scale_factor = 1 << lod_delta;
+        let src_shape_per_point = PointN::fill(lod_scale_factor);
+
+        let dst_shape = src_chunk.extent().shape >> lod_delta;
+        debug_assert!(dst_shape > PointN::ZERO);
+
+        for p_dst in ExtentN::from_min_and_shape(PointN::ZERO, dst_shape).iter_points() {
+            let src_min = p_dst << lod_delta;
+            let src_extent = ExtentN::from_min_and_shape(src_min, src_shape_per_point);
+
+            let mut frequencies = HashMap::new();
+            for p_src in src_extent.iter_points() {
+                *frequencies.entry(src_chunk.get(Local(p_src))).or_insert(0) += 1;
+            }
+            let (voxel, _frequency) = frequencies.iter().max_by(|a, b| a.1.cmp(b.1)).unwrap();
+            *dst_chunk.get_mut(Local(dst_min.0 + p_dst)) = *voxel;
+        }
+    }
+}
+
 /// A `ChunkDownsampler` that takes the mean of each `2x2x2` region of a signed distance field. It also renormalizes the values
 /// to lie in the range `[-1.0, 1.0]`.
 pub struct SdfMeanDownsampler;