christoph/cpp-masssprings
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replace recursive octree implementation with morton code version (FML)

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Christoph Urlacher
2026-03-06 02:20:28 +01:00
parent 9f31d4e6ef
commit c060cfd35d
8 changed files with 427 additions and 200 deletions
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145
include/octree.hpp
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@ -8,6 +8,7 @@
#include <raylib.h>
#include <raymath.h>
#include <libmorton/morton.h>
class octree
{
@ -16,24 +17,21 @@ class octree
public:
Vector3 mass_center = Vector3Zero();
float mass_total = 0.0;
Vector3 box_min = Vector3Zero(); // area start
Vector3 box_max = Vector3Zero(); // area end
uint8_t depth = 0;
float size = 0.0f; // Because our octree cells are cubic we don't need to store the bounds
std::array<int, 8> children = {-1, -1, -1, -1, -1, -1, -1, -1};
int mass_id = -1;
bool leaf = true;
public:
node(const Vector3& _box_min, const Vector3& _box_max) : box_min(_box_min), box_max(_box_max) {}
};
public:
static constexpr int MAX_DEPTH = 20;
private:
// 21 * 3 = 63, fits in uint64_t for combined x/y/z morton-code
static constexpr int MAX_DEPTH = 21;
std::vector<node> nodes;
// This approach is actually slower than the array of nodes
// beacuse we access all the attributes in the same function
// std::vector<Vector3> mass_centers;
// std::vector<float> mass_totals;
// std::vector<Vector3> box_mins;
@ -50,22 +48,58 @@ public:
octree(octree&& move) = delete;
auto operator=(octree&& move) -> octree& = delete;
public:
auto clear() -> void;
auto reserve(size_t count) -> void;
[[nodiscard]] auto empty() const -> bool;
private:
[[nodiscard]] INLINE static inline auto get_octant(const Vector3& box_min,
const Vector3& box_max,
const Vector3& pos) -> int;
[[nodiscard]] INLINE static inline auto get_child_bounds(const Vector3& box_min,
const Vector3& box_max,
int octant) -> std::pair<Vector3, Vector3>;
INLINE inline auto create_empty_leaf(const Vector3& box_min, const Vector3& box_max) -> int;
[[nodiscard]] auto get_child_count(int node_idx) const -> int;
auto insert(int node_idx, int mass_id, const Vector3& pos, float mass, int depth) -> void;
static auto build_octree(octree& t, const std::vector<Vector3>& positions) -> void;
[[nodiscard]] auto calculate_force(int node_idx, const Vector3& pos) const -> Vector3;
// Map a floating point coordinate to a discrete integer (so its morton-code can be computed)
// The "bits" parameter determines the discrete axis resolution
[[nodiscard]] INLINE static inline auto quantize_axis(float coordinate,
float box_min,
float box_max,
int bits) -> uint32_t;
[[nodiscard]] INLINE static inline auto pos_to_morton(const Vector3& p,
const Vector3& root_min,
const Vector3& root_max) -> uint64_t;
[[nodiscard]] INLINE static inline auto jitter_pos(Vector3 p,
uint32_t seed,
const Vector3& root_min,
const Vector3& root_max,
float root_extent) -> Vector3;
// Use this to obtain an ancestor node of a leaf node (on any level).
// Because the morton codes (interleaved coordinates) encode the octree path, we can take
// the morten code of any leaf and only take the 3*n first interleaved bits to find the
// leaf ancestor on level n.
// Leaf Code: [101 110 100 001] -> Ancestors (from leaf to root):
// - [101 110 100]
// - [101 110]
// - [101] (root)
[[nodiscard]] INLINE static inline auto path_to_ancestor(uint64_t leaf_code, int leaf_depth, int depth) -> uint64_t;
// Use this to obtain the octant a leaf node is contained in (on any level).
// The 3 interleaved bits in the morten code encode the octant [0, 7].
// Leaf Code: [101 110 100 001] -> Octants:
// - [100] (Level 2)
// - [110] (Level 1)
// - [101] (Level 0)
[[nodiscard]] INLINE static inline auto octant_at_level(uint64_t leaf_code, int level, int leaf_depth) -> int;
public:
auto clear() -> void;
auto reserve(size_t count) -> void;
[[nodiscard]] auto empty() const -> bool;
[[nodiscard]] auto root() const -> const node&;
// Morton/linear octree implementation
static auto build_octree_morton(octree& t, const std::vector<Vector3>& positions) -> void;
[[nodiscard]] auto calculate_force_morton(int node_idx, const Vector3& pos, int self_id) const -> Vector3;
};
INLINE inline auto octree::get_octant(const Vector3& box_min, const Vector3& box_max, const Vector3& pos) -> int
@ -101,18 +135,75 @@ INLINE inline auto octree::get_child_bounds(const Vector3& box_min,
return std::make_pair(min, max);
}
INLINE inline auto octree::create_empty_leaf(const Vector3& box_min, const Vector3& box_max) -> int
INLINE inline auto octree::quantize_axis(const float coordinate,
const float box_min,
const float box_max,
const int bits) -> uint32_t
{
nodes.emplace_back(box_min, box_max);
return static_cast<int>(nodes.size() - 1);
const float extent = box_max - box_min;
if (extent <= 0.0f) {
return 0;
}
// mass_centers.emplace_back(Vector3Zero());
// mass_totals.emplace_back(0);
// box_mins.emplace_back(box_min);
// box_maxs.emplace_back(box_max);
// childrens.push_back({-1, -1, -1, -1, -1, -1, -1, -1});
// mass_ids.emplace_back(-1);
// leafs.emplace_back(true);
float normalized = (coordinate - box_min) / extent; // normalize to [0,1]
normalized = std::max(0.0f, std::min(normalized, std::nextafter(1.0f, 0.0f))); // avoid exactly 1.0
// bits up to 21 => (1u << bits) safe in 32-bit
const uint32_t grid_max = (1u << bits) - 1u;
return static_cast<uint32_t>(normalized * static_cast<float>(grid_max));
}
INLINE inline auto octree::pos_to_morton(const Vector3& p, const Vector3& root_min, const Vector3& root_max) -> uint64_t
{
const uint32_t x = quantize_axis(p.x, root_min.x, root_max.x, MAX_DEPTH);
const uint32_t y = quantize_axis(p.y, root_min.y, root_max.y, MAX_DEPTH);
const uint32_t z = quantize_axis(p.z, root_min.z, root_max.z, MAX_DEPTH);
return libmorton::morton3D_64_encode(x, y, z);
}
INLINE inline auto octree::jitter_pos(Vector3 p,
const uint32_t seed,
const Vector3& root_min,
const Vector3& root_max,
const float root_extent) -> Vector3
{
// Use a hash to calculate a deterministic jitter: The same position should always get the same jitter.
// We want this to get stable physics, particles at the same position shouldn't get different jitters
// across frames...
uint32_t h = (seed ^ 61u) ^ (seed >> 16);
h *= 9u;
h = h ^ (h >> 4);
h *= 0x27d4eb2du;
h = h ^ (h >> 15);
// finest cell size at depth L
const float finest_cell = root_extent / static_cast<float>(1u << MAX_DEPTH);
const float s = finest_cell * 1e-4f; // small pp
p.x += (h & 1u) ? +s : -s;
p.y += (h & 2u) ? +s : -s;
p.z += (h & 4u) ? +s : -s;
// clamp back into bounds just in case
p.x = std::max(root_min.x, std::min(p.x, root_max.x));
p.y = std::max(root_min.y, std::min(p.y, root_max.y));
p.z = std::max(root_min.z, std::min(p.z, root_max.z));
return p;
}
INLINE inline auto octree::path_to_ancestor(const uint64_t leaf_code, const int leaf_depth, const int depth) -> uint64_t
{
// keep top 3*depth bits; drop the rest
const int drop = 3 * (leaf_depth - depth);
return (drop > 0) ? (leaf_code >> drop) : leaf_code;
}
INLINE inline auto octree::octant_at_level(const uint64_t leaf_code, const int level, const int leaf_depth) -> int
{
// level 1 => child of root => topmost 3 bits
const int shift = 3 * (leaf_depth - level);
return static_cast<int>((leaf_code >> shift) & 0x7ull);
}
#endif