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beevee: bvh: add build implementation

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Bruno BELANYI 2020-03-24 17:53:15 +01:00
parent 8b3cbccb75
commit 566a9b198d
1 changed files with 260 additions and 2 deletions
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262
beevee/src/bvh/tree.rs
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@ -1,2 +1,260 @@
/// The BVH containing all the objects.
pub struct BVH {}
use crate::aabb::{Bounded, AABB};
use crate::Axis;
/// An enum representing either an internal or a leaf node of the [`BVH`]
///
/// [`BVH`]: struct.BVH.html
#[derive(Clone, Debug, PartialEq)]
enum NodeEnum {
Internal { left: Box<Node>, right: Box<Node> },
Leaf,
}
/// A node representing either an internal or a leaf node of the [`BVH`]
///
#[derive(Clone, Debug, PartialEq)]
struct Node {
bounds: AABB,
begin: usize,
end: usize,
kind: NodeEnum,
}
/// The BVH containing all the objects of type O.
/// This type must implement [`Bounded`].
///
/// [`Bounded`]: ../aabb/trait.Bounded.html
#[derive(Clone, Debug, PartialEq)]
pub struct BVH<'o, O: Bounded> {
objects: &'o [O],
tree: Node,
}
impl<'o, O: Bounded> BVH<'o, O> {
/// Build a [`BVH`] for the given slice of objects.
/// Each leaf node will be built in a way to try and contain less than 32 objects.
///
/// # Examples
/// ```
/// use beevee::{Point, Vector};
/// use beevee::aabb::{AABB, Bounded};
/// use beevee::bvh::BVH;
///
/// struct Sphere {
/// center: Point,
/// radius: f32,
/// }
///
/// impl Bounded for Sphere {
/// fn aabb(&self) -> AABB {
/// let delt = Vector::new(self.radius, self.radius, self.radius);
/// AABB::with_bounds(self.center - delt, self.center + delt)
/// }
/// fn centroid(&self) -> Point {
/// self.center
/// }
/// }
///
/// let spheres: &mut [Sphere] = &mut [Sphere{ center: Point::origin(), radius: 2.5 }];
/// let bvh = BVH::build(spheres);
/// ```
pub fn build(objects: &'o mut [O]) -> Self {
Self::with_max_capacity(objects, 32)
}
/// Build a [`BVH`] for the given slice of objects, with an indicated max capacity per
/// leaf-node. The max capacity is not respected when the SAH heuristic indicate that it would
/// be better to iterate over all objects instead of splitting.
///
/// # Examples
/// ```
/// use beevee::{Point, Vector};
/// use beevee::aabb::{AABB, Bounded};
/// use beevee::bvh::BVH;
///
/// struct Sphere {
/// center: Point,
/// radius: f32,
/// }
///
/// impl Bounded for Sphere {
/// fn aabb(&self) -> AABB {
/// let delt = Vector::new(self.radius, self.radius, self.radius);
/// AABB::with_bounds(self.center - delt, self.center + delt)
/// }
/// fn centroid(&self) -> Point {
/// self.center
/// }
/// }
///
/// let spheres: &mut [Sphere] = &mut [Sphere{ center: Point::origin(), radius: 2.5 }];
/// let bvh = BVH::with_max_capacity(spheres, 32);
/// ```
pub fn with_max_capacity(objects: &'o mut [O], max_cap: usize) -> Self {
let tree = build_node(objects, 0, objects.len(), max_cap);
Self { objects, tree }
}
/// Return the true if the [`BVH`] has been built soundly:
/// * Each child node is contained inside the parent's bounding box.
/// * Each object in a leaf node is inside the node's bounding box.
/// * There is no missing object indices.
///
/// # Examples
/// ```
/// # use beevee::{Point, Vector};
/// # use beevee::aabb::{AABB, Bounded};
/// # use beevee::bvh::BVH;
/// #
/// # struct Sphere {
/// # center: Point,
/// # radius: f32,
/// # }
/// #
/// # impl Bounded for Sphere {
/// # fn aabb(&self) -> AABB {
/// # let delt = Vector::new(self.radius, self.radius, self.radius);
/// # AABB::with_bounds(self.center - delt, self.center + delt)
/// # }
/// # fn centroid(&self) -> Point {
/// # self.center
/// # }
/// # }
/// #
/// // Using the same sphere definition than build
/// let spheres: &mut [Sphere] = &mut [Sphere{ center: Point::origin(), radius: 2.5 }];
/// let bvh = BVH::with_max_capacity(spheres, 32);
/// assert!(bvh.is_sound());
/// ```
pub fn is_sound(&self) -> bool {
fn check_node<O: Bounded>(objects: &[O], node: &Node) -> bool {
if node.begin > node.end {
return false;
}
match node.kind {
NodeEnum::Leaf => objects[node.begin..node.end]
.iter()
.all(|o| node.bounds.union(&o.aabb()) == node.bounds),
NodeEnum::Internal {
ref left,
ref right,
} => {
check_node(objects, left.as_ref())
&& check_node(objects, right.as_ref())
&& node.bounds.union(&left.bounds) == node.bounds
&& node.bounds.union(&right.bounds) == node.bounds
}
}
};
check_node(self.objects, &self.tree)
}
}
fn bounds_from_slice<O: Bounded>(objects: &[O]) -> AABB {
objects
.iter()
.map(|o| o.aabb())
.fold(AABB::empty(), |acc, other| acc.union(&other))
}
fn build_node<O: Bounded>(objects: &mut [O], begin: usize, end: usize, max_cap: usize) -> Node {
let aabb = bounds_from_slice(objects);
// Don't split nodes under capacity
if objects.len() <= max_cap {
return Node {
bounds: aabb,
begin,
end,
kind: NodeEnum::Leaf,
};
}
// Calculate the SAH heuristic for this slice
let (split, axis, cost) = compute_sah(&mut objects[begin..end], aabb.surface(), max_cap);
// Only split if the heuristic shows that it is worth it
if cost >= objects.len() as f32 {
return Node {
bounds: aabb,
begin,
end,
kind: NodeEnum::Leaf,
};
}
// Avoid degenerate cases, and recenter the split inside [begin, end)
let split = if split == 0 || split >= (end - begin - 1) {
begin + (end - begin) / 2
} else {
begin + split
};
// Project along chosen axis
pdqselect::select_by(objects, split, |lhs, rhs| {
lhs.centroid()[axis]
.partial_cmp(&rhs.centroid()[axis])
.expect("Can't use Nans in the SAH computation")
});
// Construct children recurivsely on [begin, split) and [split, end)
let left = Box::new(build_node(objects, begin, split, max_cap));
let right = Box::new(build_node(objects, split, end, max_cap));
// Build the node recursivelly
Node {
bounds: aabb,
begin,
end,
kind: NodeEnum::Internal { left, right },
}
}
/// Returns the index at which to split for SAH, the Axis along which to split, and the calculated
/// cost.
fn compute_sah<O: Bounded>(objects: &mut [O], surface: f32, max_cap: usize) -> (usize, Axis, f32) {
// FIXME(Bruno): too imperative to my taste...
let mut mid = objects.len() / 2;
let mut dim = Axis::X; // Arbitrary split
let mut min = std::f32::INFINITY;
// Pre-allocate the vectors
let mut left_surfaces = Vec::<f32>::with_capacity(objects.len() - 1);
let mut right_surfaces = Vec::<f32>::with_capacity(objects.len() - 1);
// For each axis compute the cost
for &axis in [Axis::X, Axis::Y, Axis::Z].iter() {
left_surfaces.clear();
right_surfaces.clear();
// Sort in order along the axis
// FIXME: use parallel sort
objects.sort_by(|lhs, rhs| {
lhs.centroid()[axis]
.partial_cmp(&rhs.centroid()[axis])
.expect("Can't use NaNs in the SAH computation")
});
// Compute the surface for each possible split
{
let mut left_box = AABB::empty();
let mut right_box = AABB::empty();
for i in 0..(objects.len() - 1) {
left_box.union_mut(&objects[i].aabb());
left_surfaces.push(left_box.surface());
right_box.union_mut(&objects[objects.len() - 1 - i].aabb());
right_surfaces.push(right_box.surface());
}
}
// Calculate the cost
for left_count in 1..objects.len() {
let right_count = objects.len() - left_count;
let cost = 1. / max_cap as f32
+ (left_count as f32 * left_surfaces[left_count - 1]
+ right_count as f32 * right_surfaces[right_count])
/ surface;
if cost < min {
min = cost;
dim = axis;
mid = left_count
}
}
}
(mid, dim, min)
}