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fix: BvhNode::scale now handles negative scale components correctly #406

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fix: BvhNode::scale now handles negative scale components correctly #406

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20 changes: 12 additions & 8 deletions src/partitioning/bvh/bvh_tree.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -811,8 +811,12 @@ impl BvhNode {
/// - [`Bvh::scale`] - Scale an entire BVH tree
#[inline]
pub fn scale(&mut self, scale: Vector) {
self.mins *= scale;
self.maxs *= scale;
let new_mins = self.mins * scale;
let new_maxs = self.maxs * scale;
// When scale has negative components, mins/maxs swap on those axes.
// Use component-wise min/max to maintain the AABB invariant (mins <= maxs).
self.mins = new_mins.min(new_maxs);
self.maxs = new_mins.max(new_maxs);
}

/// Calculates the volume of this node's AABB.
Expand Down Expand Up @@ -1995,18 +1999,18 @@ impl Bvh {

/// Scales all AABBs in the tree by the given factors.
///
/// This multiplies all AABB coordinates (mins and maxs) by the corresponding components
/// of the scale vector. This is useful when scaling an entire scene or changing coordinate
/// systems.
/// Each AABB's coordinates are multiplied by the corresponding scale components, with
/// mins and maxs swapped on any axis where the scale is negative to preserve the
/// AABB invariant. This is useful when scaling an entire scene or changing coordinate
/// systems, including reflections.
///
/// # Arguments
///
/// * `scale` - Per-axis scale factors (must all be positive)
/// * `scale` - Per-axis scale factors. Each component must be non-zero.
///
/// # Panics
///
/// This function has undefined behavior if any scale component is negative or zero.
/// Always use positive scale values.
/// Undefined behavior if any scale component is zero (degenerate AABB).
///
/// # Example
///
Expand Down