CNA-BUG-071: BoundingFrustum::Intersects differs from XNA 4.0: box and sphere overloads use a conservative plane test, and the Plane overload counts an on-plane corner as Intersecting

CNA snapshot 009d40f5  ·  Known Issues › Current bugs  ·  source links pinned to 009d40f5

The box and sphere overloads report true for volumes just outside an edge or corner where XNA's exact GJK query reports false, and the Plane overload treats a zero corner distance as Intersecting (XNA: Back).

Identifier

CNA-BUG-071

Category

Bug

Subsystem

Math & geometry

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Severity

Low (a triage suggestion, not a project priority)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

Yes: see Current tests

Affected contract

Microsoft::Xna::Framework::BoundingFrustum::Intersects(BoundingBox), Intersects(BoundingSphere), Intersects(Plane); BoundingBox/BoundingSphere::Intersects(BoundingFrustum)

Expected behaviour

XNA 4.0 (IL): BoundingFrustum.Intersects(ref BoundingBox) and Intersects(ref BoundingSphere) run the Gjk helper, an exact convex-intersection query. Intersects(ref Plane) classifies each corner with dot + D > 0 as Front and everything else (including exactly 0) as Back.

Actual behaviour at TARGET

BoundingFrustum.cpp implements the box and sphere overloads as Contains(...) != Disjoint; Contains is a per-plane test that only returns Disjoint when the volume is wholly in front of one plane, so a box diagonally outside a frustum corner (in front of no single plane) is reported as intersecting. BoundingBox::Intersects(BoundingFrustum), BoundingSphere::Intersects(BoundingFrustum) and graphics-ext's FrustumCullerEXT forward to these. The Plane overload uses Plane::IntersectsPoint, which returns Intersecting for a zero distance, so a frustum touching a plane from behind is Intersecting in CNA and Back in XNA.

Source locations

• modules/math/src/BoundingFrustum.cpp — BoundingFrustum::Intersects(const BoundingBox&, bool&), Intersects(const BoundingSphere&, bool&), Intersects(const Plane&, PlaneIntersectionType&), Contains(const BoundingBox&, ContainmentType&)
• modules/math/src/BoundingBox.cpp — BoundingBox::Intersects(BoundingFrustum) forwards to the frustum
• modules/graphics-ext/src/FrustumCullerEXT.cpp — culling helper forwards to BoundingFrustum::Intersects

Evidence

Checked by reading the named sources at 009d40f5; nothing was built or executed for this entry. XNA behaviour from the genuine XNA 4.0 IL (Gjk references; the Plane overload's comparison). The conservative answer is never a false negative for culling, so visible objects are not dropped.

Focused reproduction

No focused reproduction is known. Nothing has been invented here; the evidence above is what exists.

Current tests

BoundingFrustumTests.cpp covers clearly inside/outside volumes, not corner-adjacent ones or an on-plane corner.

Regression test

A test with a box just outside a frustum corner (outside, but in front of no single plane) expecting false, and a frustum with one corner exactly on a plane expecting Back.

Blast radius

Culling (extra draws, not missing ones), picking and gameplay tests that use frustum intersection as an exact predicate.

Workaround

No workaround is known.