CNA-BUG-071: BoundingFrustum::Intersects(Plane) counts a corner lying on the plane as Intersecting, where XNA 4.0 answers Back

CNA snapshot 3b43bd86  ·  Known Issues › Current bugs  ·  source links pinned to 3b43bd86

The Plane overload treats a corner at zero distance as Intersecting, so a frustum touching a plane from behind is Intersecting where XNA (distance > 0 is Front, everything else Back) answers Back. The box and sphere overloads now match XNA.

Identifier

CNA-BUG-071

Category

Bug

Subsystem

Math & geometry

Status

Narrowed (partially fixed; describes only what survives)

Verified against

CNA 3b43bd86 (3b43bd8687ed46523d873f9bec4b32bd8481dcb5)

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(Plane) and its out-parameter overload

Expected behaviour

XNA 4.0 (IL): BoundingFrustum.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 Plane overload with Plane::IntersectsPoint for every corner, which returns Intersecting for a zero distance (Plane.cpp), and reports Intersecting as soon as two corners differ. A frustum with some corners behind a plane and the others exactly on it is therefore Intersecting in CNA and Back in XNA.

Source locations

• modules/math/src/BoundingFrustum.cpp — BoundingFrustum::Intersects(const Plane&, PlaneIntersectionType&); the box and sphere overloads (the fixed half) call ConvexIntersects
• modules/math/src/Plane.cpp — Plane::IntersectsPoint returns Intersecting for a zero distance
• modules/math/tests/Microsoft/Xna/Framework/FrustumSeparationOracleTests.cpp — the convex-separation oracle tests for the fixed half

Evidence

Checked by reading the named sources at 3b43bd86; nothing was built or executed for this entry. XNA behaviour from the genuine XNA 4.0 IL (the Plane overload's comparison).

Narrowed at c6dec91f: 5ab1c7729 replaced the conservative plane test of the box and sphere overloads with a convex-separation query: Intersects(const BoundingBox&, bool&) and Intersects(const BoundingSphere&, bool&) now call ConvexIntersects over the corners with a support function, pinned by FrustumSeparationOracleTest against CNA's recorded oracle of genuine XNA shape pairs (FrustumSeparationOracleTests.cpp) and by GeometryContainmentCompatibilityTest.FrustumIntersectionRequiresActualConvexSeparation. The Plane overload is unchanged.

Focused reproduction

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

Current tests

BoundingFrustumTests.cpp covers frustum-plane cases with clear separation, not a corner exactly on the plane.

Regression test

A frustum with one corner exactly on a plane and the others behind it, expecting Back.

Blast radius

Code that classifies a frustum against a plane and treats Intersecting and Back differently when the frustum only touches the plane.

Workaround

No workaround is known.