Replaces the box-box collider's manifold generation and post-filtering with a
single structured implementation, and deletes the accumulated repair logic it
obsoletes. Net 319 lines out of the engine.
Algorithm:
- The separating-axis test keeps the closed-form support evaluation and chooses
the axis of maximum separation among the 15 candidates by plain argmax.
Edge-cross axes whose cross product has norm below rounding are skipped: in
the nearly-parallel regime their direction is cancellation noise, previously
the source of arbitrary-normal contacts with box-scale spurious depth. A
winning edge axis within eight degrees of the best face axis is replaced by
that face unless it is better by five percent (ODE's classic fudge): resting
stacks otherwise flip between the edge and face contact codes by rounding
noise from step to step, thrashing the solver warm start until the stack
explodes. The substitution runs after the search rather than filtering during
it, so a worse non-aliasing edge cannot steal the contact the substitution
meant to give to the face.
- Face contacts clip the incident face against the reference face's side planes
(Sutherland-Hodgman). Depth is measured along the reference normal only,
never as a Euclidean distance between unrelated points. Contact position is
midway between the surfaces along the normal, so its distance to either box
is bounded by half the contact depth. Every surviving vertex of the clipped
polygon becomes a contact, so the manifold is the actual contact patch, at
most eight points as before.
- Edge contacts use the closest-point pair between the two supporting edge
segments. A near-zero axis component makes the support-corner sign ambiguous;
both signs are enumerated and the closest witness pair wins.
- Margin is an acceptance band throughout: SAT early-out and clip acceptance.
- The rounding thresholds are stated per precision. The separation tests are
the ones that cost correctness: comparing exactly against the margin reports
a pair overlapping by less than the rounding error of its own support
evaluation as separated, and the boxes pass through each other. Over 239k
overlapping pairs that is eight misses under mjUSESINGLE and none in double;
the collider this replaces misses the same eight. Slack proportional to the
summed half-sizes leaves five, which overlap by 7e-9 to 3e-8 of their own
scale, below single-precision epsilon, where the boxes are not distinguishable
from touching. Erring toward contact is the safe direction: the driver already
excludes a contact whose distance reaches the margin.
Deleted: the conditional acceptance cascade keyed on how many points earlier
generators emitted, the u/v clamping that fabricated contacts from out-of-range
projections, the outside-box removal filter and its missing-fallback hole,
exact-floating-point deduplication, and the edge-path depth clamp. The
structure makes those bug classes unrepresentable rather than filtered: depth
is a projection by construction. Every reported depth is the exact support
overlap along the contact's own normal, verified over 246k overlapping poses to
within two ulps; the face preference costs direction, not depth, deviating from
the minimum-translation axis by at most 8.1 degrees and 5.3% of its depth.
The previous implementation is preserved verbatim as mjc_BoxBoxLegacy in
test/engine/boxbox_legacy.c, a static library that only the box-box tests link,
so the claims above are measured rather than asserted. It needs no private
engine symbols. Three tests compare against it:
- NearAlignedManifoldIsExact sweeps the relative angle of a resting pair across
the regime where the edge-cross axes degenerate into noise, pinning the full
clipped polygon and a contact normal equal to the face normal exactly, where
the previous collider drifts off it.
- AlignedTowerStands settles a twenty-box tower, which comes to rest four
million times quieter than under the previous collider, which never settles
and eventually topples.
- ShallowOverlapSurvivesRounding pins a pair overlapping by 7e-8 of its scale,
reported as separated under mjUSESINGLE without slack on the separation tests.
On stacks of plates across aspect ratios from 4:1 to 25:1, five layouts each,
the collider settles into a tight band of 1e-4 to 3e-4 while the previous one
intermittently blows up to as much as 2.6e-2.
engine_collision_box_fuzz_test.cc cross-validates randomized poses against
GJK/EPA on identical box meshes and against a spherical-Fibonacci support
sweep, with hard gates per sample: no phantom penetration, no missed contact at
zero margin, no contact deeper than the true depth, contacts within half their
own depth of both boxes, and one normal per manifold. Both invocations run in
about a second.
EdgeContactAtDepthBound's tolerance widens to the five percent design band; the
three-orders-of-magnitude depth bug it pins is still caught, the deviation
being 0.13 percent of the depth.
The 100-box pile benchmark steps about 7% faster with 1.6% fewer contacts.
PiperOrigin-RevId: 965114952
Change-Id: Ie98cdcce8d1aed3ff2da938cb29703fd9c241258
When penetration exceeds a box's smallest half-size, the midpoint
between the contact surfaces can land outside both boxes. The outside-
box filter in mjc_BoxBox then removed every contact of the manifold,
returning nothing for a visibly overlapping pair, letting boxes fall
through each other. Fixes#1800.
If the filter would remove all contacts, restore the penetrating ones.
This is strictly additive: configurations where any contact survives
the filter are unchanged.
The removed midpoints are bitwise-identical to the witness midpoints
computed by mj_geomDistance for these configurations, so the positions
follow the engine-wide contact position convention; re-anchoring them
onto a box surface would not.
Fixes#1800
PiperOrigin-RevId: 957867315
Change-Id: Ia9c858661d4badeb2a832d25455e33402936011d
In the edge-edge path of the box-box collider, when line clipping yields
no points, the corner generators accept points whose projection
parameters are out of range and clamp them into the valid range. The
depth of such a point is the Euclidean distance between two unrelated
points, mixing lateral offset into penetration depth, and on the
penetrating side it is admitted with no margin check. For thin boxes
meeting edge-to-face within margin, this produced a contact with
penetration three orders of magnitude larger than the boxes' true
separation, exploding resting stacks.
No contact can penetrate deeper than the support-interval overlap along
the separating axis, which the SAT stage has already computed. Enforce
this bound on all points emitted by the edge-edge path. The bound
carries margin plus relative and size-scaled slack covering rounding
error: the depth of the deepest legitimate point is algebraically equal
to the bound, so an exact comparison would drop real contacts. The
slack is precision-dependent: in mjUSESINGLE builds the two
computations of the same overlap disagree by tens of ulps, and slack
calibrated for double precision rejects real single-precision contacts.
Differential fuzzing against the nativeccd oracle over 200k random
near-contact thin-box configurations, in both precisions: impossibly
deep contacts drop from 1018 to 4 (worst excess from 2.5x the bounding
diameter to 0.001x), with no legitimate shallow-penetration contact
lost.
PiperOrigin-RevId: 957628830
Change-Id: Iaa1f10742f91c55bf831296cba0936eb50ea09b0
That change removed some spurious contacts returned by mjc_BoxBox, but also removed some desirable contacts that occur during very deep penetration of two boxes (when one box is completely inside another box). This is now fixed.
PiperOrigin-RevId: 591036252
Change-Id: I84b51f2179fd6fe29618a4e908e86934c6fc6941
The box-box collider sometimes outputs bad contact points which are far outside one of the two boxes. This happens at configurations which are very rare numerically, but rare configurations (e.g. perfect box alignment) are often induced by the designer and the physics, making them actually not-so-rare.
Since the box collision functions are very difficult to understand, we circumvent the issue by extending the post processing code to detect and remove such contacts.
PiperOrigin-RevId: 589178197
Change-Id: I82b8a888ef04f9196111c960757e3314d0ab6d8e
Yuval Tassa