Stabilize PACS convex-corner sliding for static and border contacts

[Copilot]

PACS movement could stutter when sliding along convex border corners, while concave corners remained smooth. This was caused by unstable normal-axis corrections during corner-adjacent contacts and by overly aggressive static collision time backoff on tangential outer-edge glancing.

• Collision-response stability update (PACS)

  • In CPrimitiveWorldImage::reaction(...), clamp remaining collision time used in response math:
    • safeRemainingTime = max(dt - contactTime, 1e-6)
  • This avoids pathological impulse magnitudes near end-of-step contacts.

• Slide behavior correction at corner contacts

  • For Slide reactions, apply distance-back normal push only when velocity is directed into the contact normal (speedProj < 0).
  • If already moving tangentially / away from the surface normal, do not add extra normal push-back.
  • This reduces oscillation between adjacent convex-edge contacts.

• Static collision scheduling correction (placed landscape objects / outer edges)

  • In CMoveContainer::newCollision(...) (static surface path), collision-time backoff is no longer based on total speed magnitude for all contacts.
  • Backoff is now applied only when movement is inward along the contact normal, using:
    • inwardVelocity = -(ContactNormal · Speed)
    • thresholded by NELPACS_COLLISION_TIME_EPSILON
  • This prevents unnecessary time rewinds on tangential/glancing outer-edge contacts that can re-trigger near-immediate re-collisions and visible stutter.

• Scope

  • Localized changes in:
    • nel/src/pacs/primitive_world_image.cpp
    • nel/src/pacs/move_container.cpp
  • No API/protocol/data-format changes.

const double collisionTimeEpsilon = 1e-6;
const double safeRemainingTime = std::max(dt - surfaceDesc.ContactTime, collisionTimeEpsilon);

double speedProj = surfaceDesc.ContactNormal * _Speed;
double distBackVelocity = speedProj < 0.0 ? NELPACS_DIST_BACK / safeRemainingTime : 0.0;
_Speed -= surfaceDesc.ContactNormal * (speedProj - distBackVelocity);

#define NELPACS_COLLISION_TIME_EPSILON 1e-6

const double inwardVelocity = -(desc.ContactNormal * wI->getSpeed());
if (inwardVelocity > NELPACS_COLLISION_TIME_EPSILON)
    time -= NELPACS_DIST_BACK / inwardVelocity;

Original prompt

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This section details on the original issue you should resolve

<issue_title>PACS gets stuck when sliding around convex corners</issue_title>
<issue_description>When running against a PACS border, the character stutters around convex corners. Concave corners are smooth.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes PACS gets stuck when sliding around convex corners #633

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[kaetemi]

@copilot Can you give a primer/walkthrough on why the original implementation had this behavior, and how your adjustment actually fixes this?

[Copilot]

@copilot Can you give a primer/walkthrough on why the original implementation had this behavior, and how your adjustment actually fixes this?

In the original code, slide always injected a normal “push-back” term (NELPACS_DIST_BACK/(dt-contactTime)) even when the primitive was already moving tangentially or slightly away from that normal. At convex corners, you can hit adjacent borders in rapid succession with changing normals, so that unconditional normal injection keeps reintroducing alternating normal velocity components, which looks like stutter/jitter.

In c13b09c, the slide branch now computes the normal projection first and only applies that push-back when speedProj < 0 (actually moving into the surface normal). If motion is tangential/away, no extra normal velocity is injected. That removes the corner-to-corner normal oscillation while preserving penetration prevention when moving into a wall.

The same commit also clamps remaining time with an epsilon (safeRemainingTime) so near-end-of-step contacts don’t create extreme impulses from division by a tiny value.

[kaetemi]

@copilot I tried this and it does not fix the issue. For clarity, this occurs when running/sliding against objects (buildings, etc) that are placed in the landscape. Particularly, the outer edge.

[Copilot]

@copilot I tried this and it does not fix the issue. For clarity, this occurs when running/sliding against objects (buildings, etc) that are placed in the landscape. Particularly, the outer edge.

I’ve added a follow-up specifically on the static/landscape collision path used by placed objects in 938f61d.

The static collision scheduling in CMoveContainer::newCollision(...) used to always rewind collision time by NELPACS_DIST_BACK / |speed|, which also affects tangential/glancing contacts at outer convex edges. That can re-queue near-immediate re-collisions and produce the stutter you described.

It now only applies that time backoff when movement is actually inward along the contact normal (with a small epsilon), so tangential edge sliding no longer gets unnecessary time rewinds.