Procedural Hand Placement on Irregular Rock Faces

Calculating procedural hand placement offsets for climbing irregularly angled rock faces relies on a pipeline of geometric tracing, normal vector analysis, and Inverse Kinematics (IK). Instead of relying on static animations, game engines dynamically detect contact points on uneven geometry, calculate the surface orientation, and blend the character's limbs to match the terrain. This technique ensures that hands wrap naturally around jagged ledges, slopes, and crevices without clipping through meshes or floating in mid-air.

Environment Probing via Raycasting and Shape Sweeps

The calculation begins by casting traces from the character toward the rock surface. Developers typically use a combination of forward raycasts and downward sphere sweeps originating from virtual reference points near the character’s shoulders.

  1. Forward Traces: Cast directly toward the rock face to determine distance and detect vertical or overhang surfaces.
  2. Downward Traces: Cast from above the target area down onto the ledge to pinpoint the exact upper lip or flat edge where the fingers should rest.
  3. Sphere Sweeps: Used instead of thin line rays to prevent missing thin ledges and to account for the physical volume of the character's hand.

When a sweep hits the geometry, the engine returns a hit result containing the exact world-space impact point and the hit normal (the vector perpendicular to the surface at the hit point).

Calculating Surface Alignment and Rotation

Once the impact point is determined, the hand’s rotation must align with the rock's slope. Simply placing the hand at the hit location will cause it to clip through angled rock unless its orientation matches the local geometry.

Developers compute the target rotation using the hit normal:

  • The Normal Vector: Defines the "up" or "outward" direction for the palm (e.g., the palm faces opposite to the normal vector).
  • The Tangent Vector: Cross-products between the surface normal and the character’s world-up axis determine the lateral alignment of the hand along the ledge.
  • Rotational Clamping: To prevent unnatural arm twisting, algorithms constrain the rotation using dot-product thresholds. If a rock angle exceeds human wrist flexibility, the target rotation is clamped to an anatomically plausible limit.

Determining Depth and Pivot Offsets

A raw raycast hit lands on the outer collision hull of the mesh. Placing the hand’s wrist joint directly at this hit point causes the hand to float or clip. Developers apply static and procedural offsets to compensate:

  • Palm Offset: A fixed vector based on the hand mesh dimensions that pushes the wrist back along the hand's local normal, ensuring the palm surface touches the rock.
  • Finger Margin: A forward offset along the ledge to ensure fingers curl over the edge rather than penetrating through it.
  • Angle-Dependent Adjustment: On steep overhangs, the offset is shifted downward along the rock face to simulate the character hanging by their fingertips rather than supporting weight on a flat palm.

Inverse Kinematics (IK) Solving

With the target location and rotation established, the final transform is passed to an IK solver—most commonly a Two-Bone IK solver or a Fabrik solver governing the shoulder, elbow, and wrist.

  1. Target IK Goal: The calculated position and rotation are assigned to the wrist (hand) bone.
  2. Pole Vectors: A pole vector target is placed outside the elbow to control the bending direction, preventing elbows from flipping inward toward the torso or hyper-extending.
  3. Finger Deformation: High-fidelity systems cast individual micro-rays from each finger knuckle. If a finger detects stone beneath it, a secondary finger IK chain curls or straightens that specific digit to match the rock's micro-bumps.

Temporal Smoothing and Interpolation

Because raw terrain hit locations can shift erratically between frames on jagged geometry, developers interpolate the final transforms. Functions such as Vector Interp To and Slerp (Spherical Linear Interpolation) smooth the transition of the hand from one grip to the next. This prevents jitter, ensures natural weight transfer, and keeps hand movement fluid as the player traverses complex rock surfaces.