HairUpdateVelocity.h

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// Jolt Physics Library (https://github.com/jrouwe/JoltPhysics)
// SPDX-FileCopyrightText: 2026 Jorrit Rouwe
// SPDX-License-Identifier: MIT
 
#include "HairApplyGlobalPose.h"
 
void ApplyCollisionAndUpdateVelocity(uint inVtx, JPH_IN_OUT(JPH_HairPosition) ioPos, JPH_IN(JPH_HairPosition) inPreviousPos, JPH_IN(JPH_HairMaterial) inMaterial, float inStrandFraction, JPH_OUT(JPH_HairVelocity) outVel)
{
    // Update velocities
    outVel.mVelocity = (ioPos.mPosition - inPreviousPos.mPosition) / cDeltaTime;
    outVel.mAngularVelocity = cTwoDivDeltaTime * JPH_QuatMulQuat(ioPos.mRotation, JPH_QuatConjugate(inPreviousPos.mRotation)).xyz;
 
    if (inMaterial.mEnableCollision)
    {
        // Calculate closest point on the collision plane
        JPH_HairCollisionPlane plane = gCollisionPlanes[inVtx];
        float distance_to_plane = JPH_PlaneSignedDistance(plane.mPlane, ioPos.mPosition);
        float3 contact_normal = JPH_PlaneGetNormal(plane.mPlane);
        float3 point_on_plane = ioPos.mPosition - distance_to_plane * contact_normal;
 
        // Calculate how much the plane moved in this time step
        JPH_HairCollisionShape shape = gCollisionShapes[plane.mShapeIndex];
        float3 plane_velocity = shape.mLinearVelocity + cross(shape.mAngularVelocity, point_on_plane - shape.mCenterOfMass);
        float plane_movement = dot(plane_velocity, contact_normal) * cAccumulatedDeltaTime;
 
        float projected_distance = -distance_to_plane + plane_movement + GradientSamplerSample(inMaterial.mHairRadius, inStrandFraction);
        if (projected_distance > 0.0f)
        {
            // Resolve penetration
            ioPos.mPosition += contact_normal * projected_distance;
 
            // Only update velocity when moving towards each other
            float3 v_relative = outVel.mVelocity - plane_velocity;
            float v_relative_dot_normal = dot(contact_normal, v_relative);
            if (v_relative_dot_normal < 0.0f)
            {
                // Calculate normal and tangential velocity (equation 30)
                float3 v_normal = contact_normal * v_relative_dot_normal;
                float3 v_tangential = v_relative - v_normal;
                float v_tangential_length = length(v_tangential);
 
                // Apply friction as described in Detailed Rigid Body Simulation with Extended Position Based Dynamics - Matthias Muller et al. (modified equation 31)
                if (v_tangential_length > 0.0f)
                    outVel.mVelocity -= v_tangential * min(inMaterial.mFriction * projected_distance / (v_tangential_length * cDeltaTime), 1.0f);
 
                // Apply restitution of zero (equation 35)
                outVel.mVelocity -= v_normal;
            }
        }
    }
}
 
void LimitVelocity(JPH_IN_OUT(JPH_HairVelocity) ioVel, JPH_IN(JPH_HairMaterial) inMaterial)
{
    // Limit linear velocity
    float linear_velocity_sq = dot(ioVel.mVelocity, ioVel.mVelocity);
    if (linear_velocity_sq > inMaterial.mMaxLinearVelocitySq)
        ioVel.mVelocity *= sqrt(inMaterial.mMaxLinearVelocitySq / linear_velocity_sq);
 
    // Limit angular velocity
    float angular_velocity_sq = dot(ioVel.mAngularVelocity, ioVel.mAngularVelocity);
    if (angular_velocity_sq > inMaterial.mMaxAngularVelocitySq)
        ioVel.mAngularVelocity *= sqrt(inMaterial.mMaxAngularVelocitySq / angular_velocity_sq);
}