Approach 1: VDB + Gradient

// Adjust the VDB points based on gradient
float sign = volumesample(1, 0, v@P);  // Sample the volume for a signed distance
vector grad = volumesamplev(2, 0, v@P); // Sample the gradient at the point

// If outside the volume (sign > 0), move the point away along the gradient
if (sign > 0) v@P -= sign * normalize(grad) * 1.1;  

Explanation:

1. Sample Volume: volumesample(1, 0, v@P) takes a volume (channel 1), mipmap level 0, and the point’s position (v@P) to determine if the point is outside the volume (sign > 0).
2. Sample Gradient: volumesamplev(2, 0, v@P) samples the gradient vector (channel 2) at the point’s position. This gradient indicates the direction of the closest surface.
3. Adjust Position: If the point is outside the volume, we move it to the surface along the normalized gradient direction (normalize(grad))

Approach 2: VEX fixed box

float bounds = chf("bound_size");

if(abs(@P.x) > bounds / 2.0){
    v@vel.x *= -1;
}

if(abs(@P.y) > bounds / 2.0){
    v@vel.y *= -1;
}

if(abs(@P.z) > bounds / 2.0){
    v@vel.z *= -1;
}