Collider.ClosestPointOnBounds

Description

The closest point to the bounding box of the attached collider.

This can be used to calculate hit points when applying explosion damage.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public float hitPoints = 100.0F;
    public Collider coll;
    void Start()
    {
        coll = GetComponent<Collider>();
    }

    void ApplyHitPoints(Vector3 explosionPos, float radius)
    {
        // The distance from the explosion position to the surface of the collider.
        Vector3 closestPoint = coll.ClosestPointOnBounds(explosionPos);
        float distance = Vector3.Distance(closestPoint, explosionPos);

        // The damage should decrease with distance from the explosion.
        float damage = 1.0F - Mathf.Clamp01(distance / radius);
        hitPoints -= damage * 10.0F;
    }
}