Smooth grenade trajectory - code + explanation

Like many of you I had issues with the grenade trajectory, especially the very beginning where you see a trail shooting straight up and then following the arcAnimationCurve. Even my arcAnimation curve was choppy. Root cause was that it was hard to align the starting position of the grenade’s transform with the arc anim curve plus potentially an overconstrained calculation. Potentially aggravated by having a slower PC.

Related Post: Trajectory of grenade stopped working

Solution - Use the parametric form of the parabola + basic trajectory physics. You know two points of the parabola already (launch point and destination point). You need a third constraint (e.g. launch velocity) to get the actual equation. Use the force of gravity in the equation to get a realistic looking curve. The parametric form works beautifully here by since we are trying to replicate a physical effect and care especially about computing the Y separate from XZ.

Code below. If you have some familiarity with the above stuff, it should be easy to figure out what the code is doing. If anyone wants a detailed explanation of the math just reply here.

Hugo / Brian - let me know if I am allowed to post external links. There’s some great content on the web.

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UIElements;

public class GrenadeProjectile : MonoBehaviour
    public static event EventHandler OnAnyGrenadeExploded;

    [SerializeField] private Transform grenadeExplodeVfxPrefab;
    [SerializeField] private TrailRenderer trailRenderer;
    [SerializeField] private AnimationCurve arcYAnimationCurve;

    [SerializeField] bool useArcAnimCurve; //if you want to experiment with the two methods 

    private Vector3 targetPosition;
    private Action onGrenadeBehaviourComplete;
    private float totalDistance;
    private float grendadeLaunchHeight = 1.25f; // the height where the grenade is launched from
    private Vector3 positionXZ; //current XZ position of the projectile

    private float xzVelocity = 15f; // this is the same concept as moveSpeed in Hugo's code. We need is here for computing yVelocity in setup
    private float yVelocity; // computed in setup then used in Update()
    private const float g = 9.8f; // force of gravity in m/s^2
    private float launchStartTime; //captured in setup

    [SerializeField] private float explosiveForce;
    [SerializeField] private float upwardForce;

    private void Update()
        Vector3 moveDir = (targetPosition - positionXZ).normalized; // works because targetPosition also has 0 for y value
        float moveSpeed = xzVelocity;
        positionXZ += moveDir * (moveSpeed * Time.deltaTime);
        float positionY;

        if (useArcAnimCurve)
            float distance = Vector3.Distance(positionXZ, targetPosition);
            float distanceNormalized = 1 - distance / totalDistance; // how far along from start to destination

            float maxHeight = totalDistance / 4f;
            positionY = arcYAnimationCurve.Evaluate(distanceNormalized) * maxHeight; // gets the requisite Y value.
        } else
            float t = Time.fixedTime - launchStartTime;
            positionY = grendadeLaunchHeight + (yVelocity * t) - (g * t * t) / 2f;
        transform.position = new Vector3(positionXZ.x, positionY, positionXZ.z);

        Vector3 nextFramemoveDir = targetPosition - positionXZ;

        if (Vector3.Dot(moveDir, nextFramemoveDir) <0)
            float damageRadius = 4f; //note this is 4 units in WORLD position not grid space
            Collider[] colliderArray = Physics.OverlapSphere(targetPosition, damageRadius);

            foreach (Collider collider in colliderArray)
                if (collider.TryGetComponent<Unit>(out Unit targetUnit))
// see this post for explanation of what's happening here.  You can also use Hugo's code.
                    DamageContext damageContext = new DamageContext();
                    damageContext.SetUpAsExplosion(explosiveForce, targetPosition + Vector3.up * 1f, damageRadius, upwardForce);
                    targetUnit.Damage(30, damageContext);
                if (collider.TryGetComponent<DestructibleCrate>(out DestructibleCrate destructibleCrate))
            OnAnyGrenadeExploded?.Invoke(this, EventArgs.Empty);

            trailRenderer.transform.parent = null; 
            Instantiate(grenadeExplodeVfxPrefab, targetPosition + Vector3.up * 1f, Quaternion.identity);


    public void Setup(GridPosition targetGridPosition, Action onGrenadeBehaviourComplete)
        this.onGrenadeBehaviourComplete = onGrenadeBehaviourComplete;
        targetPosition = LevelGrid.Instance.GetWorldPosition(targetGridPosition);

        positionXZ = transform.position;
        positionXZ.y = 0;
        transform.position = new Vector3(positionXZ.x, grendadeLaunchHeight, positionXZ.z); 
        totalDistance = Vector3.Distance(positionXZ, targetPosition);
// yVelocity is a precise calculation from parametric form of the parabola.
// Ok to change field level constants above, but don't change this formula.
        yVelocity = g * totalDistance / 2f / xzVelocity - grendadeLaunchHeight * xzVelocity / totalDistance;
        launchStartTime = Time.time;

Phenomenal job for figuring it out! Thank you for sharing the code and you walkthrough of the solution. This will help out many more who will struggle with it :pray:t3:

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