Shooting projectiles in a 2d top down shooter with multiplayer

Questions & Answers
1

Hi everyone!

So, I am currently making a top-down shooter with multiplayer using Mirror. Here is my projectile script:

using UnityEngine;
using Mirror;

public class ShootProjectile : NetworkBehaviour
{
    public GameObject projectile;
    public float projectileSpeed;

    void Update()
    {
        if (isLocalPlayer && Input.GetMouseButtonDown(0))
        {
            Vector3 mouseDir = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position;
            mouseDir.z = 0;
            mouseDir = mouseDir.normalized;

            CmdShoot(mouseDir);
        }
    }

    [Command]
    void CmdShoot(Vector3 mouseDir)
    {
        GameObject shot = Instantiate(projectile, transform.position, Quaternion.identity);
        shot.transform.position += mouseDir;
        shot.GetComponent<Rigidbody2D>().velocity = mouseDir * projectileSpeed;
        NetworkServer.Spawn(shot);
    }
}

I have a prefab Projectile which contains a Rigidbody2D, Circle Collider 2D, and a NetworkTransform. This script is on my player object. The problem is, if I move the player right after I click the mouse button and shoot, the server spawns the projectile on the player’s old position, so I get the behaviour shown below (on Client):
172779-shoot-lag.gif
However, this barely happens when I shoot on a Server+Client and doesn’t happen when I don’t move while shooting, making me think that this has to do with the server not creating the projectile fast enough and not realizing the player has moved in the time it took for it to create it. Here is an example on a Server+Client instance:
172780-no-shoot-lag.gif
How do I remove this lag completely/tell the server to spawn the projectile in the new position?

Thanks in advance for your help!

NOTE: I ran the Server+Client and Client on the same machine in both tests, meaning the problem probably wasn’t because of my internet.

2

At a glance it feels like the kind of problem where you need some dead-reckoning (Googleable term!).

Basically, the network will always lag a bit. The client needs to update per-frame, and guesstimate where things should be inbetween packets. It’s normal for any multiplayer game to not be consistent between clients - if you look at two screenshots of two players at any given time, they won’t see exactly the same thing.

What matters, as you’ve picked up on, is the client experience. What you want to do is spawn the shot ‘correctly’ (visually) on the client, and use the server data to inform and update it’s trajectory when it spawns, rather than directly set its position (which should always be at the intended origin). This will mean the shot appears to happen later on the client than it actually did, but has a higher velocity to compensate - but will look ‘correct’ to a player.

I realise this isn’t as useful as a code implementation, but hope it’s some limited help!