Is there any way to render a ParticleSystem -immediately- ? [Solved]

I'm building a shooter, and some projectiles start with particle systems attached to them (set to emit over distance). This happens when the projectile has special effects like Fire or Poison. The projectile is created during Unity's Update step.

When the player fires a projectile, it is required that the projectile move one frame immediately, so that it is visible emerging from the weapon. If I do not move the projectile forward by one frame, then projectiles appear to lag one frame behind when the player fired them. I do this by using SendMessage on all objects in the projectile - Update, LateUpdate, RenderNow(0) - this renders everything that isn't a ParticleSystem, but particles aren't visible for the first frame of movement.

Is there some way to force a particle system to render or otherwise "trick" particles into appearing in the indicated space immediately? I can't have them be delayed by one frame, it has to be rendered immediately. If there's no way to do this with particles, is there some kind of replacement effect I could use that will render immediately?

Here's a screenshot of the frame immediately following me firing a very fast projectile (the issue happens on everything, but this is the one where it's most noticeable):

 

Thanks for the prompt reply - that looks promising, but I wasn't able to get it to behave. It looks like the system is just simulating two frames at the end point (using the Rate Over Time emission and not the Rate Over Distance emission), rather than running from the spawn point to the end point.



This code occurs during Unity's Update, immediately after the projectile has spawned and attached particles to itself.

Code (CSharp):

1.         // P = the projectile
2.         // Order = list of all transforms in the projectile & its descendants
3.  
4.         var systems = p.GetComponentsInChildren<ParticleSystem>();
5.         Debug.Log("Simulating " + systems.Length + " systems @ " + p.transform.position);
6.         foreach (var ps in systems) {
7.             ps.Simulate(Time.deltaTime);
8.             ps.Play(); // tried with and without this line
9.         }
10.  
11.         // FIXME: Start may not have run by this point, but calling it twice is bad
12.         // foreach (var el in order) if (el) el.SendMessage("Start", SendMessageOptions.DontRequireReceiver);
13.         // Provide a custom step for objects that use Soco's update (e.g. constantmotion) or which need to update before other projectile components
14.         foreach (var el in order) if (el) el.SendMessage("GH_InitialProjectileUpdate", SendMessageOptions.DontRequireReceiver);
15.         // Enforce a Unity update to drive e.g. SmoothTrails and motion components
16.         foreach (var el in order) if (el) el.SendMessage("Update", SendMessageOptions.DontRequireReceiver);
17.         // Enforce a late-update as well (for complex motion components like Helix)
18.         foreach (var el in order) if (el) el.SendMessage("LateUpdate", SendMessageOptions.DontRequireReceiver);
19.  
20.         Debug.Log("Second particle simulation @ " + p.transform.position);
21.         foreach (var ps in systems) {
22.             ps.Simulate(Time.deltaTime);
23.             ps.Play();
24.         }
25.  
26.         // Forcibly render the object for e.g. trails
27.         foreach (var el in order) if (el) el.SendMessage("RenderNow", 0, SendMessageOptions.DontRequireReceiver);
28.  
29.         UnityEditor.EditorApplication.isPaused = true;
30.  

I'll try manually emitting the particles. This one just looked easier so I was hopeful

 

Manual emission did the trick!



It needs some straightening up (for some reason it did 100 particles per unit instead of 10), but it's doing what I need.

Code (CSharp):

1.         // P = the projectile
2.         // Order = list of all transforms in the projectile & its descendants
3.  
4.         var systems = p.GetComponentsInChildren<ParticleSystem>();
5.         var from = p.transform.position;
6.  
7.         // FIXME: Start may not have run by this point, but calling it twice is bad
8.         // foreach (var el in order) if (el) el.SendMessage("Start", SendMessageOptions.DontRequireReceiver);
9.         // Provide a custom step for objects that use Soco's update (e.g. constantmotion) or which need to update before other projectile components
10.         foreach (var el in order) if (el) el.SendMessage("GH_InitialProjectileUpdate", SendMessageOptions.DontRequireReceiver);
11.         // Enforce a Unity update to drive e.g. SmoothTrails and motion components
12.         foreach (var el in order) if (el) el.SendMessage("Update", SendMessageOptions.DontRequireReceiver);
13.         // Enforce a late-update as well (for complex motion components like Helix)
14.         foreach (var el in order) if (el) el.SendMessage("LateUpdate", SendMessageOptions.DontRequireReceiver);
15.  
16.         Debug.Log("Particle emission from " + from + " to " + p.transform.position);
17.         foreach (var ps in systems) if (ps) SimParticles(from, p.transform.position, ps);
18.  
19.         // Forcibly render the object for e.g. trails
20.         foreach (var el in order) if (el) el.SendMessage("RenderNow", 0, SendMessageOptions.DontRequireReceiver);
21.  
22.         UnityEditor.EditorApplication.isPaused = true;
23.  
24.         /**/
25.        
26.         return p;
27.     }
28.  
29.     void SimParticles(Vector3 from, Vector3 to, ParticleSystem ps) {
30.         var dif = to - from;
31.         var dist = dif.magnitude;
32.  
33.         var perDist = ps.emission.rateOverDistanceMultiplier * ps.emission.rateOverDistance.constant;
34.         var pCount = Mathf.RoundToInt(dist * perDist);
35.         Debug.Log("Emitting " + perDist + " particles per distance (total:" + pCount + ") on " + ps, ps);
36.  
37.         ps.Emit(pCount);
38.  
39.         var particles = new ParticleSystem.Particle[ps.main.maxParticles];
40.         var pExist = ps.GetParticles(particles);
41.         if (pExist != pCount) {
42.             Debug.LogWarning("Unexpected - particle count != desired count");
43.         }
44.  
45.         float pExistF = pExist;
46.         for (int i = pExist - 1; i >= 0; i--) {
47.             // position = from + a ratio of the travel vector "dif"
48.             particles[i].position = from + dif * (i / pExistF);
49.         }
50.  
51.         ps.SetParticles(particles, pExist);
52.     }
53.