Particle lifetime and speed in Unity

Introduction

Particle lifetime and speed control how long particles last and how fast they move. This helps make effects like fire, smoke, or magic look real and interesting.

When creating a fire effect that fades out after a few seconds.

When making smoke that slowly rises and disappears.

When simulating sparks that shoot out quickly and vanish.

When adjusting rain particles to fall at different speeds.

When customizing magic spell effects with varying particle durations.

Syntax

Unity

var main = particleSystem.main;
main.startLifetime = 2.0f; // seconds
main.startSpeed = 5.0f; // units per second

You access lifetime and speed through the main module of the ParticleSystem.

Both startLifetime and startSpeed can be set to a constant or a range.

Examples

Sets all particles to live 3 seconds and move at speed 4.

Unity

var main = particleSystem.main;
main.startLifetime = 3.0f;
main.startSpeed = 4.0f;

Particles will have random lifetimes between 1 and 5 seconds, and speeds between 2 and 6.

Unity

var main = particleSystem.main;
main.startLifetime = new ParticleSystem.MinMaxCurve(1.0f, 5.0f);
main.startSpeed = new ParticleSystem.MinMaxCurve(2.0f, 6.0f);

Particles live half a second and move very fast.

Unity

var main = particleSystem.main;
main.startLifetime = 0.5f;
main.startSpeed = 10.0f;

Sample Program

This script sets the particle system so each particle lasts 2.5 seconds and moves at speed 3 when the game starts.

Unity

using UnityEngine;

public class ParticleControl : MonoBehaviour
{
    public ParticleSystem particleSystem;

    void Start()
    {
        var main = particleSystem.main;
        main.startLifetime = 2.5f; // particles live 2.5 seconds
        main.startSpeed = 3.0f;    // particles move at speed 3
    }
}

Important Notes

Changing lifetime affects how long particles stay visible before disappearing.

Speed controls how fast particles move away from their source.

You can use ranges to add variety and make effects look more natural.

Summary

Particle lifetime controls how long particles last.

Particle speed controls how fast particles move.

Both are set through the particle system's main module.

Practice

1. What does the startLifetime property of a ParticleSystem control in Unity?

easy

A. How long each particle stays alive before disappearing

B. The speed at which particles move when emitted

C. The color of the particles over time

D. The size of the particles when they spawn

Solution

1. Step 1: Understand the meaning of startLifetime

   The startLifetime property sets the duration each particle exists after being emitted.

2. Step 2: Compare with other properties

   Speed controls movement, color controls appearance, size controls scale. Only lifetime controls duration.

3. Final Answer:

   How long each particle stays alive before disappearing -> Option A

4. Quick Check:

   Particle lifetime = duration alive [OK]

Hint: Lifetime means how long particles live before vanishing [OK]

Common Mistakes:

• Confusing lifetime with speed
• Thinking lifetime affects color or size
• Mixing up startLifetime with emission rate

2. Which of the following is the correct way to set the particle speed to 5 in a Unity ParticleSystem script?

easy

A. particleSystem.startSpeed = 5f;

B. particleSystem.speed = 5;

C. particleSystem.setSpeed(5);

D. particleSystem.main.startSpeed = 5f;

Solution

1. Step 1: Recall ParticleSystem API structure

   In Unity, startSpeed is inside the main module, accessed as particleSystem.main.startSpeed.

2. Step 2: Check syntax correctness

   Options A and B are incorrect because startSpeed is not a direct property of ParticleSystem. particleSystem.setSpeed(5); uses a non-existent method.

3. Final Answer:

   particleSystem.main.startSpeed = 5f; -> Option D

4. Quick Check:

   Use main module to set startSpeed [OK]

Hint: Use particleSystem.main.startSpeed to set speed [OK]

Common Mistakes:

• Trying to set startSpeed directly on particleSystem
• Using incorrect method names
• Forgetting to use the main module

3. Consider this Unity C# code snippet:

var ps = GetComponent<ParticleSystem>();
var main = ps.main;
main.startLifetime = 2f;
main.startSpeed = 3f;
Debug.Log(main.startLifetime + ", " + main.startSpeed);

What will be printed in the console?

medium

A. 2.0, 3.0

B. 2f, 3f

C. 2, 3

D. Error: Cannot assign to startLifetime

Solution

1. Step 1: Understand property types and output

   The startLifetime and startSpeed are floats. Assigning 2f and 3f sets them to 2.0 and 3.0 internally.

2. Step 2: Check Debug.Log output format

   Logging floats with string concatenation prints "2, 3" because ToString() on whole number floats omits the decimal.

3. Final Answer:

   2, 3 -> Option C

4. Quick Check:

   Whole number floats print without decimals [OK]

Hint: Whole number floats print without decimal in Debug.Log [OK]

Common Mistakes:

• Expecting decimal output like 2.0, 3.0
• Thinking 'f' suffix prints in output
• Assuming assignment causes error

4. This code tries to set particle lifetime but causes an error:

var ps = GetComponent<ParticleSystem>();
ps.startLifetime = 4f;

What is the main problem?

medium

A. startLifetime must be set via the main module, not directly

B. startLifetime is a read-only property

C. ParticleSystem component is missing

D. startLifetime requires integer value, not float

Solution

1. Step 1: Understand ParticleSystem property access

   In Unity, startLifetime is inside the main module, so it cannot be set directly on ps.

2. Step 2: Identify correct way to set lifetime

   Correct syntax is var main = ps.main; main.startLifetime = 4f;. Direct assignment causes error.

3. Final Answer:

   startLifetime must be set via the main module, not directly -> Option A

4. Quick Check:

   Use main module to set startLifetime [OK]

Hint: Always set startLifetime via particleSystem.main [OK]

Common Mistakes:

• Trying to set startLifetime directly on ParticleSystem
• Assuming startLifetime is read-only
• Confusing missing component with syntax error

5. You want to create a particle effect where particles live for 3 seconds and move faster over time, starting at speed 2 and increasing to 6. Which approach correctly sets this behavior in Unity?

hard

A. Set startLifetime = 3f and set startSpeed = 6f only

B. Set startLifetime = 3f and use velocityOverLifetime module to increase speed from 2 to 6

C. Set startLifetime = 6f and startSpeed = 2f

D. Set startLifetime = 3f and change speed in Update() manually

Solution

1. Step 1: Set particle lifetime correctly

   Particles should live 3 seconds, so startLifetime = 3f is correct.

2. Step 2: Use velocityOverLifetime for speed change

   To increase speed over time from 2 to 6, use the velocityOverLifetime module with a curve, not just startSpeed.

3. Step 3: Evaluate other options

   Set startLifetime = 3f and set startSpeed = 6f only sets constant speed 6, not increasing. Set startLifetime = 6f and startSpeed = 2f has wrong lifetime. Set startLifetime = 3f and change speed in Update() manually is inefficient and unnecessary.

4. Final Answer:

   Set startLifetime = 3f and use velocityOverLifetime module to increase speed from 2 to 6 -> Option B

5. Quick Check:

   Use velocityOverLifetime for speed changes over time [OK]

Hint: Use velocityOverLifetime to change speed during particle life [OK]

Common Mistakes:

• Setting only startSpeed for changing speed over time
• Confusing lifetime and speed values
• Trying to update speed manually every frame