Color and size over lifetime in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Unity particle color code?

Consider this Unity C# code snippet that changes a particle's color over its lifetime. What color will the particle have at half its lifetime?

Unity

var main = particleSystem.main;
main.startColor = new ParticleSystem.MinMaxGradient(
    new Gradient {
        colorKeys = new GradientColorKey[] {
            new GradientColorKey(Color.red, 0.0f),
            new GradientColorKey(Color.blue, 1.0f)
        },
        alphaKeys = new GradientAlphaKey[] {
            new GradientAlphaKey(1.0f, 0.0f),
            new GradientAlphaKey(1.0f, 1.0f)
        }
    }
);
// Particle lifetime is 1 second

A The particle color will be purple (a mix of red and blue).

B The particle color will be red.

C The particle color will be blue.

D The particle color will be green.

💡 Hint

Think about how gradients blend colors over time.

The gradient blends from red at time 0 to blue at time 1. At half lifetime (0.5), the color is a mix of red and blue, which results in purple.

❓ Predict Output

intermediate

What size will the particle have at 75% of its lifetime?

Given this Unity C# code controlling particle size over lifetime, what is the particle size at 0.75 lifetime?

Unity

var sizeOverLifetime = particleSystem.sizeOverLifetime;
sizeOverLifetime.enabled = true;
sizeOverLifetime.size = new ParticleSystem.MinMaxCurve(1.0f, new AnimationCurve(
    new Keyframe(0.0f, 0.5f),
    new Keyframe(1.0f, 2.0f)
));

A The particle size will be 1.0.

B The particle size will be 0.5.

C The particle size will be 2.0.

D The particle size will be approximately 1.625.

💡 Hint

The size changes linearly from 0.5 to 2.0 over the lifetime.

At 75% lifetime, size is 0.5 + 0.75*(2.0 - 0.5) = 1.625.

🔧 Debug

advanced

Why does this particle color over lifetime code cause an error?

This Unity C# code snippet tries to set a particle system's color over lifetime but causes a compile error. What is the cause?

Unity

var colorOverLifetime = particleSystem.colorOverLifetime;
colorOverLifetime.enabled = true;
colorOverLifetime.color = new Gradient();
colorOverLifetime.color.colorKeys = new GradientColorKey[] {
    new GradientColorKey(Color.green, 0.0f),
    new GradientColorKey(Color.yellow, 1.0f)
};

A Gradient.colorKeys is a read-only property and cannot be assigned directly.

B GradientColorKey requires three parameters, but only two are provided.

C colorOverLifetime.color must be assigned a MinMaxGradient, not a Gradient.

D The particle system must be stopped before changing colorOverLifetime.

💡 Hint

Check if you can assign to colorKeys directly.

Gradient.colorKeys is a property with only a getter. You must create a new Gradient, set its colorKeys, then assign the Gradient to a MinMaxGradient.

📝 Syntax

advanced

Which option correctly sets size over lifetime with a curve in Unity C#?

Choose the correct code snippet that sets a particle system's size over lifetime using an AnimationCurve.

A sizeOverLifetime.size = new AnimationCurve(new Keyframe(0, 1), new Keyframe(1, 0));

B sizeOverLifetime.size = new ParticleSystem.MinMaxCurve(1.0f, new AnimationCurve(new Keyframe(0, 1), new Keyframe(1, 0)));

C sizeOverLifetime.size = new ParticleSystem.MinMaxCurve(new AnimationCurve(new Keyframe(0, 1), new Keyframe(1, 0)));

D sizeOverLifetime.size = new ParticleSystem.MinMaxCurve(1.0f, 1.0f);

💡 Hint

MinMaxCurve constructor can take a multiplier and an AnimationCurve.

Option B correctly uses the MinMaxCurve constructor with a multiplier and an AnimationCurve. Option B is invalid because size expects a MinMaxCurve, not an AnimationCurve directly. Option B uses a wrong constructor. Option B uses two floats which is invalid.

🚀 Application

expert

How to create a particle system that fades color from white to transparent and grows in size over lifetime?

You want a Unity particle system where particles start white and fade to transparent, while their size grows from 0.1 to 1.0 over their lifetime. Which code snippet achieves this?

A

var col = ps.colorOverLifetime;
col.enabled = true;
var grad = new Gradient();
grad.SetKeys(
    new GradientColorKey[] { new GradientColorKey(Color.white, 0f), new GradientColorKey(Color.white, 1f) },
    new GradientAlphaKey[] { new GradientAlphaKey(1f, 0f), new GradientAlphaKey(0f, 1f) }
);
col.color = new ParticleSystem.MinMaxGradient(grad);
var size = ps.sizeOverLifetime;
size.enabled = true;
size.size = new ParticleSystem.MinMaxCurve(1f, new AnimationCurve(new Keyframe(0f, 0.1f), new Keyframe(1f, 1f)));

B

var col = ps.colorOverLifetime;
col.enabled = true;
col.color = new ParticleSystem.MinMaxGradient(Color.white);
var size = ps.sizeOverLifetime;
size.enabled = true;
size.size = new ParticleSystem.MinMaxCurve(0.1f, 1f);

C

var col = ps.colorOverLifetime;
col.enabled = true;
var grad = new Gradient();
grad.SetKeys(
    new GradientColorKey[] { new GradientColorKey(Color.white, 0f), new GradientColorKey(Color.clear, 1f) },
    new GradientAlphaKey[] { new GradientAlphaKey(1f, 0f), new GradientAlphaKey(0f, 1f) }
);
col.color = new ParticleSystem.MinMaxGradient(grad);
var size = ps.sizeOverLifetime;
size.enabled = true;
size.size = new ParticleSystem.MinMaxCurve(1f, new AnimationCurve(new Keyframe(0f, 0.1f), new Keyframe(1f, 1f)));

D

var col = ps.colorOverLifetime;
col.enabled = true;
col.color = new ParticleSystem.MinMaxGradient(Color.white);
var size = ps.sizeOverLifetime;
size.enabled = true;
size.size = new AnimationCurve(new Keyframe(0f, 0.1f), new Keyframe(1f, 1f));

💡 Hint

Use GradientColorKey with Color.clear for transparency and set size with MinMaxCurve and AnimationCurve.

Option C correctly sets the color gradient from white to transparent (Color.clear) and uses a size curve from 0.1 to 1.0. Option C uses white for both color keys, so no fade. Options B and D misuse size or color assignments.