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Explore the essential guidelines on multiplying by `Time.deltaTime` in Unity. Learn about instantaneous and continuous operations to create smooth gameplay in your projects.
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Understanding When to Multiply by Time.deltaTime in Unity

In the world of game development, particularly in using the Unity game engine, developers often grapple with the question of when to use Time.deltaTime. This small yet powerful variable is vital for ensuring that actions in your game run smoothly, regardless of the frame rate. However, the advice surrounding it can be conflicting.

The Core of the Issue

Many developers find it challenging to identify the right situations for multiplying by Time.deltaTime. Notably, this concern arises when dealing with Unity's physics system, specifically:

When applying changes to rigidbody.velocity in FixedUpdate.

Using rigidbody.AddForce and how the different ForceMode types affect this.

This article aims to clarify these scenarios, breaking them down into intuitive concepts that can be universally applied.

The Intuition Behind Time.deltaTime

Multiplying by Time.deltaTime is best understood in the context of the difference between instantaneous and continuous operations.

Instantaneous Operations: These operations cause a sudden change to a value.

Examples include:

Rigidbody.[position|velocity|rotation|angularVelocity] + = x;

Rigidbody.Add[Force|Torque](x, ForceMode.[Impulse|VelocityChange]);

Continuous Operations: These are applied over a duration and do not cause abrupt changes.

Examples include:

Rigidbody.Add[Force|Torque](x, ForceMode.[Force|Acceleration]);

Rigidbody.velocity = x;

Understanding these distinctions will guide your usage of Time.deltaTime.

When to Multiply by Time.deltaTime

General Guidelines

Continuous Operations: Do not multiply by Time.deltaTime. They are designed to be smooth and frame-rate independent by default.

Instantaneous Operations: Multiply by Time.deltaTime if you're applying the change continuously.

Practical Examples

Example 1: Explosion Effect

[[See Video to Reveal this Text or Code Snippet]]

In this case, you should not multiply by Time.deltaTime since Explode() is called only once and does not span multiple frames.

Example 2: Moving an Object

[[See Video to Reveal this Text or Code Snippet]]

Here, it is necessary to multiply by Time.deltaTime, as you want the movement to occur over time, ensuring smooth motion.

Alternative Continuous Operation:

[[See Video to Reveal this Text or Code Snippet]]

This removes the need for multiplying by Time.deltaTime.

Example 3: Applying Acceleration

[[See Video to Reveal this Text or Code Snippet]]

In this situation, multiplying by Time.deltaTime is important to maintain consistent velocity changes across frames. However, you could also write it as:

[[See Video to Reveal this Text or Code Snippet]]

This is cleaner and equally effective, applying acceleration over the entire frame duration.

The Role of FixedUpdate

It’s common to assume that using FixedUpdate removes the need to multiply by Time.deltaTime, but this is a misconception. While FixedUpdate runs at a constant timestep, relying solely on it means all values must be tied to the fixed frame rate; changes in frame rates would necessitate manual adjustments.

Key Takeaways:

Just because you are in FixedUpdate, you still need to consider the implications of frame independence.

There’s no need to replace Time.deltaTime with Time.fixedDeltaTime as Unity manages this internally for you.

Conclusion

In summary, multiply by Time.deltaTime only when you need an instantaneous operation to appear smooth across frames. Using the right type of operation — continuous or instantaneous — can help you avoid unnecessary complications and enhance the clarity of your code.

By applying these principles, you will create a more fluid gameplay experience in yo