Slow Motion

Part of the Building Real Games section of Coddy's Unity journey. Lesson 34 of 56.

A Time.timeScale between 0 and 1 is slow motion. At 0.25, Time.deltaTime is a quarter of the real frame time, so a car moved by speed * Time.deltaTime covers a quarter of its distance each real second, Rigidbodies fly and fall at a quarter of their speed, and WaitForSeconds(1f) lasts 4 real seconds. Above 1 the game runs fast: 2 is double speed.

Physics needs one more line. Each physics step still moves the game on by Time.fixedDeltaTime, 0.02 seconds of game time, and at 0.25 those take 0.08 real seconds: FixedUpdate() runs only 12.5 times a real second, so bodies move on one frame in five and stutter. Shrink the step by the same factor, and put both values back afterwards, since Unity never resets fixedDeltaTime on its own. This script slows the game while Space is held:

float normalStep;

void Awake()
{
    normalStep = Time.fixedDeltaTime;   // 0.02 in a new project
}

void Update()
{
    if (Input.GetKeyDown(KeyCode.Space))
    {
        Time.timeScale = 0.25f;
        Time.fixedDeltaTime = normalStep * Time.timeScale;
    }
    if (Input.GetKeyUp(KeyCode.Space))
    {
        Time.timeScale = 1f;
        Time.fixedDeltaTime = normalStep;
    }
}

With the smaller step, physics still steps 50 times a real second and each step covers a quarter of the game time, so the slow motion stays smooth. The step is in game seconds, so code in FixedUpdate() that uses Time.deltaTime keeps working unchanged.

Challenge

Medium

A comet crosses the sky, and Space launches it from the left (Launcher.cs, ready). On its way it flies through the SlowZone, a trigger box, and that part of its flight should play in slow motion. Write SlowZone.cs:

When something enters the zone, game time runs at slowScale (0.4 in the Inspector), and the physics step shrinks by the same factor from 0.02, so the comet still moves smoothly.
When it leaves the zone, time runs at normal speed again, with the physics step back at 0.02.

Press Play, press Space and watch the comet pass through, then Check.

Try it yourself

using UnityEngine;

public class SlowZone : MonoBehaviour
{
    public float slowScale = 0.4f;

    // Slow motion while something is inside the zone
}