Develop Reference

How to make Unity gravity and AddForce accelerate

I'm making a clone of Hollow Knight, and my character is falling at a constant rate instead of accelerating. I tried changing the gravity scale and using Addforce instead of rigidbody gravity.
This is the code I tried for the gravity
public Rigidbody2D rb;
void FixedUpdate(){
rb.AddForce(-transform.up*100f * Time.fixedDeltaTime, ForceMode2D.Impulse);
}

I've done a lot of testing and it doesn't seem like what you say is true.
In a new project, I simply created a rigidbody2D gameobject and added this script to it.
public void Start()
{
StartCoroutine(PrintDistance());
}
IEnumerator PrintDistance()
{
float p = 0;
for (; ; )
{
p = transform.position.y;
yield return new WaitForSeconds(1);
print("Distance per second: " + (transform.position.y - p));
}
}
As you can see from the console, every second, the distance traveled increases, so the speed is not constant but accelerated.
How can you see that the falling speed is not constant?
Try adding this script and tell me the results.
I can speculate that some other part of your project is causing this problem, or you have changed some properties in the Physics panel in Project Settings, but that is not likely.
if you think my answer helped you, you can mark it as accepted. I would very much appreciate it :)

How to reduce 'speed' of a rigidbody without changing the distance it has to cover?

I am moving a rigidbody using rb.AddForce(force,ForceMode.Impulse) where force is the target position the rigidbody have to reach.
Now the speed it goes directly depends on the distance it has to cover.
Let's say the time taken to reach the target position is 3sec. I need the rigidbody to cover the same target pos in 5sec.
I dont want to change the timescale as it affects my gameflow
On Changing the velocity of rigidbody it fails to reach the target position

Some basic physics/math:
velocity = change-in-position / travel-time
force = mass * change-in-velocity / acceleration-time
For ease, we're going to call change-in-position as distance, and change-in-velocity/acceleration-time as acceleration
Now, since the acceleration-time component is effectively zero because you're using Impulse, we're going to remove it from the equation (in math terms, we set it at '1')
force = mass * change-in-velocity
Assuming your object starts at zero velocity, we can simplify change-in-velocity to just velocity
force = mass * velocity
force = mass * distance / travel-time
To bring that back into Unity code:
var mass = rb.mass;
var distance = destination.position - transform.position;
var travelTime = 5f; // seconds
var force = mass * distance / travelTime;
rb.AddForce(force, ForceMode.Impulse);
Note that this assumes a frictionless transfer and constant velocity.

If you ignore gravity, this code solves the problem, here I changed the drag according to weight and distance, it may be a little bit away from the destination at the end, the reason should be higher drag friction.
public void ForceToTarget(Transform target, float time = 1f)
{
var rb = GetComponent<Rigidbody>();
var vector = target.position - transform.position;
var distance = vector.magnitude;
rb.drag = distance/time;
rb.AddForce(vector*rb.mass*distance/time, ForceMode.Impulse);
}

If you want precise control over your speed, then stop using ForceMode.Impulse because other physics effects like drag will make your answers wrong. Instead, just set the speed yourself. You can do this with a Coroutine to control timing and ForceMode.VelocityChange to control the speed. Basically, just look at where you are, where the target is, how much time is left, and apply the speed directly.
private bool canMove = true;
public void MoveTo(Vector3 targetPosition, float targetTime)
{
if(canMove)
{
StartCoroutine(MoveToCoroutine(targetPosition,targetTime));
}
}
private IEnumerator MoveToCoroutine(Vector3 targetPosition, float time)
{
canMove = false;
while(time > 0)
{
var positionDelta = transform.position - targetPosition;
var targetSpeed = positionDelta / time;
var speedDelta = targetSpeed - rb.velocity;
rb.AddForce(speedDelta , ForceMode.VelocityChange);
yield return null;
time -= Time.deltaTime;
}
// Bring the object to a stop before fully releasing the coroutine
rb.AddForce(-rb.velocity, ForceMode.VelocityChange);
canMove = true;
}
I wrote this here into the text editor, no IDE and haven't tested it, but I'm pretty sure this'll do what you want.

Assuming you're using the target position as-is then larger vectors will cause larger force to be applied than smaller vectors. Similarly, if using a direction vector as-is then as the rb gets closer to the target the magnitute of the vector gets smaller and thus less force is applied.
To get a constant speed use the direction to the target and Normalise it instead. Regardless of the distance the direction vector will always have a magnitude of 1 so you can multiply it by any value to accurately control the speed of the object:
Rigidbody rb;
public Transform target;
public float dist;
public float speed = 2f; // or whatever
public float targetDistance = 40f; // or whatever
private void Start()
{
rb = GetComponent<Rigidbody>();
StartCoroutine("IMove");
}
IEnumerator IMove()
{
dist = Vector3.Distance(transform.position, target.position);
while (dist > targetDistance)
{
dist = Vector3.Distance(transform.position, target.position);
rb.AddForce(Vector3.Normalize(target.position - transform.position) * speed, ForceMode.Impulse);
yield return new WaitForFixedUpdate();
}
}

Without getting too much into the physics and maths, if you want it to travel slower but the same distance you need to reduce the gravity on it and the initial force.
Note in this example I am assuming the weight is 1 to make the calculation a bit easier for force.
public class TrevelSpeedAdjusted
{
public float speedFactor = 1;
void FixedUpdate()
{
// Reduce the gravity on the object
rb.AddForce(-Physics.gravity * rigidbody.mass * (1 - speedFactor));
}
public float AddAdjustedForce(Vector3 force, ForceMode forceMode)
{
rb.AddForce(force * speedFactor, forceMode);
}
}

So you can try DoTween package to do this pretty easily and its very convenient to use a package instead of using Unity's inbuilt system.
With doTween use this:
DOMove(Vector3 to, float duration, bool snapping) condition to tween your physics Gameobject to a given target position in the duration you require.
Here's documentation you can refer to if you want: http://dotween.demigiant.com/documentation.php
Let me give you an example:
Install the doTween Package. http://dotween.demigiant.com/download
Import it to unity.
Go to your script where you want to achieve the functionality you mentioned on your question and add this header "using DG.Tweening".
Now get access of your RigidBody.
For Example Lets say: I have a cube gameobject with rigidbidy and this script attached.
The Cube Initial Position is at 0,0,0.
And I want it to move to 5,5,5 in 3 seconds or 5 seconds as per your questions request. And lets say I want this to happen when I click SpaceBar on keyboard.
So I would simply do.
Rigidbody rb;
void Start()
{
rb= GetComponent<Rigibody>();
}
void Update()
{
if(Input.GetButtonDown(Keycode.Space))
{
MoveCube(new Vector3(5,5,5),5);
}
}
void MoveCube(Vector3 inTargetPosition , float durationToReachTheTarget)
{
//What this line does is first take in the target position you want your physics object to reach as it first parameter, Then takes in the duration in which you want it to reach there.
rb.DoMove(inTargetPosition,durationToReachTheTarget);
}
This should help you. But remember this is only if you okay with adding an extra package. Personally this package is very good and I would recommend you this.

Why is this transform.position reset between frames

So, my issue is as follows:
I'm developing a very simple 2D game as I'm quite new to Unity. Basically, there is a knight, and there is a troll. I'm developing my custom AI for the troll and got stumped at the very beginning. I tried to make the troll sprite correctly rotate and walk towards the player (in Update()), but then it turned out that the troll game object doesn't move as much as a millionth of a unit (although it rotates correctly)!
Now, what is special about my problem is the following: it's not about using localPosition instead of position (the troll has no parents – pun intended), it's not about referencing the transform in the script via "[SerializeField]", it's not about working with static objects or messed-up prefab coordinates or any of the other issues I've found in my desperate 10-hour search on the Internet. No, the issue is that I HAVE the troll transform in my code, I modify it successfully, but then it fails to be applied to the in-game troll object. Let that sink in – I am correctly modifying the correct transform (checked multiple times via debugging) but at the beginning of the next frame, all changes get magically reverted and the transform doesn't move the slightest.
I've made almost twenty modifications to my code in order to try and make it work (including separating stuff in different files as was suggested in a particular forum topic) but it just doesn't, and I can't think of anything else. Any help is very, very much appreciated.
public class EnemyController : MonoBehaviour
{
[SerializeField]
private Animator animator;
[SerialiseField]
private float walkingSpeed;
[SerializeField]
[Header("The aggro range of the enemy in Unity units.")]
private float aggroRange;
private float dir;
private Transform playerTransform;
void Update()
{
if (Aggro(playerTransform, aggroRange))
{
//WALKING:
WalkTowards(playerTransform, walkingSpeed);
}
}
private void WalkTowards(Transform targetTransform, float walkingSpeed)
{
animator.SetBool("IsRunning", false);
dir = 0.0f;
//Get the direction (left or right)
if (targetTransform.position.x < transform.position.x) //Left
{
dir = -1.0f;
//Flip the sprite to the left
transform.rotation = Quaternion.Euler(0.0f, 180.0f, 0.0f);
}
else //Right
{
dir = 1.0f;
//Flip the sprite to the right
transform.rotation = Quaternion.Euler(0.0f, 0.0f, 0.0f);
}
transform.position = new Vector3(
transform.position.x + (walkingSpeed * dir * Time.deltaTime),
transform.position.y,
transform.position.z
);
Debug.Log(
string.Format("Troll 1 position:\nx: {0:#.##}", transform.position.x) +
string.Format(" y: {0:#.##}", transform.position.y) +
string.Format(" z: {0:#.##}", transform.position.z)
);
}
}
(NOTE: Aggro() is a custom method that determines when the enemy aggros; not included in the code as it is irrelevant to the issue)
So basically, transform.position gets changed correctly in the script and stays that way until the end of the method, but at the beginning of the next frame – i.e. when WalkTowards() gets called again, the transform.position remains the same as it was at the beginning of the last frame – as if no calculations or changes were ever made.

#Ruzihm,
You were right!!! Turns out the Animator component of the troll was causing the issue – "Apply root motion" was off, I turned it on and the position gets correctly updated now. Thank you ever so much for the help!
Example:

Unity- Player Jump Height / Public, Private, Static, nothing

I have been following a tutorial on 2D Player Controller in Unity (It's 'Live Training 16 Dec 2013 - 2D Character Controllers' video).
I was able to implement everything the tutorial showed successfully with some edits to make it work in Unity 5. Afterwards, I decided to play around with it so that I can gain a better understanding. One thing I tried to do was changing the jump height when pressing the Space key. Here is the code:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class RobotControllerScript : MonoBehaviour {
public float maxSpeed = 10f;
bool facingRight = true;
Animator anim;
bool grounded = false;
public Transform groundCheck;
float groundRadius = 0.2f;
public LayerMask whatIsGround;
public float jumpForce = 700f;
// Use this for initialization
void Start () {
anim = GetComponent<Animator>();
}
// Update is called once per frame
void FixedUpdate () {
grounded = Physics2D.OverlapCircle(groundCheck.position, groundRadius, whatIsGround);
anim.SetBool("Ground", grounded);
//vSpeed = vertical speed
anim.SetFloat("vSpeed", GetComponent<Rigidbody2D>().velocity.y);
float move = Input.GetAxis("Horizontal");
anim.SetFloat("Speed", Mathf.Abs(move));
GetComponent<Rigidbody2D>().velocity = new Vector2(move * maxSpeed,
GetComponent<Rigidbody2D>().velocity.y);
if (move > 0 && !facingRight)
{
Flip();
}
else if (move < 0 && facingRight)
{
Flip();
}
}
void Update()
{
if(grounded && Input.GetKeyDown(KeyCode.Space))
{
anim.SetBool("Ground", false);
GetComponent<Rigidbody2D>().AddForce(new Vector2(0, jumpForce));
}
}
void Flip()
{
facingRight = !facingRight;
Vector3 theScale = transform.localScale;
theScale.x *= -1;
transform.localScale = theScale;
}
}
Looking at the code and the tutorial explanations, jumpForce is the variable that controls how high the character will jump (force applied). So, I changed 700f to a 5f. I expected the character to make a really small jump, but this was not the case. It jumped at the same height as 700f.
public float jumpForce = 700f;
After playing around with the code, I was able to have the expected result by removing the 'public' next to jumpForce. Other ways to fix this was setting it to private or static. I remember I had a similar problem when making a temperature widget on QT Creator. I had to set a variable to static or else it would not return to the default value after C to F conversion, but I don't remember exactly why.
Can anyone explain why 'public' does not work and why a private/static/nothing might? What is the best/efficient solution for this problem? Thank you very much.

When a field is public (or serializable), it is shown in inspector. When it is shown in inspector, you tweak it's value, a you expect the tweaks you make to the value, in the inspector, to be preserved between play mode runs.
When you declare a variable in the code, you may specify an initial value, like this:
public float jumpForce = 700f;
So, the first time this object gets inspected, after created, it has a reasonable value. Then, you tweak and choose a better value (or back to the same, whatever). That's the value you preferred! You want the object to keep functioning like this while you go ahead and finish making your game.
So, once a field is tweaked in Inspector, it doesn't respect the initial value you hardcoded anymore, and that's usually what everyone want. Imagine how annoying would be if everytime you make a change in your class, all the tweaked fields got back to the initial hardcoded value...
If a field was previously public, and then you make it private (or hidden in inspector by any mean), then the hardcoded initial value will stand.
But, what if I really want my object to get back to it's initial/default configuration? That's why the Reset method and the reset button in inspector were made!

Unity 2D Colliders Stuck to Walls?

I have just updated Unity to version 5.2.1 from 5.0.0 this morning and I experienced a few problems. After Unity translated the game to work for the new version, I tested the game. Everything worked fine, except that when I shot a bullet in my game, they would stick to the walls instead of bouncing off of them, but only at certain angles and distances. It was like the bullet was so fast that it skipped hitting the collider and got stuck inside of the collider. That would make a little bit of sense, but the weird part is that I have a C# script for slow motion in the game. Whenever I turn the slow motion on and then turn it off again, the problem with bullets sticking goes away. I cannot seem to figure out what is causing the problem and it definitely wasn't there before I updated the software. Can anyone help me out with this? Thanks. I'll post the bullet script and the slow motion script below. The bullet is instantiated inside the player script by the way.
Bullet Script:
using UnityEngine;
using System.Collections;
public class Bullet : MonoBehaviour {
public float bulletSpeed;
public float bulletOpacity;
public bool fadeOut;
Animator anim;
Rigidbody2D rigidbod;
SpriteRenderer spriterend;
public GameObject player;
public float aimRotation;
// Use this for initialization
void Start () {
anim = GetComponent<Animator> ();
rigidbod = GetComponent<Rigidbody2D> ();
spriterend = GetComponent<SpriteRenderer> ();
rigidbod.velocity = transform.right * bulletSpeed;
rigidbod.gravityScale = 0;
bulletOpacity = 1;
}
// Update is called once per frame
void Update () {
Destroy (gameObject, 3f);
spriterend.color = new Color (1f, 1f, 1f, bulletOpacity);
if (fadeOut == true)
bulletOpacity -= 0.03f;
if (bulletOpacity <= 0)
Destroy (gameObject);
aimRotation = player.GetComponent<Player> ().aimRotation;
}
void OnTriggerEnter2D (Collider2D bulletHit) {
/*if (bulletHit.gameObject.layer == LayerMask.NameToLayer ("vulnerableLayer")) {
}*/
rigidbod.gravityScale = 1;
rigidbod.drag = 1;
fadeOut = true;
}
}
Slow Motion Script:
using UnityEngine;
using System.Collections;
public class SlowMotion : MonoBehaviour {
public float currentLongevity = 0f;
public float slowAmount = 0.2f;
public float normalTime = 1f;
public float slowLongevity = 0.4f;
// Use this for initialization
void Start () {
}
// Update is called once per frame
void Update () {
if (Input.GetMouseButtonDown (1)) {
Time.fixedDeltaTime = 0.0f * Time.timeScale;
if (Time.timeScale == normalTime)
Time.timeScale = slowAmount;
}
if (Time.timeScale == slowAmount) {
currentLongevity += Time.deltaTime;
}
if (currentLongevity > slowLongevity) {
currentLongevity = 0;
Time.timeScale = normalTime;
}
}
}

Since you indicated that the 'weird' bullet behavior ceases after the SlowMotion script is run, I would suggest that the issue has to do with setting Time.fixedDeltaTime as you are doing in the Update() method of the SlowMotion script. This is also supported by your later comment that the 'weird' behavior no longer appears when you set the rigidBody2D's collision detection to continuous (the bullet's RigidBody2D I assume).
Continuous RididBody2D collision detection enables determining if contact occurred between updates, where as Discrete collision detection registers a collision during a physics update. In your SlowMotion script, the following line (in the Update() method) sets fixedDeltaTime to zero:
Time.fixedDeltaTime = 0.0f * Time.timeScale;
Since Time.fixedDeltaTime is the interval in seconds at which physics and other fixed frame rate updates are performed, there must be some minimum value at which the frame rate is realistically run, (0.0 cannot be an actual frame-rate update interval). When Time.fixedDeltaTime == 0.0, Unity may use a default minimum value or use this to indicate that frame updates run as often as possible, (though I have not tested this). Since the Bullet script calls Update() rather than FixedUpdate(), there is no guarantee that the actual interval between the frame updates is uniform. I suspect that when you run the SlowMotion script, and set Time.fixedDeltaTime to 0.0, Unity runs with the smallest possible frame update interval and following this, it does render the expected bullet behavior.
One way to test this would be to set your initial Time.fixedDeltaTime to 0.0 in the editor (and your collision detection behavior back to discrete for the bullet's RididBody2D).
To set Time Manager fixedDeltaTime in the editor: main menu -> Edit -> Project Settings -> Time
If you find a fixedDeltaTime that works, you can set this in the editor and retrieve the initial value in your SlowMotion script in the Awake() method to initialize the base fixedDeltaTime. I would not recommend using fixedDeltaTime == 0.0 since there is no indication in the documentation of a consistent behavior for a time interval of 0.0 and it may be bound more to the underlying hardware and could result in more unexpected behavior.
Hope this helps, cheers!