koobas.hobune.stream
Why are my Vector3s returning inaccurate floats? I don't think it's the debug .ToString() issue.
I am trying to simulate a free-fall in a vacuum with an acceleration of -9.81m/s^2 along the y axis.
This is my code:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class behavBall : MonoBehaviour
{
//!!! Declare Variables:
[SerializeField] private GameObject goWorldManager;
private static Vector3 v3InitialRestingVelocity = new Vector3(0f, 0f, 0f);
private Vector3 v3MyGravityAcceleration;
private Vector3 v3CumulativeAcceleration;
private Vector3 v3PreviousPosition;
private Vector3 v3PreviousVelocity;
float fTimeStepScaled;
void PerSecond()
{
// : Print out the displacement and previous velocity every second.
Debug.Log("TIME: "+Time.time+", DISPLACEMENT: "+transform.position.y+", PRE-VELOCITY: "+v3PreviousVelocity.y);
}
void Start()
{
//!!! Initialize Variables:
v3MyGravityAcceleration = new Vector3(0f, -9.81f, 0f);
v3PreviousVelocity = v3InitialRestingVelocity;
v3PreviousPosition = transform.position;
InvokeRepeating("PerSecond", 1f, 1f);
}
void FixedUpdate()
{
fTimeStepScaled = Time.fixedDeltaTime;
v3CumulativeAcceleration = v3MyGravityAcceleration;
Vector3 v3CurrentVelocity = v3PreviousVelocity + (v3CumulativeAcceleration*fTimeStepScaled);
Vector3 v3Displacement = ((v3CurrentVelocity+v3PreviousVelocity)/2f)*fTimeStepScaled;
transform.position = v3PreviousPosition + v3Displacement;
v3PreviousVelocity = v3CurrentVelocity;
v3PreviousPosition = transform.position;
}
}
By my calculations my gameobject should have travelled from 0units to -4.905units in the first second and its previous velocity should be -9.81m/s^2, but when I print out the displacement after 1 second it has travelled -5,1units.and has a previous velocity of -10.
It looks like the floats are being rounded inside the calculations I'm doing with the Vectors, but if I do the same calculations using just floats the values are accurate.
What am I missing here?
EDIT: It turned out that I wasn't accounting for the initial execution of FixedUpdate at the start of run-time. Once I did that the values were accurate.
by Streeetwalker:
"OK, it must be that the fixedDeltTime you are using is not in sync with your repeating function - it's producing larger values than you expect. I bet if you use Time.deltaTime in your fixed update instead you will get more accurate values in accord with your manual calculations."
- Using Time.deltaTime did not make a difference but it did turn out to be a syncing issue between game-time and real time.
This, shared by Streeetwalker, helps: https://johnaustin.io/articles/2019/fix-your-unity-timestep
Call the ToString method of a float explicitly using the number of decimals parameter. For example:
v3PreviousVelocity.y.ToString("F7");
to my knowledge, 7 decimals is the maximum a float can have.
This doesn't fix the problem, the values are still way off. It's like is rounded up by about an integer.
OK, it must be that the fixedDeltTime you are using is not in sync with your repeating function - it's producing larger values than you expect. I bet if you use Time.deltaTime in your fixed update ins$$anonymous$$d you will get more accurate values in accord with your manual calculations.
https://docs.unity3d.com/ScriptReference/Time-fixedDeltaTime.html?_ga=2.197035881.456281214.1584718044-143348933.1578040369
see this also: https://johnaustin.io/articles/2019/fix-your-unity-timestep
Okay, so I changed the timescale to Time.deltaTime and it still gives me the same value. Then I tried debugging at 50 iterations of FixedUpdate() because there should be roughly 50 per second, when I do that I get the correct values happening at 0.98 seconds. You are right, it is a syncing issue. Thank you :)
