Smooth object moving. Lerp issue

Questions & Answers
1

Hello. Sorry for my bad English. I’m making 2d game and I need to realize projectile motion in it. I wrote this code to make it work:

	private float angle;
	private float velocity;
	private float g = 9.80665f;
	private float t;
	private float flightTime;
	
	void Start()
	{
		// scale object
		var scale = CameraSetter.YMax * 2 / 4;
		transform.localScale = new Vector3(scale, scale, 0);

		//set desirable parameters for projectile motion
		t = 0;
		var maxHeight = CameraSetter.YMax + CameraSetter.YMax/2;
		var range = CameraSetter.XMax*2;
		angle = CalculateAngle(maxHeight, range);
		velocity = CalculateVelocity(range);
		flightTime = CalculateFlightTime();
	}
	
	private float CalculateFlightTime()
	{
		return (2*velocity*Mathf.Sin(angle*Mathf.Deg2Rad))/g;
	}

	private float CalculateVelocity(float range)
	{
		return Mathf.Sqrt((range * g) / Mathf.Sin(angle * 2 * Mathf.Deg2Rad));
	}

	private float CalculateAngle(float maxHeight, float range)
	{
		return Mathf.Atan(4*maxHeight/range)*Mathf.Rad2Deg;
	}
	
	void Update()
	{
		t += Time.deltaTime;
		var x = GetXv();
		var y = GetYv();
		transform.position = new Vector3(x, y, 0);
	}

	private float GetYv()
	{
		return ((velocity * t * Mathf.Sin(angle * Mathf.Deg2Rad)) - 0.5f * (g * (t * t))) - CameraSetter.YMax;
	}

	private float GetXv()
	{
		return (velocity * t * Mathf.Cos(angle * Mathf.Deg2Rad)) - CameraSetter.XMax;
	}

This works fine to me: object starts moving from left bottom corner to right bottom corner with parabolic trajectory and maximum height in center (with coordinates x=0, y=orthographicSize/2). However, it’s moving not smoothly, so i decided to do Lerp like this:

transform.position = Vector3.Lerp(transform.position, new Vector3(x, y, 0), Time.deltaTime);

instead of

transform.position = new Vector3(x, y, 0);

and this actually ruined the whole thing!! Trajectory now ends in somewhere near the center bottom with max height just about y=-orthographicSize/3 or something like that.
Please explain me why Lerp is ruining trajectory and how can i fix it (i need working trajectory with smooth movement).

2

That’s because you’re using Time.deltaTime as the fraction parameter in Lerp(). This is NOT an actual time value but the time in seconds it took to complete the last frame.

I didn’t read up intensively on your code, but you could use your “t” variable instead as an example. In this case the object would move to the target position in exactly 1 second.

EDIT:

Try this out:

private const float MOVEMENT_TIME = 5.0f; // Should move for 5 seconds

void Update()
{
    t += Time.deltaTime;
    float lerpFraction = t / MOVEMENT_TIME;
    var x = GetXv();
    var y = GetYv();
    transform.position = Vector3.Lerp(transform.position, new Vector3(x, y, 0), lerpFraction);
}