How to animate objects with bobbing up and down motion in Unity?

Question

I have created a 2D platform game with Unity and C# where I have collectable items that can be picked up by the player. What I want is to give the items more focus by having them bobbing up and down.

I came up with the following version:

Vector2 floatY;

public float FloatStrength; // Set strength in Unity

void Update () {
    floatY = transform.position;
    floatY.y = (Mathf.Sin(Time.time) * FloatStrength);
    transform.position = floatY;
}

But, this makes the object float in the middle of the level instead of where the object is positioned. How can I fix that?

Here is a screenshot of the prototype of the game:

The player can collect the wood stumps which should be bobbing up and down in space just above the ground.

Answer 1

You have a pretty simple solution. What your problem here is that you are not preserving the original y position. Essentially, you need to store the original y position in a variable, that would look like this: (Don't forget to set the variable in the Start() function.)

Vector2 floatY;
float originalY;

public float floatStrength;

void Start ()
{
    this.originalY = this.transform.position.y;
}

void Update () {
    floatY = transform.position;
    floatY.y = (Mathf.Sin(Time.time) * floatStrength);
    transform.position = floatY;
}

After that, I suggest that instead of directly setting the y value of the floatY to Mathf.Sin(Time.time) * floatStrength, you set it based on the sum of that value and the original transform position. The final code should look something like this:

Vector2 floatY;
float originalY;

public float floatStrength;

void Start ()
{
    this.originalY = this.transform.position.y;
}

void Update () {
    floatY = transform.position;
    floatY.y = originalY + (Mathf.Sin(Time.time) * floatStrength);
    transform.position = floatY;
}

Finally, instead of storing floatY as a Vector, you could store it as a float. Alternatively, you could use transform.position directly, doing something like this:

Vector2 floatY;
float originalY;

public float floatStrength;

void Start ()
{
    this.originalY = this.transform.position;
}

void Update () {
    /* Old code:
    floatY = transform.position;
    floatY.y = originalY + (Mathf.Sin(Time.time) * floatStrength);
    transform.position = floatY;
    */
    transform.position = new Vector2(transform.position.x, 
        originalY + (Math.Sinf(Time.time) * floatStrength));
}

Finally, the last step is to do some minor refactoring as to remove some errors that have popped up along the way. Math.Sin (Note: not Math.Sinf) yields a double instead of the desired float, so we have to explicitly convert that value. Next, we can remove the Vector2 floatY completely. Note that this would need to actually be a Vector3 to store transform.position, as that also has a z component, if we were to actually have kept using that. Finally, we must fix that same problem in the Update() function, and change:

    transform.position = new Vector2(transform.position.x, 
        originalY + (Math.Sinf(Time.time) * floatStrength));

to:

    transform.position = new Vector3(transform.position.x,
        originalY + (Math.Sinf(Time.time) * floatStrength), transform.position.z);

That code segment does not implement the other changes that I mentioned. In the end the complete code is as follows:

using UnityEngine;
using System;
using System.Collections;

public class FloatBehavior : MonoBehaviour
{
    float originalY;

    public float floatStrength = 1; // You can change this in the Unity Editor to 
                                    // change the range of y positions that are possible.

    void Start()
    {
        this.originalY = this.transform.position.y;
    }

    void Update()
    {
        transform.position = new Vector3(transform.position.x,
            originalY + ((float)Math.Sin(Time.time) * floatStrength),
            transform.position.z);
    }
}

Note that this code can be attached to an object either through New Object >> 2D >> Sprite and then by adding this code as a script, or through the following code programmatically:

    GameObject object = new GameObject();
    object.name = "Floating Box";
    object.AddComponent<SpriteRenderer>().sprite = SOME_SPRITE;
    object.AddComponent<FloatBehavior>();

Here's an example of what this could look like:

Edit: Fixed compile errors