Question about rigidbody2d movement calculations

Hi there,

So I'm making a simple 2D space game, and I have an orthographic camera (the main camera) that moves by the following:

FixedUpdate - Camera Movement()

Code (CSharp):

1. theCamera.position = new Vector3(theCamera.position.x + 0.02f, theCamera.position.y, theCamera.position.z);

For my ship movement, because of the camera moving (used for parallax stuff), I had to move the ship to keep up with the camera movement.

Start()

Code (CSharp):

1. prevPosX = theCamera.transform.position.x;

FixedUpdate Ship Movement()

Code (CSharp):

1. var cameraPos = theCamera.transform.position.x;
2. float translationX = Input.GetAxis("Horizontal");
3. float translationY = Input.GetAxis("Vertical");
4. float calculatedKeepUp = (cameraPos - prevPosX) * 7f;
5. Vector3 movement = new Vector3(translationX + calculatedKeepUp, translationY, 0.0f);
6. prevPosX = cameraPos;
7. rigidBody2D.velocity = movement * speed;

So basically the calculatedKeepUp is 0.02f * 7, which is 0.14f

The problem is, the ship still drifts with that value.
If I set the calculatedKeepUp to 0.1430f the ship is moving perfectly with the camera movement.

So, my question is...where in all that's holy is the extra 0.003f coming from??

 

I think it's a floating point precision problem...

I added some logs to my camera position and the ship position, and saw the following:

Fixed update 1:
Camera Pos: 3.14
Ship Pos: 0

Fixed update 2:
Camera Pos: 3.16
Ship Pos: 0.01959997

So, I was expecting the pos to move by 0.02f, but it did not.
Maybe due to it being set by rigidbody velocity maybe.

0.01959997, let's say 0.0196 x 7(ship speed) = 0.1372
0.14 (what I was expecting) - 0.1372 = 0.0028 or 0.003...Mystery somewhat solved!

This probably won't bug the player as they wont keep the ship still long enough to notice such a minor drift, but I don't like leaving things unknown (even though the velocity calculation is still a mystery)

 

Yup, it's exactly that.
Precision loss is an inevitability in any floating-point number calculations.

 

Yeah, I'll stop being so picky about decimal places unless it's absolutely necessary

I just checked, and the line:
float calculatedKeepUp = (cameraPos - prevPosX) * 7f;

actually returns 0.1399999...I really thought it would have been 0.14!

So there we go...Thanks floats !