How do I rotate and object around itself, and translate it at the same time?

Question

I have this animated gif of my problem.

I have a sprite object with a box collider, and I'm trying to rotate it around its center.

My rotation script consists of this :

this.transform.Rotate (0, 0, Time.deltaTime * rotateSpeed);

and in the translation script this is the code :

this.transform.Translate (0, -tranSpeed * Time.deltaTime, 0);

but as you see in the gif it does not give me the effect I desire. Can anyone give me what am I supposed to do to be able to rotate it around its center? Thank you.

Answer 1

From the docs on transform.Translate:

public void Translate(float x, float y, float z, Space relativeTo = Space.Self);

If relativeTo is left out or set to Space.Self the movement is applied relative to the transform's local axes. (the x, y and z axes shown when selecting the object inside the Scene View.)

So because you have not specified a space in which to apply the translation, it is applied relative to the object's current facing direction. As the object turns, so does its direction of travel.

You can fix this by setting transform.position directly, or telling transform.Translate to move relative to Space.World:

this.transform.Translate (0, -tranSpeed * Time.deltaTime, 0, Space.World);

Answer 2

It seems like your transformations might be accumulating over time. As repeatedly "stepping forward" and "rotating around your axis" will cause the black bar to walk in circles. I'm unfamiliar with Unity, but there are some general approaches that might be applicable:

1. You could try recalculating the object's position from its starting position, rather than from its current position.
2. You could try to make the translation transformation independant from the object's orientation. Right now, it seems the translation is using the black bar's orientation to calculate the local X-axis. Instead of a local coordinate system, you'll want to use the world coordinate system.
3. As a general remark, it is important to know that transformations are not commutative! This means that the order in which you apply them, can cause different results. As a general rule, transformations are mostly applied in this order: 1. scaling -> 2. rotation -> 3. translation. This makes sure your scaling and rotation don't cause a change in position. It also makes sure your scaling aligns with the local axes of the object.