Rotation of sprite is not smooth

Question

I am making a speed meter of car. Its pretty simple when ever speed increases or decreases the meter pin moves accordingly. I have done the movement part of pin but I am unable to get a smooth movement.

    public Image meterPin;

    const float maxPinAngle = -116f;
    const float minPinAngle = 16f;

    float meterMinSpeed = 10f;
    float meterMaxSpeed = 210f;


    void Update()
    {
        changeMeterSpeed();
    }
    void changeMeterSpeed()
    {
        meterMinSpeed = userSpeed;
        meterMinSpeed = Mathf.Clamp(meterMinSpeed, 0f, meterMaxSpeed);

        meterPin.transform.eulerAngles = new Vector3(0, 0, getTransformRotation());
    }

    float getTransformRotation()
    {
        float totalAngle = minPinAngle - maxPinAngle;

        float speedNormalized = meterMinSpeed / meterMaxSpeed;

        return minPinAngle - speedNormalized * totalAngle;
    }

The userSpeed value is calculated in another function in this script. The only issue is that the movement is not smooth. Any ideas if I can fix the smoothness issue in this script.

EDIT:

Here is a gif to explain whats happening.

Thanks

Answer 1

You change the arrow rotation immediately when speed changes. To make the transition smooth, you have to store the arrow current state and change it slowly towards the target state.

// arrow shows a car's speed. These are bounds for what it can display
const float meterMinSpeed =  10f;
const float meterMaxSpeed = 210f;

const float minPinAngle =   16f; // rotation for  10 units of speed, degrees
const float maxPinAngle = -116f; //          for 210

// arrow's current rotation in degrees (z component of euler angles)
float currentArrowEulerZ;

void Update() {
    // find relative arrow position between min and max, it will be from 0 to 1
    float targetState = Mathf.InverseLerp (meterMinSpeed, meterMaxSpeed, userSpeed);
    // no need to clamp, it's already in Lerp

    // find arrow rotation which it will try to reach
    float targetEulerZ = Mathf.Lerp (minPinAngle, maxPinAngle, targetState);

    // move the arrow towards target, but only 20% this frame
    // so it will move smoothly, first fast then slower
    currentArrowEulerZ = Mathf.Lerp (currentArrowEulerZ, targetEulerZ, 0.2f);
    // change that number to make arrow faster or slower

    // actually change rotation
    arrowTransform.eulerAngles = new Vector3 (0, 0, currentArrowEulerZ);
}

Answer 2

I understand that no smooth movement = some times it moves slower and some times it moves faster.

It is a case of Update vs FixedUpdate. Unity has both. In Update you need to get the delta time since last frame. In FixedUpdate you can forget about that.

https://unity3d.com/es/learn/tutorials/topics/scripting/update-and-fixedupdate

The answer is then: use FixedUpdate or calculate your transformation including Time.deltaTime.

My personal preference: FixedUpdate.