Scroll screen in XNA - Easing math (flick gesture)

Question

I've been trying back and forth, but I can't figure out the math on how to scroll my view (or rather offset all objects) when using the flick gesture. I would like the scrolling to have some kind of ease-out.

    public override void Update(GameTime gameTime, bool otherScreenHasFocus, bool coveredByOtherScreen)
    {
        float elapsed = (float)gameTime.ElapsedGameTime.TotalSeconds;

        //Some math to change 'matrixOffsetY'
        //I use 'matrixOffsetY' to offset my objects in Draw()

        base.Update(gameTime, otherScreenHasFocus, coveredByOtherScreen);
    }

Here's the gesture event

    public override void HandleInput(InputState input)
    {
        if (input == null)
            throw new ArgumentNullException("input");

        while (TouchPanel.IsGestureAvailable)
        {
            GestureSample gesture = TouchPanel.ReadGesture();

            switch (gesture.GestureType)
            {
                case GestureType.Flick:
                    {
                        //Set a variable with some math? Using:
                        //gesture.Delta

                        //gesture.Delta gives us pixels/sec
                        break;
                    }
                default: return;
            }
        }
    }

This shouldn't be that hard, but I have a brain-freeze :) Please help me out!

Answer 1

You can "lerp" (linear interpolate) between two values to gradually get closer (starting fast, ending slowly).

I assume you're scrolling in 2D. So the position is a Vector2.

Try something like this:

position; //current position of the object.
targetPosition; //set by scrolling, position should gradually come close to this
float weigth; //how much the weight is of the targetPosition compared to the position
//practically this means how fast position approaches targetPosition. 
//First try values between 0 and 1, for example 0.8f for starters.
public void Scroll(Vector2 ammount)
{
   //We assume scrolling the scrollbar 1 pixel down, means the object should go 1 pixel up.
   //So the targetPosition should move the same ammount opposite.
   //If you don't want scrolling to correspond 1:1, you can multiply ammount by a float.
   targetPosition -= ammount;   
}

public void Update(GameTime gameTime)
{
    //..
    //Executed every update, position closes in on targetPosition pretty fast.     
    position = Vector2.Lerp(position, targetPosition, weigth);

    //Because we Lerp position will only get extremely close to targetPosition. Never exactly at it.
    //most of the time this is good enough, if this isnt use the following code

    float omega = 0.05f; // the minimum dinstance between position and targetPosition before we clamp
    if(Vector2.Distance(position, targetPosition) < omega)
    {
        position = targetPosition;
    }
    //..
}

public void Draw(GameTime gameTime)
{
    //..
    spriteBatch.Begin();
    spriteBatch.Draw(texture, position, Color.White);
    spriteBatch.End();
    //..
}