Question

I have a point that moves (in one dimension), and I need it to move smoothly. So I think that it’s velocity has to be a continuous function and I need to control the acceleration and then calculate it’s velocity and position.

The algorithm doesn’t seem something obvious to me, but I guess this must be a common problem, I just can’t find the solution.

Notes:

• The final destination of the object may change while it’s moving and the movement needs to be smooth anyway.
• I guess that a naive implementation would produce bouncing, and I need to avoid that.

Answer 1

This is a perfect candidate for using a “critically damped spring”.

Conceptually you attach the point to the target point with a spring, or piece of elastic. The spring is damped so that you get no ‘bouncing’. You can control how fast the system reacts by changing a constant called the “SpringConstant”. This is essentially how strong the piece of elastic is.

Basically you apply two forces to the position, then integrate this over time. The first force is that applied by the spring, Fs = SpringConstant * DistanceToTarget. The second is the damping force, Fd = -CurrentVelocity * 2 * sqrt( SpringConstant ).

The CurrentVelocity forms part of the state of the system, and can be initialised to zero.

In each step, you multiply the sum of these two forces by the time step. This gives you the change of the value of the CurrentVelocity. Multiply this by the time step again and it will give you the displacement.

We add this to the actual position of the point.

In C++ code:

float CriticallyDampedSpring( float a_Target,
                              float a_Current,
                              float & a_Velocity,
                              float a_TimeStep )
{
    float currentToTarget = a_Target - a_Current;
    float springForce = currentToTarget * SPRING_CONSTANT;
    float dampingForce = -a_Velocity * 2 * sqrt( SPRING_CONSTANT );
    float force = springForce + dampingForce;
    a_Velocity += force * a_TimeStep;
    float displacement = a_Velocity * a_TimeStep;
    return a_Current + displacement;
}

In systems I was working with a value of around 5 was a good point to start experimenting with the value of the spring constant. Set it too high will result in too fast a reaction, and too low the point will react too slowly.

Note, you might be best to make a class that keeps the velocity state rather than have to pass it into the function over and over.

I hope this is helpful, good luck 🙂

EDIT: In case it’s useful for others, it’s easy to apply this to 2 or 3 dimensions. In this case you can just apply the CriticallyDampedSpring independently once for each dimension. Depending on the motion you want you might find it better to work in polar coordinates (for 2D), or spherical coordinates (for 3D).