Question

I’m trying to animate a 2d sprite in my XNA 2D game, by the force of gravity. I’ve developed a very basic class to achieve the simulation effect. This is my sample code.

namespace Capture
{
class PhysX
{
    static Vector2 g = new Vector2(0.0f, 10.0f);
    public Vector2 pos, vel, acc, accumForce;
    public float mass;
    public bool USE_GRAVITY = true;

    /* Constructor */
    public void Engage(ref GameTime gameTime, uint maxX, uint maxY)
    {

        Vector2 F = Vector2.Zero;
        if (USE_GRAVITY)
        {
            F = accumForce + mass * g;
        }

        acc = F / mass;//This is the Net acceleration, Using Newtons 2nd Law
        vel += acc * gameTime.TotalGameTime.Seconds;// v = u + a*t
        pos += vel * gameTime.TotalGameTime.Seconds;// s = u*t + 0.5*a*t*t, 
        pos.X %= maxX;
        pos.Y %= maxY;

    }

    public void ApplyForce(ref Vector2 f)
    {
        accumForce += f;
    }
}
}

I’m calling the PhysX#Engage() method in Game#Update(GameTime gt) method.
The problem is this I’m not getting a smooth animation. This is because the position gets very large soon. To, overcome that I tried to take modulus, as show in the code with the Viewport.Width, Viewport.Height but still the position coordinates are not at all smooth. What should I do. How to make animation smooth? Please help.

Answer 1

I think that’s happening because you are taking elapsed time value as integer.

Try use double value of seconds that takes into account even the fractionary part:

flaot dt = (float)GameTime.ElapsedGameTime.TotalSeconds;
vel += acc * dt;// v = u + a*t
pos += vel * dt;// s = u*t + 0.5*a*t*t,

In addition:

This is because the position gets very large soon.

Well, you should keep the float value as close as possible to [-1,+1] range, otherwise you will get a loose of precision.
My advice is:

1. keep the float properties of your objects in a small range([-1,+1])
2. transform your values before drawing depending on your resolution