Can I swap out Farseer physics fixtures at runtime?

Question

I am working on a 2D side-scroller using Farseer Physics Engine v3.3.1.

In order to create a realistic physical skeleton for the player, I am using a method similar to the one explained here (See "Texture to polygon" section).

Below is my code for creating the player body:

uint[] data = new uint[Source.Width * Source.Height];

mTexture.GetData(0, Source, data, 0, Source.Width * Source.Height);

Vertices textureVertices = PolygonTools.CreatePolygon(data, Source.Width, false);

Vector2 centroid = -textureVertices.GetCentroid();
textureVertices.Translate(ref centroid);

mPhysicalDrawOrigin = -centroid;

textureVertices = SimplifyTools.ReduceByDistance(textureVertices, 2.5f);

List<Vertices> list = BayazitDecomposer.ConvexPartition(textureVertices);

Vector2 vertScale = new Vector2(ConvertUnits.ToSimUnits(1)) * Scale;
foreach (Vertices vertices in list)
{
    vertices.Scale(ref vertScale);
}

mBody = BodyFactory.CreateCompoundPolygon(mWorld, list, 1f, BodyType.Dynamic);

The player's sprite is animated, though, so the problem arises when switching states - I.e. the player is crouching or jumping. The physical body created above only reflects one state (currently standing).

Is there a way I could somehow create a body for each sprite and swap them out later?

---

UPDATE:

As suggested by @Nick Wiggill, I am creating one physical body per player state (I.e. walking, standing, jumping, etc) when the player is loaded, and saving them within my class. All the bodies have .Enabled = false; initially, and are only enabled when the player is in the state that corresponds to that physical body. The Bodies and their corresponding states are stored in parallel lists.

Currently, my class has a function SetState which takes care of changing the physical body when the state is changed:

new protected void SetState(object nState)
{
    //If mBody == null, the player is being loaded for the first time
    if (mBody == null)
    {
        mBody = mBodies[mStates.IndexOf(nState)];
        mBody.Enabled = true;
    }
    else
    {
        //Get the body for the given state
        Body nBody = mBodies[mStates.IndexOf(nState)];
        //Disable the current body
        mBody.Enabled = false;
        //Copy the current body's attributes to the new one
        nBody.Position = mBody.Position;
        nBody.LinearVelocity = mBody.LinearVelocity;
        nBody.AngularVelocity = mBody.AngularVelocity;
        //Set the current body to the new body
        mBody = nBody;
        //Enable the new body
        nBody.Enabled = true;
    }
}

Currently, when the game loads, a NullReferenceException is thrown when calling World.Step: mWorld.Step(Math.Min((float)nGameTime.ElapsedGameTime.TotalSeconds, (1f / 30f)));

In the above function, mBody is just a variable I have to keep track of the current active body. I'm not modifying the body in any other way outside of the above function, and none of the m* variables are referenced or saved anywhere outside of my class. Perhaps I am swapping the bodies incorrectly?

---

UPDATE: Created a new question with the newer, slightly more specific issue: Farseer Physics - IndexOutOfRangeException when calling World.Step after Enabling/Disabling a body

Answer 1

You can have more than one b2World (i.e. standalone Box2D simulation) in place at a given time... this is also used to implement wrap-around worlds. You could store spare bodies there. If you disable/sleep them all, it won't cost much, just a bit of memory.

The other alternative is to create the spare bodies (yech!) in a "no-fly zone", a remote part of the map that will never be traversed in normal play.

The cost per body, as you've probably guessed, is very light. You should be able to safely simulate several thousand bodies at a given time. Unless you are thinking about having huge armies of units in motion at once (which, given the area of interest around the player being limited, would be unlikely anyway after basic optimisation), I wouldn't worry about duplicating bodies too much; it all depends on your figures though.

I remember dealing with a similar issue to yours when prototyping once, and a similar solution came to mind. Can't recall if I got around to trying it, though.

Answer 2

It is actually really easy

for (int i = 0; i < origin.body.FixtureList.Count; i++)
{
     origin.body.DestroyFixture(origin.body.FixtureList[i]);
}
FixtureFactory.AttachCompoundPolygon(verticeslist, 1f, origin.body, null);

return origin.body;

Answer 3

Your sprites are drawn separate from your physics engine, so unless you need to provide proper collision detection when crouching, you shouldn't have to swap the body. Just draw the crouched texture on top of it.

If you really need to replace the body, you can consider creating all the bodies you need, and then set their position to something like Int.Min so that it doesn't draw on your screen. Then you can simply swap positions with the active body when needed.

Might be a few issues with this method, considering your body is dynamic.

The point though, is to try not to destroy/create bodies everytime your character crouches.