Box2d and sprites

Question

I have very little experience with box2d, but basically it seems to me that it is possible to attach to it a sprite (that is, a bounding box enclosing the sprite itself) and let box2d to check for collisions.

Now, suppose that our animation is composed of N sprite (for example, a character walking) and that the sprites are not equal in terms of width and/or height of their minimal bounding box.

How do you do with box2d? Compute an average box to enclose the sprites or change the box width/height at every sprite change? The first method seems to fit well only those situations where the sprites bounding boxes are similar.

If that is not the case, the second way is surely the most precise, but I wonder if that is computationally heavy (I mean, to change the size of a box everytime we change the animation frame).

Any advice/ideas are welcome!

Answer 1

It depends on the game. In most cases you'll be fine with just some simple shapes that average the sprite over all animation frames. Something as shown here (image from Ray Wenderlichs/Andreas Loews MonkeyJump tutorial) :

You see that three circle shapes are good enough as average for this sprite (the two small circles left and right are just sensors to see at which side a collision happened).

If your sprites do extrem transformations from one stage of animation to another, or if you really need a high accuracy (think a beat-em-up, or some slow-motion close-up), then you should create separate bodies per frame (or animation phase). Then you add/remove the physics bodies at the appropriate time of your animation.

Answer 2

If that is not the case, the second way is surely the most precise, but i wonder if that is computationally heavy (i mean, to change the size of a box everytime we change the animation frame).

Boxes in Box.2D are represented as polygons. You can easily alter the size by getting the Fixture of the Body and then the Shape of the Fixture. The PolygonShape has a property that lets you access of the polygon data representing the shape and alter it however you need.

I do this often with ~100 bodies and have had no performance issues so far.

Edit: Bummzack is correct. However, after a bit of Googling about, if you call ResetMassData() and then relocate the box to its current rotation/position both the AABB and Mass data will be recalculated. Probably still faster and more flexible than swapping out new fixtures constantly.

Body.ResetMassData();
Body.SetTransform(Body.Position, Body.Rotation);

Example manipulation of verts. Remember that they are represented in local space around the position of the body.

var poly1 = (PolygonShape)Body.FixtureList[0].Shape;
var poly2 = (PolygonShape)Body.FixtureList[1].Shape;

poly1.Vertices[0] = End + perpNormal - center + normal;
poly1.Vertices[1] = End - perpNormal - center + normal;
poly1.Vertices[2] = Start + perpNormal - center - normal;

poly2.Vertices[0] = Start + perpNormal - center - normal;
poly2.Vertices[1] = Start - perpNormal - center - normal;
poly2.Vertices[2] = End - perpNormal - center + normal;