Question

I’m trying to make a system of “chunks” in Box2D – i.e. shapes that are attached together in a big lump, so that you can break chunks off into their own body.

Currently, my system is very basic (just starting really) and modelled after the “Breakable” test in the Box2D testbed. It’s a body with quite a few basic box fixtures (which will later become polygons). When you click a body, it deletes the fixture and creates a new body with the same fixture, rotation, etc in the same place as the old “chunk”.

However, I can only get it to create the new body at the big body’s origin
(->GetWorldCenter). What I want to do is find the global location of the fixture to be deleted and replaced, so I can make the new chunk there.

I’ve read some stuff about b2PolygonShape having a “centroid”, but I’m not sure how to access and use that…

To put it simply, I want to find the global location (as x+y or a b2Vec2) of a FIXTURE, NOT a body (that’s easy). The fixture will move with the body it’s attached to, but I only currently need the position for one frame (having it update would be useful too though).

Answer 1

For Box2D position of the body not so important. Body position is only shift for fixture coordinates. It do not impact to simulation process. So, I don’t understand, why you so care about this.

I made something like this, and can say, that using big body’s origin works fine. You can see into Box2D testbed to Breakable test. There also used big body’s origin. Only reason to calculate new coordinates that I see is strange behavior of SetAngle method.

If you still want centroid, look at ComputeCentroid, located at b2PolygonShape.cpp. Pay attention, method don’t declared at b2PolygonShape.h. You can copy code from cpp and use it like this:

b2Fixture* chunk = ...;
b2Vec2 chunkCentroidLocal = ComputeCentorid(chunk->m_vertices, 
    chunk->m_vertexCount);
b2Vec2 chunkCentroidWorld = bigBody->GetWorldPoint(chunkCentroidLocal);

p.s. Don’t forget to transform polygon points, when you will create new body. Simply find difference between big body and new body, and subtract it from every point of polygon.

b2Vec2 shift = chunkCentroidWorld - bigBody->GetWorldCenter();
b2Vec2 newVertices = new b2Vec2[chunk->m_vertexCount];
for(int i = 0; i< chunk->m_vertexCount; i++)
{
   newVertices[i] = chunk->m_vertices[i] - shift;
}