Question

I have a simply Box2d body with a triangular polygon shape and I’m trying to rotate the body. The code below appears to work; I’ve checked the body angle before and after and it has changed as I expect.

body.setTransform(body.getPosition(), body.getAngle()+TURN_ANGLE);

The problem is when I draw the polygon the vertices remain unchanged. Is this how it should be? I did wonder whether I need to apply a transform to the vertices but can’t find a way to do it.

Any pointers would be very much appreciated.

Answer 1

Originally my drawing method did something like this:

    PolygonShape src = (PolygonShape) f.getShape();

    for (int i = 0; i < src.getVertexCount(); ++i)
    {
      Vec2 srcPt = src.getVertex(i);

      // build polygon up of these points and draw ...

I’ve managed to get it working now by changing the code above to this:

    PolygonShape src = (PolygonShape) f.getShape();

    Transform t = new Transform();
    t.set(new Vec2(0,0), body.getAngle());

    for (int i = 0; i < src.getVertexCount(); ++i)
    {
      Vec2 srcPt = Transform.mul(t, src.getVertex(i));

      // build polygon up of these points and draw ...

Seems to work as expected.