How to set centers of shapes/fixtures/bodies in Box2D

c++ object center box2d centering
Griffin picture Griffin · Jul 8, 2011 · Viewed 8k times · Source

Hey i'm trying to integrate SFML and Box2D, and SFML has made setting centers for sprites, shapes, etc. very easy. Box2D, on the other hand, I'm having trouble with as i can't figure out how to set or even find the center of a shape or fixture.

It seems to me that when adding vertices manually to a b2_PolygonShape the center is set to the first vertex in the vertex array, but the results are very different when using the shortcut functions SetAsBox() or any other SetAs__(). The Center is the middle of the shape, or half extent(s) of the box/shape.

I need to commonize box2D's and SFML's centering system, but i can't figure out how the local coordinate systems work for objects.

How do I set/get the centers of the many objects such as shapes, fixtures, bodies, etc. in Box2D?

Answer

Griffin picture Griffin · Jul 13, 2011

Ok I realized that SFML and Box2D really weren't all that different, but i just wasn't thinking about how shapes are made/ rendered correctly in world coordinates.

Unless otherwise specified, (0,0) is always used as the refrence point/ center of the object when its position is being moved around/ drawn, and each vertex is drawn in relation to that point.

the SFML tutorial was a bit confusing as it said setting the center of a shape/sprite was the offset from the top left corner of the object, not (0,0).

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