c++ - Rectangles render with gaps in between them (Box2D)

Right now I create my boxes where 1 meter is 85 pixels. Gravity is 10. And

                        fixtureDef.restitution = 0.1f;
            fixtureDef.friction = 0.5f;

            fixtureDef.density = 1.0f;

The problem I'm having is illustrated in the image I have provided:

As you can see, there is a small gap between many, but not all the crates. What could cause this?

Thanks

Answer

I guess that's some issue in box2d, there is such a bug to increase performance. since more accuracy needs much more computational power. to fix that issue I guess you only need reduce box physical sizes (for example you can create a 84px*84px rectangle for a 86px*86px image). you can also fade the pixels in the edges to get better results.