Loop shapes

Choosing the loop shape type for a fixture will make a closed loop of edges from the vertices.

Setting a non-zero radius for the shape will make a 'fat' loop, constructed of circles and polygons.

Under the Display tab of the Options dialog, you can alter the opacity of the internal lines to make them either more or less prominent - an opacity of zero makes the internal lines fade out completely:

Ideally, loop shapes should not have self-intersecting segments as this may cause collision detection to behave incorrectly. Self-intersecting segments will be shown in red if they exist.