ofxBox2d : How to disable collisions between two shapes?

Can you explain how to disable collision effect between two shape with ofxBox2d?
Thanks you!

Collision filtering is a system for preventing collision between fixtures. Collision groups let you specify an integral group index. You can have all fixtures with the same group index to never collide if you use a negative index:

fixture1Def.filter.groupIndex = -8;
fixture2Def.filter.groupIndex = -8;