title: categoryBits/maskbits filter for LiquidFun
body: <p>My game has particle fluids using LiquidFun. Now, I want to have 1 physics object that only collides with particle systems and not with any other Box2D/LiquidFun fixtures (like an invisible shield around the hero that repulses water).</p>
  
  <p>I couldn't find any b2Filter parameters for <code>ParticleGroup</code> or <code>ParticleSystem</code> in the documentations and according to <a href="https://groups.google.com/forum/#!topic/liquidfun/HTwiNckBBzg" rel="nofollow">this</a> post custom filters doesn't even work.</p>