piston.filter.categoryBits = 1;
piston.filter.maskBits = 2;
body.createFixture(piston);
RevoluteJointDef rjd = new RevoluteJointDef();
rjd.initialize(prevBody, body, new Vec2(0.0f, 17.0f));
world.createJoint(rjd);
PrismaticJointDef pjd = new PrismaticJointDef();
pjd.initialize(ground, body, new Vec2(0.0f, 17.0f), new Vec2(0.0f, 1.0f));