Collision with only certain objects?

[Jeeper]

I am confused by how people seem to think that the movement and other behavior in box2d can not be changed to whatever you want. I do agree that bumb is a great library. But I do not agree that box2d is this magically impossible thing that can only do 1 thing and is 100% impossible to use. Box2d is extremely solid, super easy to tweak and has a bunch of advanced features that are not present in other options.

[kikito]

I am confused by how people seem to think that the movement and other behavior in box2d can not be changed to whatever you want. [...] I do not agree that box2d is this magically impossible thing that can only do 1 thing and is 100% impossible to use.

Your confusion comes from a false premise. No one said it was "impossible" to modify box2d, or that "you could not change it". We said it is not the best tool for one particular job.

super easy to tweak

Well, I have do disagree with you on that. Granted that my experience is with box2d is limited - last time I used was before it had fixtures. But on that limited experience, implementing a platformer on top of it felt like swimming against the tide. I reflected upon that and came to the conclusion I wrote above (platformer physics aren't as realistic as box2d physics, and that difference requires significant work and tweaking to be surpassed).

It is doable. But not simple or easy.

[adnzzzzZ]

I have to agree with Jeeper here. The warning is silly and there's absolutely nothing wrong with making a platformer with box2d. You can control your physics calculations internally entirely if you want and just use body:setLinearVelocity(your_velocity_x, your_velocity_y). The only real things you have to learn is stuff like the question in this thread, but for that many resources already exist, like box2d's manual which is great, and this site: http://www.iforce2d.net/b2dtut/, which is also great. For the question in this thread specifically, this should help: http://www.iforce2d.net/b2dtut/collision-filtering, http://www.iforce2d.net/b2dtut/sensors