# Particles doesn't collide

Hi, everyone.
I cannot understand how collide system in Particle CHOP works: sometimes it works well, like in video tutorial, but sometimes it doesn’t and I can’t understand what I’m doing wrong. Can you help me with that?

In attached file, particles moves in XY-plane, their Z-coord doesn’t change. And there is a box, which i want to be a wall (so particles should rebound from it), but particles somehow simple goes inside it. Why?
I tried with 2D-grid unstead of box as well with same result
Collide.toe (3.85 KB)

Interesting, I think you found a bug.
It seems to be related to the limit planes. Try to put them back to 1000 1000 1000 -1000 -1000 -1000,
voila, here it works. (?)

hm, thinking about it again, I don’t think it’s a bug.
Your negative z limiting plane was at 0, I guess outside or on the limiting plane the collisions are not calculated. If you give it a little more room that should do it.

Thanks, giving some value to Z-limits helps in my situation.

But it is still strange behaviour when you use rectangle unstead of 3D-box (file attached). So, if I need only 2D picture of collided particles I still need 3D-wall for them to rebounce.
Collide.2.toe (3.96 KB)

hm, doesn’t matter right?
Also makes more sense to me to use a 3d thing to limit another 3d thing although you are not making use of one dimension. It’s still a 3d thing.

But is there to make both: particles and wall in 2D?
Just to save calculating resources, because with a simple box obstacle and points as particles it’s easy to calculate graphics, but I’m planning to use dynamic countor (from camera) and make particles with different color and size and in this case, probably, 3D-scene will reduce fps compare to 2D.

hm, everything will still be calculated in 3d, so I doubt there are any benefits to sticking to 2d.
Of course if you were working in 2d completely(I mean not using SOPs but coding a shader for it or the like) you would have a much more efficient system and could do more than in 3d.

Maybe someday I’ll be able to do it, but for now for me even visual coding (like in TD or vvvv) isn’t simple, so it will take long time 'till I’ll start to write shaders =)