Filtering Collisions - how to?

cocos2d-x C++
#1

I’m reading Programming Guide Ch12 - Physics and I’m slightly confused how to setup and use CategoryBitmask and CollisionBitmask to filter collisions properly.

Say, I have the following physics bodies:

Body player, dumb, edge, barrier, bonus, tile

  • player can collide with dumb, edge and bonus
  • dumb can collide with dumb, edge and barrier
  • barrier can collide with dumb
  • bonus can collide with player
  • tile can collide with nothing

How to setup bitmasks and filter body collisions in such scenario?

#2

Have you checked them? good articles to tackle these mind boggling issues.
http://www.cocos2d-x.org/wiki/Physics#Collision
http://www.iforce2d.net/b2dtut/collision-filtering

if you can implement collision grouping system from box2d then please do share them i also need them :smile: for my next prototype.

Collision Detection not triggering
#3

Easiest way to think of this is to give each body a power-of-two number
player=1
dumb=2
edge=4
barrier=8
bonus=16
tile=32
This will be their CategoryBitMask

Now to calculate the CollisionBitMask

You want player to collide with dumb, edge and bonus
so the CollisionBitMask is 2 + 4 + 16 = 22.
dumb can collide with dumb edge and barrier
so the CollisionBitMask is 2 + 4 + 8 = 14
barrier collides with dumb
so the collisionBitMask = 2
bonus collides with player
so the collisionBitMask = 1
tile collides with nothing
so the CollisionBitMask = 0

Simple!

The way to works is because the numbers are powers of two, they represent a bit-pattern
00000001 = 1
00000010 = 2
00000100 = 4
00001000 = 8
00010000 = 16

So the collisionBitMask for player (22) is
00010110 = 22

Now, when you logically AND to binary numbers, the result is a 0 in every position except where BOTH numbers have a 1

so

0001 & 0001 = 0001
0010 & 0001 = 0000
0011 & 0001 = 0001

So, if I take the player bitmask (22) and AND with barrier category (8)
00010110 & 00001000 = 00000000 - which is zero - so they do not collide
AND with the Edge category (4)
00010110 & 00000100 = 00000100 - which is not zero, so they do collide.

9 Likes
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[SOLVED] Can someone please point out what i did wrong in my code for detecting collision in cocos2dx?
#4

Thanks, @Maxxx!

This post should be added to Programmers Guide :smiley:

#5

Happy to have it added if @slackmoehrle thinks it would be useful!

I guess having all this sort of stuff in one place would be good!

#6

Good ideas, @Maxxx Can you create an issue here: https://github.com/chukong/programmers-guide/issues

I am working on the v3.10 version and I will get this info included.

#7

I’ve posted a slightly updated version to the issue - feel free to use how you wish.

Cheers

#8

Thank you!

#9

Always a pleasure; I’ve knocked the documentation previously so only fair!!

Merry Xmas

#10

For more information on collision bitmasks you can check this video of ours out http://www.sonarlearning.co.uk/coursepage.php?course=cocos2d-x-v3-cplusplus-physics

#11

EDIT: Sorry Now I understand why power of two is required.

#12

So according to this how should onContactBegin be implemented?

#13

I’d start here …

http://cocos2d-x.org/docs/programmers-guide/physics/index.html