Last modified: Mon, 8 Apr 2013
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Collision filtering

Actually this has been explained already in Getting started, but collision filtering is relatively complex and apparenty not everyone understands the short explanation. This tutorial gives a more detailed explanation on how ep_shape_set_collision works.

// pseudo-code
if (body1.mask1 & body2.mask2 != 0 or body1.mask2 & body2.mask1 != 0) and (body1.group == 0 or body1.group != body2.group) {
    // collision
} else {
    // no collision
}

In the following examples, X = (1,2,3) means:

• The first collision mask of a body of type X is 1 (binary 00000000 00000000 00000000 00000001)

• The second collision mask of a body of type X is 2 (binary 00000000 00000000 00000000 00000010)

• The group of a body of type X is 3.

Most of the time you don't need groups. You can indicate you don't want to use groups by setting group to 0.

A = (1,1|2|4,0)
B = (2,1|2,0)
C = (4,1,0)

This means:

• A collides with A, B and C

• B collides with A and B

• C collides with A

Icuurd12b42 wrote an excellent explanation of this example:

You can use groups to filter collisions between small groups of bodies:

A1 = (1,1,1)
A2 = (1,1,2)
A3 = (1,1,3)
...

This means:

• A1 collides with A2, A3, ... (but not with other bodies of type A1)

• A2 collides with A1, A3, ... (but not with other bodies of type A2)

• A3 collides with A2, A3, ... (but not with other bodies of type A3)

• ...

This is very useful if your game uses lots of small groups of bodies that can't collide, because the number of groups is unlimited (1 to 2^32-1, actually). So if you have a car and you don't want the wheels to collide with the body of the (same) car, groups are a good choice to achieve this. Just put the wheels in the same group as the car.

You can also combine collision masks and groups:

A1 = (1,1|2|4,1)
A2 = (1,1|2|4,2)
A3 = (1,1|2|4,3)
B = (2,1|2,0)
C = (4,1,0)

This means:

• A1 collides with A2, A3, B and C

• A2 collides with A1, A3, B and C

• A3 collides with A1, A2, B and C

• B collides with A1, A2, A3 and B

• C collides with A1, A2 and A3

Comments

mask = (1<<(size+1))|1;

|1

mask = (1<<(size+1))//|1;

mask = (1<<(size+1))|1;

|1

mask = (1<<(size+1))//|1;

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