# Box2D Group Indices

*12/20/2017 c85c0b21142eddd43b7189190e9da07c*

## What are Group Indices?

The group index flag can be used to override the category or mask of a *fixture*. As the name implies it can be used to group certain fixtures together to describe if they should or should not collide, a good example would be Friendly Fire, friendly players don't get hit by friendly bullets, vice versa.

A group index is an integer ranging from -32768 to 32767.

```
Fixture:setGroupIndex( index )
```

## How they work

- if either fixture has a groupIndex of
**zero**, use the category/mask rules as above - if both groupIndex values are
**non-zero**but**different**, use the category/mask rules as above - if both groupIndex values are the
**same**and**positive**, collide - if both groupIndex values are the
**same**and**negative**, don't collide

What we're really interested in are the last two points. \
I'll give an example, say we have two rectangles, `A`

and `B`

.

```
A.fixture:setGroupIndex(1)
B.fixture:setGroupIndex(1)
```

They both have the same index, `1`

, which is a positive number, therefore `A`

and `B`

will collide.

```
A.fixture:setGroupIndex(1)
B.fixture:setGroupIndex(-1)
```

Although `A`

and `B`

have the same absolute value, `1`

, they are not the same number, thus they fit into the point of non-zero but different.

```
A.fixture:setGroupIndex(-1)
B.fixture:setGroupIndex(-1)
```

Finally, both fixtures have the same index and are negative, therefore there is no collision between the fixtures.

### References

http://www.iforce2d.net/b2dtut/collision-filtering \ https://love2d.org/wiki/Fixture:setGroupIndex