Box2D Group Indices

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