Box2D Group Indices2017-12-20 02:38, 2 years and 1 month ago
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,
They both have the same index,
1, which is a positive number, therefore
B will collide.
B have the same absolute value,
1, they are not the same number, thus they fit into the point of non-zero but different.
Finally, both fixtures have the same index and are negative, therefore there is no collision between the fixtures.