Physics & Collisions - Learn SpriteBuilder for iOS Game Development - page 121

Ignoring Collisions with Categories and Masks
Before a collision callback method is sent, the physics engine first evaluates a body's
Categories and Masks properties. These are used to sort out collisions before the ac-
tual collision detection, resolve, and callback code is performed.
It is preferable to use collision Categories and Masks to filter out any noncolliding
bodies whenever you can because it's more efficient than returning NO from the Begin
contact callback method.
The masks use the same identifiers as the categories. To you better understand this, it may
help to think of Masks as a short name for “collides with categories” in the rest of the text.
There can be up to 32 unique category identifiers, which are simply arbitrary strings that
you enter in SpriteBuilder. Both Categories and Masks fields can contain multiple
identifiers. An example of these settings is shown in Figure 4-14 .
Figure 4-14 . Example usage of collision categories and masks
Tip While working on your game, you should make note of the category iden-
tifiers and their exact names in a list. This helps you avoid mistakes like incor-
rectly naming an identifier, and you can keep track of how many of the 32 iden-
tifiers you've already used. You should also stick to either plural or singular
names, so you never have to wonder whether the identifier was enemy or en-
emies . This makes it easier to recall them.
Categories and Masks are both empty by default. The confusing bit here is that an
“empty” category means that this body is actually a member of all previously defined cat-
egories. Likewise, an “empty” mask means that the body is set up to collide with all previ-
ously defined categories. That means physics bodies default to colliding with all other
bodies unless their categories and masks are set to specific identifiers.