A mask is another term for a bitfield
used to only retrieve specific values
from another bitfield.
If there is a bitfield 01010101
but only the middle 4 bits
are desired, bitwise AND (&)
it with the mask 00111100
. This works because anything ANDed with a 0 is 0, and anything ANDed with a 1 remains unchanged, allowing the unwanted bits to be stripped.
A mask is also another name for a texture
that has transparency
--a masked texture.