REJECT

From ZDoom Wiki

The Reject lump is a now mostly-obsolete lump used by the original Doom engine to cut down on the number of monster LOS (line of sight) calculations that must be performed by the engine each tic.

To create the REJECT lump, a number of bits is stored, equal to the number of sectors in the map squared. The bits are arranged in a table with each sector having a column and row, as follows (from the Doom Specs):

         sector that the player is in
              0  1  2  3  4

            +---------------
sector    0 | 0  1  0  0  0
that      1 | 1  0  1  1  0
the       2 | 0  1  0  1  0
monster   3 | 0  1  1  1  0
is in     4 | 0  0  1  0  0

A value of 1 in the table means that any LOS check between the two sectors should automatically fail.

So, for instance, assume the player is standing in sector 3 during a monster's LOS routine. The engine will run a check on the reject table, comparing the monster's sector with the player's. If it finds a 1 at the corresponding place in the table (sectors 1 - 3 in this case) then the monster will skip its LOS check, meaning it cannot be woken up by seeing the player. (It will still wake up if it hears the player, however) If there is a zero then the engine runs the line of sight calculation as normal for that monster (which still may not wake if it is facing away from the player, or if a wall blocks its view of him).

Because LOS checks are skipped entirely when a 1 is encountered in the REJECT table, special effects may be created by manually editing the table to add reject entries where they normally wouldn't occur. For example, by placing a 1 in the same column and row (essentially telling a sector to reject itself), all monsters within that sector will be unable to see the player when he's in the same sector. This can be used to create monsters that you can walk right up to (assuming they aren't already awake) without them attacking you. A one-filled reject table will make for a map full of monsters who are totally unaggressive; they'll only attack you if you attack them first.

It's important to note that the reject map applies to sight only. If a monster hears you, it will still wake up as normal (Unless it has its Ambush flag set) regardless of the REJECT entry. Monsters that take damage from player weapons will also always awaken.

Apart from the special effects previously described, the REJECT lump's only use is to reduce the number of calculations the CPU needs to make per tic. Because today's computers are far more capable than the 386/33MHz PCs originally targeted by id Software, it is not strictly necessary to include a REJECT lump, as there will be no noticible performance gain by skipping these checks. For this reason, ZDoom will work properly with a REJECT lump that is 0-length, to save space. Other ports (and Doom itself) need at least a 0-filled reject table of the correct length to function properly. You can use the utility ZDBSP to create a 0-length REJECT map by using the -r option.