Full brightness cheat

Full brightness is a multiplayer cheat which causes the screen to always be rendered at full brightness. There are two ways of doing this. The effect is similar to the lite amp power up, but permanent. This is considered a cheat as it gives the cheating player an unfair advantage over other players: in dark areas, the player will be able to see opponents perfectly.

The only conclusive way to detect the use of this cheat is visual observation of the player's screen as the game is played. However, on maps with extremely dark areas, viewing deathmatch recordings in which the cheat was loaded by one of the players could reveal circumstantial evidence.

Method 1
The cheat takes the form of a PWAD loaded at startup or a modified IWAD file, to change the COLORMAP lump. Each light level map in the colormap is changed to be equal to the full brightness colormap.

An alternative method is to repeat and duplicate the first 16 colormaps two times each (or the first 8, four times each), thereby dramatically increasing the brightness of the game, while still allowing the player to partially view partially invisible things (like spectres and players using the blur artifact). Partial invisibility is achieved via the colormap remapping the pixels behind the object to slightly darker shades, and a totally remapped colormap makes them almost totally invisible.

Method 2
A second approach is to simply edit the map being playing and set all sector light levels to 255. As long as the map's BSP tree is not rebuilt, this will not cause consistency failures. It must be done per each map play and is not a general solution for all maps. The changes can be saved into the Doom IWAD or loaded as a PWAD whenever the cheat is needed.

Related
A related multiplayer cheat is done by editing the executable file with DeHackEd (or other doom exe editor), and giving the player objects "full brightness" (causing their sprites to always be rendered on colormap 0). If done properly, this does not cause consistency failures and would dramatically increase the visibility of other players.

Source

 * This writeup includes text from Bright Doom2 cheat modification, used with permission.