Hall of mirrors effect
Under the Doom engine, if the no-clipping cheat is used to go outside the level, a shimmering mass is visible in the empty space outside, composed of parts of previous frames of animation. The same effect can be achieved by making a level with a wall which has no texture. This is called the Hall of mirrors effect. The name comes from the fact that it appears somewhat like being in a hall of mirrors. It is sometimes abbreviated HOM or HOM effect.
When the game renders the level to the screen, it draws it into a buffer, an area of memory. During gameplay, the previous contents of the buffer are overwritten by consecutive frames. However, if a player travels outside the level, there are no walls to draw, so Doom draws nothing. Instead, the previous contents of the buffer are displayed, left over from previous frames.
If a player turns around and looks back toward the level, they can see through the walls and inside the level. The floors of the level stretch outwards toward the screen edge in vertical columns. This is because the Doom floor and ceiling drawing system is like a flood fill algorithm: because there are no walls to bound them, they simply "bleed" down to the edges of the screen.
Additionally, a shimmering effect also occurs with the HOM effect. The reason for this is in the Doom engine video system. The Doom engine actually draws into four buffers in a rotating pattern. This is called page flipping. While Doom is rendering into buffer 2 or 4, the video card is displaying the contents of buffer 1 or 3 (so it is not possible to see the screen until rendering has completed). When rendering is complete, it reverses ("flips") the buffers: the video card displays the contents of buffer 2 or 4, and Doom draws the next frame into buffer 1 or 3.
Because of this, when there is nothing to draw, the leftover parts of previous frames are displayed. However, because of page flipping, the contents of two previous frames are shown, alternating back and forth. Because there is typically only very slight difference between frames of animation, there is a shimmering effect. Most newer source ports do not use page flipping, and thus lose the shimmering effect.
Deep water effect
Certain levels have exploited the HOM as a feature; for instance, it can be used to make invisible pits, for a "deep water" effect (as with the easternmost room in MAP13: The Crypt of Plutonia). The edges of the pit have no texture, so the floor bleeds over the step. If the player dies while in the water, his viewpoint falls below the waterline and the shimmering hall of mirrors effect is visible on the missing texture, similar to being underwater. However, this procedure can cause problems on modern hardware-accelerated source ports, which do not behave the same as the original software-rendered version of Doom.
HOMs can be a problem for level designers. Properly designed levels will not have any areas where a HOM is visible. For this reason, many level editors include checking facilities to search for locations where textures are not set and HOM is likely to occur.
Team TNT's Boom modification of the Doom engine includes a special feature to assist in HOM detection: the cheat code "TNTHOM" enables it. When enabled, the screen is cleared before rendering. The clear color alternates between black and red, giving a "flashing red light" effect where a HOM exists.