Hall of mirrors effect

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A hall of mirrors effect in Doom.

A shimmering HOM seen due to a missing door-side texture definition, in Doom II.

In computer graphics, the hall of mirrors (HOM) effect is a visual anomaly caused by missing or broken surfaces during 3D rendering. When a surface is not drawn, the area of the screen where it would have rendered retains the contents that it had on the previous frame. This can create a feedback loop with an effect similar to two mirrors reflecting one another, which is the origin of the term. The effect is especially dramatic when the rendering engine uses double or triple buffering, because the area will flicker between several different images even if the point of view does not change.

The concept of the hall of mirrors was popularized by the computer game Doom, where individuals have the capacity to create their own maps for play in the game. As it is easy to accidentally create maps with missing walls or broken textures, these maps often displayed the hall of mirrors effect. These holes caused a "shimmer" effect and would repeat or "mirror" whatever else appeared on the screen at the time.

The hall of mirrors effect is most commonly achieved by noclipping into an area where the character is technically not allowed to gain access to, such as map boundaries. Many modern game engines compensate for this effect by causing any area outside the bounds of the map to render a particular solid color or texture using a skybox. Also, in some games like Unreal Tournament, touching a HOM results in instant death. Unreal Engine games have the capability of using multiple skyboxes at once, however incorrect usage results in a HOM.