Why do the OOB (out-of-bounds) areas of Doom look so unusual?enter image description here As seen in the image above, the Doom engine not only incorrectly renders the scene, but also renders slices of other scenes that aren't even loaded in the map. This is not the case in other versions of doom. Just the MS-DOS original. Why does this happen and how is it possible?

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    I know, from far too many hours of my teenage years (and far too many hours of cheating through every level), that this is not a valid screenshot from any version of Doom or Doom 2 from v 1.666 or onward. -- Can this be reproduced using an earlier version? Do you have a WAD file that can generate this? – Ghedipunk Feb 26 '20 at 5:20

This is known as "Hall of Mirrors" effect.

The Doom Fandom Wiki states:

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.

Source: https://doom.fandom.com/wiki/Hall_of_mirrors_effect

The old MS-DOS release of DooM renders the scene different than newer source ports do. There are some good resources online that cover your question better than I could, but this answer may give you an idea on where to start looking :)

  • I feel like the Wiki authors have extended the meaning of the hall-of-mirrors effect there; I really think that originally referred only to the shimmering caused by buffer flipping back and forth when nothing was being redrawn. But, whatever, it's an idiomatic term and you've definitely described the correct effect. – Tommy Dec 16 '19 at 17:18
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    IIRC there were three screen buffers in the cycle, which was what gave it that quality of shimmering movement as opposed to a sense of "flashing" that two alternating buffers would have. – smitelli Dec 16 '19 at 23:39
  • @smitelli yes, sorry, that sounds more likely. But, regardless, I just meant that I feel like hall of mirrors was originally intended to describe the effects of not-fully-erased buffers, rather than the effect of flood fill on floors escaping their room bounds. Though, again, to be completely clear: it doesn't inherently mean anything, LuNa has answered the question correctly with a source that definitively proves that's not what the community means by hall of mirrors now. So positive votes all round, thanks to LuNa for the helpful and well-argued answer! – Tommy Dec 18 '19 at 19:21

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