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glBSP and Map-Tricks/Errors
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===========================
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1. Self-referencing Lines
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This trick involves linedefs with the same sector reference
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on both sidedefs. In the original DOOM renderer, segs from
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these lines are virtually ignored (though any mid-masked
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texture is still drawn). Hence by using a sector which is
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different from the surrounding sector, people could create
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the following special physics effects:
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1. invisible walkways (self-ref sector floor is higher)
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2. deep water (self-ref sector floor is lower)
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3. force fields (self-ref sector blocks movement)
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4. railings you can jump over.
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glBSP detects these lines and does two things: prevents
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spurious warnings about mismatched sectors, and adjusts
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the final subsector so that the FIRST seg is not one on
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a self-reference line. It does the latter because most
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engines use the very first seg to decide what sector to
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associate the subsector with.
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Engines should be aware of the trick and perform their
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own analysis to emulate it. NOTE this trick will cause
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some subsectors to contain segs from multiple sectors.
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This trick is mostly used to make higher mid-masking textures.
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glBSP can detect perfectly overlapping linedefs (ones using
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the same start and end vertices, possibly swapped), and will
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IGNORE them (producing no segs from the second and subsequent
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Engines should also detect overlapping lines and emulate
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the trick (by rendering the additional mid-masked textures).
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This trick uses a one-sided linedef where a two-sided linedef
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should be, so the back of the one-sided line faces into a
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normal sector. In the original DOOM renderer, there was no
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back-facing seg in the BSP, hence you could see through the
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line when viewed from the back. When viewed from the front,
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the one-sided linedef acted normally (blocked your view).
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glBSP can detect many usages of this trick (although not 100%
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since that would require much more expensive analysis). It
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has to be explicitly enabled with the -windowfx option (or
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for libglbsp via a field in nodebuildinfo_t).
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When found, glBSP will add a REAL seg along the back of the
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one-sided linedef. This is the only time that segs are
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placed on a linedef with no corresponding sidedef.
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Engines should detect that and emulate the trick accordingly.
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Depending on how your renderer does occlusion culling, it may
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not be necessary to do anything special.
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4. Missing Lowers/Uppers
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This trick is mentioned since it is very common, though it
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doesn't affect or interfere with node building in any way.
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When the DOOM renderer encounters a missing lower texture,
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and the floor on the back side is visible (z < camera.z)
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then the space where the lower texture should be will be
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filled by that floor texture. The same thing happens with
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This effect can be used to create a deep water effect, or
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for drawing a dark ceiling over a bright lava sector.
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When the back floor is out of view (z > camera.z), then
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the missing lower is not drawn at all. Same for missing
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uppers. This is sometimes used to create "invisible"
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PWADs often contain a few mapping errors in them, and even
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the IWADs contain some small errors, for example:
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Doom1 E3M2: Linedef #156 has a wrong sector reference.
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Doom2 MAP02: Sector #23 is not properly closed.
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Doom2 MAP14: Linedefs #964 and #966 are missing back sidedefs.
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Hexen MAP02: Vertex #1370 is not joined to linedef #668.
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glBSP is not psychic and cannot automatically fix errors
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in maps (and besides, sometimes they are used deliberately
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for special effects, as described above).
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When a linedef has a wrong sector reference, the usual
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result is that the subsector (or subsectors) near that
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spot will contain segs belonging to different sectors.
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I cannot say what best way to handle it is -- engine
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developers should at least be aware of the possibility.
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Other map errors are more serious and cause structural
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problems, like degenerate subsectors (which contain only
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one or two segs). There is nothing glBSP or the engine
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can do in these cases.
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TRICKS.txt
