Am I right in thinking that the sound-blocking flag only works in one direction?  In E1M4, monsters in sector 97 are awakened by gunfire in sector 82, despite the sound-blocking linedefs between them.    Ryan W 02:44, 4 March 2006 (UTC)

Heh, wrong.  Sounds are only blocked if they have to cross at least *two* sound-blocking linedefs to reach the "target" [1].    Ryan W 22:54, 5 September 2007 (UTC)

Flag 0x0004 isn't exactly two-sided[edit]

I did some really weird experiments (like having an open sector for example) and I can tell you that it is NOT the "two-sided" flag, it is the "transparency" flag. If you deset this flag in a level, you will find that there is a void, but it still has two sides, since you can just walk through it. If you want two-sides, make two sidedefs. --GreaseMonkey, 06:07, 6 September 2007 (UTC)

The two-sided flag controls whether the "back" side of a linedef is displayed. From p_setup.c, the code that loads the level:
       li->frontsector = sides[ldef->sidenum[side]].sector;
       if (ldef-> flags & ML_TWOSIDED)
           li->backsector = sides[ldef->sidenum[side^1]].sector;
           li->backsector = 0;
What this means is that the "front side" of the line is always set, but the "back side" of the line is only set when the two sided flag is set. Basically, the second sidedef is ignored if this bit is missing.
It's important to note that two-sidedness is not the same as impassibility. Being able to walk through something doesn't mean anything - make a level with a single empty room and turn off the "impassible" flags on the walls and you'll be able to walk through them, but they're still one-sided lines. Fraggle 07:56, 6 September 2007 (UTC)
The point I was saying was that it's a transparency flag, but thanks for that info. --GreaseMonkey, 04:24, 11 September 2007 (UTC)
Well, sort of. It's used as a hint to the rendering engine to identify the difference between walls and "joining" lines.
I'm going to change this back to "two sided" as this is the standard terminology used both in the Doom source code and in pretty much every level editor I've ever seen. Fraggle 07:23, 11 September 2007 (UTC)

Flags 9 and 10[edit]

The flags table should mention flags 9 and 10 in doom-format maps, which behave differently to flags 9 and 10 in hexen-format maps. In vanilla doom they're simply ignored. In Boom-compatible ports, flag 9 is activate switch with pass-thru (equivalent to setting flag 10+12 in Hexen I believe). In Eternity and ZDoom, flag 10 activates the 3D middle texture feature.