Hitcan-blocking Vanilla linedefs

Some time ago, I was trying out a new mapper's first map, and the author was clearly inexperienced in dealing with linedefs. There were missing textures and rendering problems, and another peculiar accidental effect. In this outdoor-themed map was a stream or pool of blood, lowered from the surrounding sectors. Here's the strange effect: the linedefs at the sides of the pool stopped all hitscan attacks. Shots fired over the pool, or outwards from inside the pool, would turn into a bullet puff at the pit's edge. The lines were passable to the player, and looked perfect in both software and openGL rendering (in Skulltag). He did have lower textures around the pit - there were no visible HOMs or bleeding flats in it. This effect didn't occur anywhere else within the level.

Now, I'm interested in replicating this effect in a wad of my own. I've lost this wad, so I unfortunately can't copy it. I'm assuming that he accidentally marked those linedefs as single-sided, but whenever I try that, the line loses all transparency and turns into a giant HOM. If I try putting transparent textures on that line, the empty parts of the texture turn to solid black.

I'd like to be able to use this in a vanilla map - you could make a new sort of safe zone without mucking with the reject table. Does anybody know this trick? As I was playing this map in Skulltag, it's possible that this same thing can't be done in Vanilla, but the author did it accidentally, so I don't think any special line types are involved.

http://doom.wikia.com/wiki/Flawed_collision_detection
http://games.moria.org.uk/doom/research/shooting-through-things

I've seen this, but mostly in flawed levels made with older editing tools. Perhaps a buggy node build.

On eternal map07 I noticed some very odd line actions. I tested in zdoom, prboom, chocolate-doom and eternity. All these ports treated the tight cluster of lines differently. Still never figured out what it was.

Window by chaingun, berserk with chaingunner behind window.

Some ports let me shoot through. Some blocked shots. Some woke up the chaingunners. Some kept chaingunner silent. Some ports blocked chaingunners shots.

Hm, perhaps it is a blockmap error (as DJShrimpy appears to be suggesting). You could possibly create an effect similar to this by manually changing the lists of linedefs present in each block. Do any utilities exist for fudging around with the blockmap to add special effects? I suppose there's no real point in trying, as ZDoom and more advanced ports probably support all the line options I'd ever want. Still, there's something enticing about making a Vanilla map where the player still wonders "how did he make that?"

Catoptromancy said:

On eternal map07 I noticed some very odd line actions. I tested in zdoom, prboom, chocolate-doom and eternity. All these ports treated the tight cluster of lines differently. Still never figured out what it was.

Window by chaingun, berserk with chaingunner behind window.

Some ports let me shoot through. Some blocked shots. Some woke up the chaingunners. Some kept chaingunner silent. Some ports blocked chaingunners shots.

Weird. This effect may not be worth replicating then, since every port will treat it differently anyway.

WRT Eternal MAP07, it's actually a fairly simple trick. Open it in an editor and you'll notice a linedef that's not connected to anything on the East side of the window. Look closely and you'll see it's a self-referencing sector that's raised to the ceiling. This has the effect of blocking objects, but allowing bullets to pass. It works because it's only one line, so the engine doesn't consider it solid(I believe).

This effect is totally dependent on the nodes, though. Sometimes the properties of the linedef will be inherited by the surrounding sector, so if the port rebuilds the nodes it can change the behavior.

Andy Olivera said:

WRT Eternal MAP07, it's actually a fairly simple trick. Open it in an editor and you'll notice a linedef that's not connected to anything on the East side of the window. Look closely and you'll see it's a self-referencing sector that's raised to the ceiling. This has the effect of blocking objects, but allowing bullets to pass. It works because it's only one line, so the engine doesn't consider it solid(I believe).

Most source ports completely ignore any linedef with both sides in the same sector. With ZDoom it's a compatibility option so normally you can't shoot through invisible constructs made out of self-referencing sectors.