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EarthQuake
9.5 on the Richter!


Posts: 2839
Registered: 05-03


Let's play a little game: get the BFG.

Yes, it's possible. The question is why?

Edit: First try with PrBoom.

Last edited by EarthQuake on 02-22-11 at 07:03

Old Post 02-22-11 06:47 #
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Maes
I like big butts!


Posts: 12666
Registered: 07-06


What version of prBoom is supposed to run this? Both latest prBoom (2.40 from 2008) and latest prBoom+ can't read the demo file.

Edit: my bad, I thought this was a direct link and tried to play back an HTML doc -_-

An educated guess is that you managed to cross the teleporter exactly on one of its sector's vertexes and thus defeated linedef detection. Was this done by TAS or just trial and error?

Last edited by Maes on 02-22-11 at 09:14

Old Post 02-22-11 09:09 #
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ArmouredBlood
Senior Member


Posts: 1911
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I mess around a bit and get it by accident, then proceed to wtf.

E: Missed the why part. Linedef skipping off of the increased speed from thingrunning. Hard to get the thing lined up right, though it helps that I'd guess there's about 20 units between the position your hitbox can hit the torch's and the position you'd trigger the teleport linedef.

Last edited by ArmouredBlood on 02-22-11 at 10:23

Old Post 02-22-11 10:18 #
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EarthQuake
9.5 on the Richter!


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Nope, no TAS. I actually found this in ZDaemon while bugtesting a teleport in a wad I'm working on. For some reason I would manage to pass through the teleport, and there were no obvious reasons why. So I decided to set up a more controlled environment and tried various things to reproduce the bug.

The decoration seems to have everything to do with it... or at least is responsible for it being so reproducible. I don't believe this is a case of moving too fast (thingrunning), thus literally skipping over the linedef entirely. I believe there is something far more sinister at work here.

I'm going with Maes' explanation; this probably has to do with hitting the vertex just right, and the torch maybe helps align the player position ever so slightly that this is more likely to happen. Although... I played back my demo with the torch removed; the demo didn't desync. Still not quite sure what it means. :p

Old Post 02-22-11 15:37 #
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Never_Again
knows his birth month


Posts: 981
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You guys make it sound more complicated than it is. You can simply skip the teleporter trigger with strafe-50, a common trick in speedrunning. Here's a vanilla-compatible demo, got it on the first try. You can watch it in the automap and see that the player never touches either torch or anything else.

edit:
tip#1: use prBoom-plus with Extended Demo Format turned on (Options -> General -> Options -> third page -> Demos section), the demos you record with it will then play back automatically, no need for frontends or command-line switches
tip#2: record with vanilla complevels unless the WAD requires a different one, your demos can then be analyzed with LMPC.

Once a properly controlled enviroment is set up, facts can be obtained easily enough. More facts -> less speculation.

Last edited by Never_Again on 02-22-11 at 17:19

Old Post 02-22-11 17:07 #
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Maes
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It's a bit disturbing to learn that teleporters -or any other linedef- can be defeated simply and effortlessly by SR50-ing. If that were the case, wouldn't it be a heavily used exploit in speedruns and the such? Or it's not really that easy to pull out consistently?

Old Post 02-22-11 20:19 #
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Wagi
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Posts: 344
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Maes said:
It's a bit disturbing to learn that teleporters -or any other linedef- can be defeated simply and effortlessly by SR50-ing. If that were the case, wouldn't it be a heavily used exploit in speedruns and the such? Or it's not really that easy to pull out consistently?
Even at max speed, it's pretty random whether or not it happens. Therefore it is rarely used in real speedruns but I've seen it exploited in TAS runs before (like Doom2 MAP08). Even so, as a general rule, the greater your speed, the more likely you are to "skip" a line action. The Fighter in Hexen has major trouble going through some portals because of this, and I've skipped over this one teleporter on Bloodfalls countless times because of all the acceleration you get from flying down the staircase.

Old Post 02-22-11 21:09 #
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Never_Again
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Although skipping linedefs is a simple trick it is not heavily used. It's not hard to pull off yet it's not guaranteed to work every time. More importantly, there are not many situations where it is useful. It's most useful in defeating ambush/obstacle triggers, I imagine, there isn't always enough room to get a run-up, though. A secondary use is getting into teleporters whose sectors have the secret attribute without teleporting. Not a common situation, mostly with old WADs, although this problem is still seen today occasionally.

Old Post 02-22-11 21:24 #
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Maes
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The "random" occurence could be explained by the necessary speed being right on the maximum attainable SR-50 speed, which then again is only consistently achieved with TAS tools, uncanny navigation, or cheating by mapping two button actions into one.

And then, you'd need to "cut" the linedef at a near-perfect perpendicular (normal) angle, or else part of the SR50 speed would get lost to sideways movement (tangential speed).

So the conditions seem to be actually two:


  • A travel speed equal or greater to the SR50 maximum
  • A linedef normal speed equal to the travel speed, with no tangential speed at all.

Old Post 02-22-11 21:35 #
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EarthQuake
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Just FYI, I never used SR50. In fact, I never even tried it with SR50 to see how much the odds increase of passing through it. Are you guys sure that's what's going on here? I cannot pull it off heading into the teleport straight on, I have to approach it from the corner...

Regardless, I find it disheartening that something I thought for years was fail-proof, in fact isn't. Thankfully it doesn't seem to be a very common occurance in normal play, and can be remedied by placing some failsafe lines on the teleport pad in case a player misses the first one.

Old Post 02-22-11 22:28 #
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Maes
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Well if that isn't the case then you might have discovered a different bug altogether -it might just have to do with a right-on corner approach, if speed alone can be definitively ruled out as the main culprit.

Old Post 02-22-11 22:45 #
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vdgg
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I'm not 100% sure, but from my experience SR50 is not a requirement, it just increases the odds. You may check the demo below for MM2, after 4 minutes of play I jump over teleporting linedef and I'm pretty sure there was no SR50 used.

http://doomedsda.us/lmps/675/1/m2141034.zip

Old Post 02-22-11 23:15 #
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Phml
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I'm 100% sure SR50 isn't a requirement, because I often skipped teleporters without trying to before I could SR50.

Old Post 02-22-11 23:18 #
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EarthQuake
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Okay, just confirmed it: it's a linedef skip.

And it only seems doable with an SR40 at max speed (and above). That seems to be the speed threshold here. Ignore all the speculative rubbish about the torches or vertex entry points; it's all irrelevant.

So I guess now the question is, how to prevent it? ;_;

Old Post 02-22-11 23:45 #
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Never_Again
knows his birth month


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EarthQuake said:
So I guess now the question is, how to prevent it? ;_;


EarthQuake said:
can be remedied by placing some failsafe lines on the teleport pad in case a player misses the first one.

Old Post 02-23-11 02:03 #
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EarthQuake
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Forgot to mention that sometimes that doesn't work. The only real solution seems to be the use of impassible lines.

Old Post 02-23-11 05:33 #
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ArmouredBlood
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Make the backup lines diagonal. More area to trip the linedef.

Old Post 02-23-11 05:43 #
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EarthQuake
9.5 on the Richter!


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I don't think that's true. The player is only going to intersect the line at one given point.

Old Post 02-23-11 05:51 #
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Never_Again
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Then build your maps the way ArmouredBlood does, using non-Euclidian geometry. ;)

Old Post 02-23-11 07:44 #
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traversd
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Switches. Lots of switches.

Old Post 02-23-11 08:29 #
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