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In a video I watched, the guy playing is doing a run of No Mercy on Solo Realism Expert mode. Near the end, where you have to wait on the helicopter, he jumped up onto a certain spot and waited, spamming his medkit. While he waited, there were practically no threats, as very few enemies spawned. What caused the minimal amount of enemies?

The video in question:

  • AFAIK, there are many factors that affect the Director AI's behavior for spawning enemies to make the game "more balanced". I'm not sure if spamming medkits may indicate that the player is currently having a difficulty, and thus the Director reduces the spawning rate... – antimo Apr 13 at 14:51
  • In this specific instance, I believe the reason few threats spawned is because the amount of zombies (and zombies per horde) is maxed in the instance. I cannot confirm this, as I can't seem to find the actual default server value for this mode. I've also encountered small horde sizes when playing on lower performance settings. – Nick S Sep 30 at 19:54
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This run uses two exploits to make the playthrough easier: a well-known glitch called the "Jesus room" that exploits the infected's ability to find of your location, and medkit exploit that abuses The Director's knowledge of your physical condition.

A brief summary of the Jesus room from GameFAQs:

"The Jesus room" is the name commonly used for the room under the satellite, on the rooftop finale of the No Mercy campaign. Once inside this room there is a glitch that makes it so no zombies can come in whether common, special, or tank. Eventually if you wait long enough both hordes of zombies will completely die, along with the two tanks prior to the helicopter's arrival...."

You can read how to access "The Jesus Room" on the Left 4 Dead wikia. Once your character is this room inside both common infected and special infect will be unable to reach you as the game believes the room is inaccessible. While the glitch was in frequent use zombies would not attempt to breach the room, and instead would turn back. Their programming would view the door in a similar way they would normally view a wall - inaccessible and cannot be broken. After a period of inactivity they would simply die off. However, the Jesus Room has since been patched out of the Rooftop Finale level and is no longer accessible.

The second, and less well-known glitch is the medkit exploit. Almost everything in Left 4 Dead is controlled by an enigmatic AI called The Director. The Director controls the quality of weapon spawns, the frequency of special zombie spawns and even the layout of some maps. The Director is capable of grace and pity in addition to it's default malice. Cooperative teams will be rewarded with more items and fewer zombie spawns when The Director see frequent assists and team members healing each other.

To exploit this glitch the speedrunner is almost continuously healing themself. Because of the constant healing The Director assumes they are in a bad physical state. The Director will soften the difficulty when players are hurting and spawn fewer zombies. In this case, now that the speedrunner has falsely convinced The Director that he is in a critical condition the game will take pity and spawn fewer zombies to harass him - making the final escape easier. To my knowledge this glitch has yet to be patched out

Both of these exploits in conjunction severely limit the number of zombies that spawn and make what would normally be a very difficult finale appear trivial.

  • There were some spots in maps (I recall a certain railing in the L4D2 concert finale) which would stop spawns and/or break the finale if all players stood on it. Wondering if this video is an example of such a spot. – Yorik Oct 1 at 16:05
  • Same thing in the lighthouse survival level with the cracked concrete column or the rock in the finale of Death Toll. AI pathing issues have existed for lots of levels in the game. Valve is pretty good at patching them out, though. – PausePause Oct 1 at 16:34
  • Jumping on the sandbags on No Mercy rooftop finale (next to the gun) prevents zombies from pathing to you. They will simply stand still. However it's a vulnerable location and SI can still get you, as can tanks. – NibblyPig Nov 19 at 11:26

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