Alien Swarm: Reactive Drop Update - February 1, 2024

This month's update includes some large changes to Heroes of the Interstellar Armed Forces, as well as some balance changes for the Infection Deathmatch mode and several other fixes.

A note on Lag Compensation

The internet doesn't transmit information instantly. Information can only travel at the speed of light, and sometimes the speed of light is just too slow. For example, if a player from Frankfurt joins a lobby hosted in Beijing, even if their computers were directly connected by a very long wire, information would take 50 milliseconds to get from Frankfurt to Beijing and back. In reality, the information needs to travel much further than the 15500 km round trip because the information doesn't move in a straight line.

When you fire a bullet, the information travels a long distance through complicated circuits before it even leaves your computer, and then it spends time traveling through a series of other computers before it finally reaches the one hosting the lobby. But that's not the end of the journey. Once the lobby host has figured out what your bullet did, that information needs to travel back to you and every other player in your lobby. By the time your bullet reaches the lobby host, the alien you were shooting might have moved! But game engines have a few tricks up their sleeves to hide this from you, and because they can't shorten the physical distance the information needs to travel, they do something easier: time travel!

The information about the bullet you fired doesn't wait until it gets to the host and back before it shows something on your screen. First, the game running on your computer pretends the information about the bullet made a round trip to the host instantly. This is called "prediction" because the game "predicts" what the host will say happened. Prediction is shown in red in the images below.

Second, when the information about the bullet reaches the host, the characters in the game move back in time to check where your bullet would have hit. This is called "lag compensation", and is shown in blue in the images below.

Alien Swarm and Alien Swarm: Reactive Drop (prior to this update) lag compensation algorithm, shown with 300ms simulated round trip ping and default settings.

That's not good! If the drone hadn't been moving toward Vegas, he would have completely missed the shot! The lobby host rewound time, but it didn't do a very good job.

Here's what that shot will look like after this update:

Alien Swarm: Reactive Drop (as of this update) lag compensation algorithm, shown with 300ms simulated round trip ping and default settings.

Much better! If you have a configuration that sets cl_lagcompensation 0, we recommend removing that command as it means the lobby host won't time travel at all when you fire a bullet. The hotfix a week ago inadvertently marked cl_lagcompensation as a cheat; it can now be changed freely when not connected to a lobby.

Want to try it out for yourself? Use the newly-added sv_showimpacts 1 command.

Missions

• Tears for Tarnor: Oasis Colony Spaceport: Fixed a piece of the ceiling being solid to grenades near the security office and metal detectors.
• Tears for Tarnor: Oasis Colony Spaceport: Made a ceiling between the hangar and the baggage claim fade out sooner to be less distracting.
• Added a workaround for missing sounds in the workshop mission The Gauntlet: Arctic.

Weapons

• Fixed an animation issue causing aim to shift to the right shortly after the reload animation started, causing missed shots after successful fast reloads on weapons like the Precision Rail Rifle.
• Fixed newly unlocked weapons having their "new" marker removed by unlocking another weapon and then closing and reopening the game rather than only removing it when the weapon is selected in briefing as intended.

Lobby

• Leader and kick votes are no longer reset on instant restart.
• Prevented instant restart from being used more than once every 3 seconds (asw_instant_restart_cooldown). Does not apply to instant restarts performed after the mission complete/failed screen starts fading in.
• Fixed a visual bug causing the challenge to appear to be changed to "cmd_change_challenge" when exiting the challenge selection submenu immediately after opening it in the create lobby screen.

Misc

• Added a notifications list and filtering settings to the main menu.
• Added support for bold and italic text in inventory item descriptions in-game.
• Added rd_vindicator_fire_rate. If 0, it will use the previous behavior of inheriting its fire rate from the shotgun.
• Fixed medics having a 1-in-25 chance of shouting that they were out of meds when using the "Zing!" chat wheel command.
• Fixed a rare crash that could happen after stats were recorded at the end of a mission related to the upcoming crafting materials beta.

Infection Deathmatch Challenge

• Rifle grenade damage bonus is now calculated after the knockback force is calculated.
• Decreased rifle grenade damage.
• Massively increased rockets' knockback force.
• Increased rocket damage.
• Damage taken during the preparation phase is now immediately healed.
• Humans' burning damage against zombies now receives damage bonus regardless of whether the source of the burn is an incendiary mine.
• Constructed sentries are now deleted immediately.
• Allowed the server console (and challenge scripts) to use the deathmatch game mode console commands (in addition to the lobby leader).