I just saw something very odd....

While playing through Map 21 of Memento Mori II, two Barons of Hell attacked and killed another Baron of Hell. I always thought monsters of the same type couldn't in-fight with each other but I just saw the contrary. It was quite entertaining to see.

When a monster somehow hits another monster of the same type with a melee attack, they will in-fight. Demons that bite other demons (pretty much only do-able with the invis sphere) will in-fight as well.

Good to know, thx.

http://doom.wikia.com/wiki/Infighting, under "Difficulties".

Also, timeline :P

Naked Snake said:

Demons that bite other demons (pretty much only do-able with the invis sphere) will in-fight as well.

Not doable at all. Monster melee attacks can never hit an unintended target. What can happen is that a Baron explodes a barrel which in turn hurts another Baron though.

Speaking of this, the wiki mention that archviles never become target of another monster... but in AV25 under vanilla, there is a room where you can provocate an infight between a cyberdemon and many archviles, how does it work ?

Graf Zahl said:

Not doable at all. Monster melee attacks can never hit an unintended target. What can happen is that a Baron explodes a barrel which in turn hurts another Baron though.

I beg to differ, as I have seen it happen.

Graf beat me to the barrel-bug post.
If you're familiar with running vanilla Doom demos, you can find examples of that bug (causing fights between Imps, Barons, Cacos and Revenants) on the OddDemos page.

Heh.

http://img230.imageshack.us/img230/9674/hellknightslt9.png

I think that under some very special cirumstances (I have absolutely no idea about what they might actually be), Vanilla DOOM can get some target pointers crossed. The best possibility for this to happen could be coincidence caused by the freeing of a Lost Soul followed by the immediate allocation of another thing in the exact same memory location. DOOM doesn't reference-count mobj_t's and so if such a substitution took place, a thing might blame the wrong object for damage done to it by a projectile or might start to target the wrong enemy somehow. It's extremely unlikely, but certainly possible, and this is just one way it could happen.

The reason I believe in this is because one time on the level of episode 4 that the demo plays on (I don't remember which E4Mx this was) I heard two spectres start to bite each other. It was like this:

*BITE*BITE*BITE*BITE*BITE*BITE*

This continued until I heard one of the spectres die, and then the other reverted to wandering around. I can't explain it, especially since it happened in an area of the map that I couldn't currently get to in order to see what was going on. There's no other plausible explanation for this other than some kind of undefined behavior. There certainly are plenty of Lost Souls on this map, which lends some support to my outlandish idea.

Naked Snake said:

I beg to differ, as I have seen it happen.

You may beg to differ but the melee attack code is written so that it is impossible to accidentally attack anything but the intended target.

Pretty much the reason, I figure, why melee attacks never miss when you're partly invisible. Monsters may seem to aim towards a different direction, but they *always* cause damage to their intended target (not counting being invulnerable, obviously).

The only way I've ever managed to have non-hitscan firing monsters of the same kind attack one another is via a barrel (before a certain doom version, can't remember which, monsters could even commit suicide this way, by accidentally hitting a barrel and causing damage to themselves).

In this light, it's true that when they fight, their projectiles still cause no damage to each other, but their melee attacks do.

Naked Snake said:
I beg to differ, as I have seen it happen.

Go to E1M8 in any version of DOOM under v1.5, use the partial invisibility cheat in the room with the demons and barrels and stand between two barrels pretty close to a demon while avoiding its attack by moving back each time, and you'll see it again.

K!r4 said:
Speaking of this, the wiki mention that archviles never become target of another monster... but in AV25 under vanilla, there is a room where you can provocate an infight between a cyberdemon and many archviles, how does it work ?

The cyberdemon will never target the arch-viles but the arch-viles will certainly target anything that hits them.

Sorry to veer off topic, but while we're on the subject of enemy behavior, there were a few other things I was curious about. Firstly, do Lost Souls infight differently than other enemies? Oftentimes, when a Lost Soul starts infighting, it will only dive bomb its demon target once or twice and then apparantly resume attacking me again without provocation or dispatching its current target. I don't believe any other enemies do this, so I was wondering if this was a special behavior like the Archie's "NoTarget" rule. Also, when a pain elemental dies, do the Lost Souls swarm at it's target or whatever killed it even if it wasn't already attacking it? I'm pretty sure they attack the Pain Elemetal's target, so whenever they go after whatever killed them, the Pain Elemental must have just switched targets as it was killed. For example, if a Revenant tracer hits a Pain Elemental and I kill the Elemental myself to avoid a swarm, will the Lost Souls spawned in death attack me or the Revenant?

See Lost soul target amnesia in the wiki. If anyone can add code related info there, be my guest (and maybe the article could also be split into two if technical aspects merit it).

Death-Destiny said:
For example, if a Revenant tracer hits a Pain Elemental and I kill the Elemental myself to avoid a swarm, will the Lost Souls spawned in death attack me or the Revenant?

They will attack you. Only the initial (spewed) attack of a spawned lost soul is determined according to the pain elemental's target. After that, its target is the player character or any monster that hits the skull (and there is only one retaliation per hit in the latter case).

Regarding the Lost Souls, it's true, and I'll add to the fact that this makes them a great nuiscense when trying to provoke infighting with other monsters, especially if the player has a short supply of ammo.

I try to get Lost Souls fighting amongst themselves whenever possible, target amnesia seems to be less of an issue that way. If you have room to manouvre they can also make good decoys for other monsters to chase.

I think I've only seen one melee that wasn't started by a barrel - between two Imps.

myk said:

The cyberdemon will never target the arch-viles but the arch-viles will certainly target anything that hits them.

It's strange because I can be far from the place and when I come back the archies are all killed by the cyberdemon... so why is he shooting on them if there isn't another target than the archies in front of him ?

The archies are probably dying from their own splash damage.

The reason for Lost Soul amnesia, which doesn't seem to be mentioned anywhere in the article, is that when a SKULLFLY'ing object hits a wall or another enemy, the game places it back into its spawn state. This is probably one reason why Lost Souls have no alert sound (the other being that it gives them stealth). The spawn state calls the A_Look codepointer, which causes the Lost Soul to search for a new player to target. In the event the Lost Soul cannot see a player or "hear" a sound that the player has made (keep in mind sectors remember any sound you've made forever), then he'll fall asleep. Since a Lost Soul charging into a wall typically won't see a player, this in turn causes the problem of Lost Souls falling asleep against walls when you haven't made any noise in their area.

Heretic Gargoyles and Maulotaurs exhibit the exact same behavior.

Grazza said:

The archies are probably dying from their own splash damage.

Oh well I totally forgot this fact, thx.