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Dec 01, 2005



There are reasons other than the computational expense to avoid doing character-to-character collision detection. In fact, I'd say that the expense isn't even the largest reason -- if it were, I think we'd see more MMOGs with collision detection, even just niche ones. The larger reason, IMO, is that character-to-character collision detection can be seriously griefed. In UO, Felucca has collision detection between characters, but Trammel does not (for those who do not know, Felucca predates Trammel and is considered the more hardcore of the two). And even in UO, at least when I was a GM there a few years ago, players would complain about collision detection griefing on the Felucca side.

Think about it in terms of WoW. What if someone could block you from getting to a quest NPC? What if the crowd in Ironforge made it impossible to get close enough to use the Auction House, the mail box, or the bank? What if four or five players could physically box you in? What if a guild could create a line of players to block everyone else from getting to a certain spawn point?

Some may say that we should just make our areas bigger to allow for more non-overlapping bodies, and let in-game Customer Service deal with the griefing aspects. To those people I would say first, social areas in MMOGs must be carefully designed to encourage, rather than discourage, social interaction. Increasing the size of the AH-to-bank layout in Ironforge could potentially destroy its usefulness as a social hub. And second, Customer Service is already a hugely complex and expensive undertaking; anything that will add to the work load of the Customer Service should add significantly to gameplay.

Is character-to-character collision detection worth losing the social hub in Ironforge, hugely increasing the Customer Service costs, and losing players who get fed up with the constant hassle and griefing? Personally, I don't think so. But then again, I'm the sort of player who goes out of my way not to run through people or stand on top of them, anyway.


To wit, I remember one of my first experiences in SL was being bumped by someone landing and getting a popup asking me whether or not I'd just been griefed. It didn't look like anything malicious, so I said no.


Guild Wars does have collision detection, and tactics involving physically blocking doorways and the like are often useful (the party size limit of 8 restricts its practicality to unusually narrow passages). Because all PVE areas are instanced, and collision detection is switched off in towns, the "griefers blocking important NPCs" issue doesn't arise.

It does cause a few problems, though, mainly due to GW's fairly dim AI. It's not unheard of to find yourself blocked in a narrow passage, not because of griefing by a player, but because the henchman AI is too stupid to get out of your way. (Necromancers, who tend to be accompanied by a swarm of undead minions as well as the standard henchmen, are particularly prone to this. You just have to wait for most of your minions to decay and fall apart.)

(What is it with MMORPGs and crappy AI anyway? I used to think Half-Life 2's AI was bad, until I tried Guild Wars. Then I thought GW's AI was bad, until I tried WOW...)


I really do miss CD in MMORPGs.

However some limits do have to be placed on it. Asheron's Call had near perfect implementation of it, where all mobs had it and you only had it against PCs that you could attack.

PC wise it only really mattered on the PvP servers, where you would have people block the doors to force others to attack them if they wanted to get by.

However agaist mobs this forces lots of strategies and makes the size of the mob matter. So it would matter that you could position yourself in a corner so that less amounts of mobs could attack but at the same time you just blocked off all forms of escape.

Also some mobs just because they were small were considered a really grave threat. Under AC you lost power each time you were attacked, and once out of power you were basicly defenseless and attackless, aka dead. With a larger mob you could use the walls and passage ways to keep some of them away from you, however you get thoses smaller mobs and they could slip through cracks and move would be on you.

With CD no just running through a dungeon or along that narrow bridge or wall if there is a mob guarding it.
Also it can be used to solves one of the biggest problems with MMORPG invisibility/sneak. If you collide with a mob your invisibility/sneak could be dropped or hampered.


Despite, or perhaps inspite of, the griefing aspects of CD it is an important factor in immersion for me personally. When I played UO (long before Trammel, up until the "noto patch") it was probably the most immersive element of the game, considering the rest of the interface. It naturally lent itself to reasonable strategies in close quarters combat (PVP) that was infinitely more "real" feeling than the strategies adopted for PVE. (e.g. A huge demon being blocked in a doorway by a small bag of flour.)

As many games today echo the lessons learned from UO, it is deeply dissatisfying to see the importance of CD being completely dismissed. When my little gnome can stand in the same spot as 20 other toons there is a significant part of my suspended disbelief being ignored by developers.


Collision detection is alive and well in City of Heroes/City of Villains. Almost TOO well: until recently wandering NPCs, the street-level citizenry that's just there for appearances, would bump player characters out of the way if they walked into them. This seemed to offend players, and was regarded as a bug by developers. In combat, mobs and players can block the doors, but once a player locks on a target then the player's ranged attacks can't be blocked by other characters, just the environment.

CD was not a part of Star Wars Galaxies at launch, and is not yet implemented in the New Gaming Experience, although its developers plan for it. The lack of collision detection in world objects has been an important boon to decorators and tinkerers -- a friend once built an espresso cart and won an unofficial Architects' contest -- and of course it simplified combat. Here's an example of a lack of collision detection fostering creativity that ultimately built immersion by allowing a high degree of player customization (and, thus, a sense of ownership). I have read a few accounts of player concern that when collision detection is implemented, this sort of thing won't be possible.


Totally off-topic, but there's a really good Aimee Mann song with the title "Lost in Space". (Why do I always have a tendency to comment on the titles of Nate's posts?)

Now back to your regularly scheduled programming.


There are a suprising number of WoW players who, for whatever reason (new to PVP games, laptops, etc.) do not use the keyboard/mouse combination to move about, or are just very inexperienced at it. In dueling and PVP combat, I have used this to my advantage in many situations, and my ability to run straight through my opponent has been a key factor in my tactics. I will often engage my opponent, run through, and quickly 180 so that I can fight facing my opponent's back. These players have a tough time turning around to face me, and by the time they do, I've already gone through them again, forcing them to start rotating the other direction. Attacking one's backside has numerous advantages: for a rogue, it's obvious, and for everyone else, melee attacks can't be blocked or parried from behind. In addition, when I attack casters, I often interrupt spells that require line of sight by quickly stepping through the caster at the last second. There are also advantages I've used against PVE mobs related to lack of CD.

Besides all the potential (inevitable) griefing, if WoW were to implement CD, most of these combat tactics would not be usable. Of course, I'm not advocating one way or the other. I'm merely saying that it does detract from the realism of close combat.

Another similar issue is the ability of some types of spells and projectiles to ignore certain objects in the game such as trees, walls, mounds of dirt, etc., and how some have taken advantage of these tricks to the point where they're considered bugs or exploits in the game.


WWIIOL is PVP by design and has always had some form of CD. It has been heavily griefed so most of the obvious stuff, (objective blocking by units difficult to kill) has been dealt with by the devs.

There are also clip griefs which really take a lot out of the game experience.

Like another poster up top, I don't think I could enjoy an OG that had little or no CD, it is just too much disbelief to be enjoyable.



Player to Player Collision Detection is one of those features that many people think would increase the realism of a game world, but in fact most of the time decreases it. In the real world, you can turn your shoulders sideways to slip by someone blocking a door. You can push them if necessary. You can slip between their legs. You can ask them nicely and the pressures of social nicety will force them to respond.

In the virtual space, two things conspire to make things worse. First off, the collision spaces must be crude. Secondly, we simply lack the fine control of our virtual avatars to allow us to, say, contort our bodies. These are surprisingly non-trivial issues, and they really bring the griefing issue in social spaces to the forefront.

On the flip side, collision on a battlefield can make tactics much more realistic. Whether or not that makes it more interesting, though, is the real question. MMO combat is full of abstractions, hit points being the primary example. Players are thinking in terms of game terms. If collision in your game's combat model makes the game richer, you should consider it. Also if your game's combat model is made hokey by non-colliding players. On the other hand, if your game's combat model is made incredibly annoying by having collision turned on (which I have seen, to be honest), then a designer shouldn't be bound by the rules of the real world to follow suit. As always, the fun's the thing.


Being inclined to explore every last polygon of WoW terrain, I can tell you that one more ramification of the introduction of collision for avatars in such a carefully balanced game would be a deluge of access exploits. The people who cite collision-based player griefing merely provide examples of the general truth. Just because you model a more realistic world doesn't make your game more fun.

Others may share my experience that the 'haptic' sensations of a game center on the crucial physical mechanic of the game (in these examples, killing). The headshot I make in any given shooter with a scoped gun 'feels' softer, better. It mirrors closely the sensation of clean follow-through motion that continues after you've hit a really good baseball pitch or a really good tennis stroke. A haptic resonance after the successful event. That solid 'whack' sound accompanied by a big floating number when you crit in WoW conjures much the same sensation, whether I'm 4 feet away from my opponent, or I'm standing halfway through them.


CD appears to be popular at NCSoft...
That Chip Guy > Collision detection is alive and well in City of Heroes/City of Villains.
Ross Smith > Guild Wars does have collision detection, and tactics involving physically blocking doorways...
Doorways are one of the few combat zones in Lineage towns, and are often used for duelling - although the shopkeepers have generally been placed such that it is possible to trade with them without entering through the door, making the duels optional even when the door is blocked. CD is a key part of the strategy in castle sieges - in fact, it would be difficult to imagine defending a castle in Lineage if the attackers could simply run through the lines of defenders and the best you could do was chase the looters.


I think it's kind of a cop-out for developers to abandon collision detection because of player greifing concerns, ESPECIALLY when those developers have built a game which divides the playerbase into friendly and enemy groups. Specifically in WoW, clients already recieve the data as to who is attackable and who isn't attackable, and the collision detection algorithm could use this data in determining collisions. I don't believe it would be too farfetched to only collide only with hostile things.

But generally, this wouldn't be an issue if the problems from a collisionless system were properly addressed to minimize a playstyle that utilizes the lack of collision. Such as the tactics Psyae has already described.

But as a support Priest that almost exclusively played WoW's PvP content, i would have really appreciated collision to help with the assist trains.


Speaking of shopkeepers and doorways, I recall how shopkeepers would stand in the doorways of stores in the old character-based Rogue/Hack game, preventing you from entering or exiting the store under certain conditions. In a single-player game, CD seems mostly to be a Good Thing.

But then there are multiplayer games, and the access-to-content griefing that seems inevitable if CD is turned on for player characters. It looks like there are basically three options:

1) Allow it. Let your players deal with content blockers however they like. (Presumably CD implies PvP -- is CD useful/necessary if you don't implement PvP?)

2) Control it. Whenever some form of CD griefing shows up, add special code to prevent it. The rules that will dieterich described above for Asheron's Call are an example of this approach.

3) Incorporate it. As in judo where you use your opponent's momentum against him, find a way to work CD griefing into the architecture of the game, so that when it happens players have some way of imposing limited in-context consequences. (This is sort of a cross between the other two approaches.)

If you're OK with (or want) a "nature red in tooth and claw" kind of game, the first approach will work, but non-PvP types will be turned off. Developers who want PvP but also want to attract newbies and/or non-PvPers seem to prefer the control approach, which hammers loose ends down without spending a lot of time thinking about redesign (but which sometimes creates other unintended problems as a result).

I'd like to see more developers going for that last option. Let physical "presence" be a part of your world, but recognize how people will abuse that effect and create physical and social abilities that other players can use to counteract or circumvent the abuses. If you can do this in a way that fits into and deepens your world's fiction, that's even better.



Curious... but couldn't the Dev implement CD where it fitted? I mean if non-combat CD causes issues, just don't implement it. And if it enhances the combat experience, place CD in instances. Would this not be an effective solution?

-Nathan J.


As an aside to the above, realism doesn't always mean fun. For example. I would have perferred a SWG with CD on the PvP, but not on the world environment. I personally don't feel that CD is needed in a non-combat situtation. In the virtual world the important places are the interfaces (doors, quest givers, chests, etc.), the buildings are just decorative pieces that enhance the overall beauty of the world. Granted this would mean inside buildings, caves, etc. CD would have to be turned on to prevent people from running through walls, and that inside of buildings would be instances. But then... would that be bad?

-Nathan J.


You certainly can have different rules in different places. One thing you have to be wary of, though, is a world that feels inconsistent. The inconsistent feel is, for obvious reasons, truly a risk for an immersion breaker. Whether or not it breaks immersion as much as collision exploits in a CD-world or walking through people in a non-CD world is open for debate.


I second the idea of hostile-only collision detection, with an extra twist for the sake of immersion. When two friendlies "collide", this could engage a "slip-by" animation wherein the colliders sidestep past each other. This could allow crowds to impede movement, as moving characters shoulder past standing ones, but wouldn't permit blocking outright. It could even open up new gameplay possibilities: imagine having to chase down a thief through a crowd of civilians. This is where those mass fear effects could really come in handy...


As far as WoW goes, I think collision detection would be great and the player griefing would actually enhance the experience by creating another layer of problems for people to solve. It would mean that a big enough band of Horde characters could literally close down Stormwind until enough players banded together to fight them off.

You would need limits like you can't block people unless you have PvP switched on and maybe sub L10 are exempt but I could see it working.


Would momentom help solve Colliosion detection griefing? 4 People may be able to block a door but if speed and strength are used to determmine who can hold their ground running headlong into them should bounce things around enough to get someone through and even provide a more realistic simulation for tactical battlefield maunvering.

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