Monday, November 2, 2009

Combat speed and Threat mechanics

Two topics I want to expand upon, both to say it 'out loud', and to keep a record of thoughts. First, movement speed in relation to combat, and second, threat mechanics. Both with respect to the skirmish game design, naturally.

First, Speed.

Ever notice how in games everyone runs everywhere, all the time? This is because walking is very, very, very slow. So slow that no one ever walks in games, unless it's a social thing. Those quests where you escort some NPC who decides to walk? Very irritating. But equally irritating are escorts where they move faster than you can and leave you in the dust. There's a reason all the escorts in WoW move at 95% run speed. So you can catch up.

Now take the perma-run speed and bring it into combat. For a game without positioning being important, no problem. For a game where you control one person, no problem. But in a game where managing formations, lines, and special maneuvers are important, and where you control a plethora of peeps, run speed is just too fast. Nothing is understandable. Small breaks in the line are immediately filled, to the point where the line break isn't noticeable. For formations, motion, and knockback to be relevant, units need to SLOW DOWN. A lot.

So we slow them down. Now we have the irritating dilemna of the slow escort. Waiting for your units to get in position is intolerable. Crossing a skirmish to setup a trap is agonizing. Exploration is, well, a slide show rather than a whirlwind tour. It's meh.

We get around one problem in the Skirmish system by the nature of its design. Exploration is not done with combat units. We can have exploring units move at a run pace, or even substantially faster if we wish, with no balance concerns, since out-of-skirmish speed has no advantageous effect in combat. But this is a digression, easily solved and set aside.

The current plan is to introduce the concept of engagement. When a unit is engaged, they are considered in combat, and their movement speed slows dramatically. Now, how engagement works depends on the offenders and defenders. Shieldmen have a small engagement radius, but may slow their opponents more. Scouts may have a large engagement radius but only slow their opponents slightly. On the receiving end, a scout may reduce the amount of motion slowed. This makes scouts faster and more nimble in combat, but all units match outside of combat, when travelling.

There are lots of open questions here. Do engagements stack? Are there disadvantages to being multiply engaged? This is sort of a flanking determination. If you're engaged by 5 enemies it is certainly worse than being engaged by 1. But this does not account for support. If you are engaged by 5 archers, but surrounded by 15 shieldmen, you are still at an advantage. So perhaps we should integrate the concept of support, which is a sort of anti-engagement.

Or perhaps this is getting too complicated. The design may need to be simplified down to understandability.

So let me review the basic tenets: In combat, units move slower. This is to promote the efficacy of formations, breaking formations, in addition to increasing the 'parsability' of combat. That is, the ability of the player to understand what is happening and make interesting combat decisions.

The second point is related: Threat mechanics. The holy trinity of MMOs is the tank/healer/dps role distinction. Tanks manage threat, Healers manage tanks, dps manage damage. Threat mechanics are imperative to this design working, even if they are quite artificial. It doesn't make much sense, really, but it makes the game fun, and makes the roles possible. Without them each role blends together. Compare class roles in Diablo 2 to WoW, for example. Diablo has no threat mechanics, and therefore you have no control over combat, except to kill things faster. In its defense, that works quite well for a brawler, but less so for a game intended to be tactical in nature.

So, threat. That is, how the AI units decide who to strike. The most basic threat mechanic is based on incoming damage. Units attack whoever hits them hardest. The next simplest is that they attack whoever is nearest. Let's consider our goals first then see what tools we need to implement to reach them.

AI behavior needs to be predictable. The user needs to be able to understand them and setup a formation and units to overcome the enemy 'strategy'. In skirmish, there are 2 seriously differing cases to consider. First, let's ponder the simple case; independent units. Secondly we'll address enemy squads, which should behave at a group level.

Certain types of player units are built to be the wall. They are the tanks of the squad. The first of these is the shieldman, naturally. We need to make sure the shieldman can do his job. His job really is to block units from getting behind him. We could enforce a sort of literal interpretation of this and eschew threat entirely, but just requiring a solid wall of shieldmen. Any independent unit that decides to get through to the nukers just keeps trying to walk around until they get there. But really, that just looks ugly. It means walls don't work at all. In reality, the shieldman would be easily able to prevent a single unit from passing, just by staying in the way. But if the AI routines and movement system cannot handle that by itself, we'll have to cheese it using something else. One solution (clearly) is a threat system. A basic proximity system would both get incoming units to stop at the shieldwall and appear to engage.

But, well, that's boring. Everything will stick to your closest units and strategy is too simple. If there are 3 incoming units and one defender, that should be a problem, and the 3 shouldn't merely stick to the 1 defender while the 15 archers plug away.

So how bout this. Earlier we introduced the concept of engagement, as part of a combat speed mechanic. Let us reintroduce that. Give each unit type an engagement radius and an engagement magnitude. Furthermore, let's make the engagement magnitude decay (optionally) with distance. Secondly, and importantly, have the magnitude of the engagement drop off by a function of the number of units engaged. Now, what do to with the engagement magnitude calculation? Let's say every tick, we recalculate engagement, perform a weighted combat-speed adjustment as before, and introduce a threat mechanic. The more engagement, the higher the threat ticks up. An example:

Let us define a shieldman's engagement: Short radius, high magnitude, but high decay. It's very strong in melee but even 10 feet away is almost nonexistant. Let us also define an archer's. Wide radius, low magnitude; they don't really slow you down since they aren't in your face.

So a unit attacks your formation from the front; the shieldman is closest and the first tick produces a lot of threat, the shieldman stays in melee with the unit and easily maintains a threat lead. Second example, 4 units attack from the front. The shieldman is closest and manages in get in melee with 2 of them. Each gets a moderate amount of threat. The decay rate on the other 2 means they go for the archer. The shieldman is able to hold a few but is overwhelmed as intended. One more case to ponder: 4 shieldmen in a line that isn't touching but overlaps engagement radii, and many attackers. As each attacker comes in, if they engage anywhere in the center of the line, they will be slowed substantially by each defender's engagement, and will likely stick to the shieldmen. However, if enough pile in on one location, they can overrun that shieldman's engagement by splitting it among themselves, and break through the line. Or perhaps they have enough knockbacks to split up the shield line, reduce engagement count in the breach, and suddenly those units are at full speed.

But... why did they try and break through at all? Well, the nukers, of course. But, we haven't accounted for them at all! We need to make sure they're on the threat table and known to be the highest threat, and therefore the intended target. So, we introduce a damage-to-threat mechanic as well. Balancing the intensity of the two will be tricky of course. Maybe there's an alternate thought here...

Consider keeping engagement threat and damage threat distinct. Damage threat will continually accrue from damage and decay slowly, while engagement threat is only based on the current state, and has no history at all. The current target is still the greater of all the threat entries, but this gives a new desirable behavior. When the line is broken, engagement threat falls off and damage threat dominates. The line falls apart and the attackers flood the nukers. Dramatic! Or, if your wall is solid, nothing will ever escape, and your formation does its job.

Food for thought!

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