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  • Something that's been on my mind recently is the  implementation of aim assist in fixed-perspective  

  • games like Hades and Death's Doorwhen aiming with an analog stick

  • When playing these games, I've had  experiences where I felt a disconnect  

  • between what I wanted to do, and what the  game's aim assist thought I wanted to do

  • Fundamentally, I felt like my issue was  that many of these games implement more  

  • of an aim automation system than aim assist. The effect is much more pronounced in Hades,  

  • where the weapon can snap by as much as 90  degrees, wrestling control away from the player

  • In Death's Door, the effect is much more  subtle, as the game never visualizes the  

  • correction it applies to the user inputand doesn't correct by more than 15 degrees

  • For the sake of this video, I reverse engineered  the aim assist code in both of these games,  

  • because I think it highlights the  types of gameplay design problems  

  • that aim assist attempts to address. I'll then present my own thoughts on  

  • how to better formulate the problem of  aim-assist in a way that makes it easier  

  • to think about how to balance things like  player intention, skill, and gameplay feel

  • Hades is a fast-paced game where players often  find themselves surrounded by fast moving enemies 

  • In general, every enemy must be dealt with  before the player can progress to the next room 

  • In this context, aiming is often secondary  to hitting, and so the aim assist in Hades  

  • appears to have been designed to ask the question  “What are the targets that the player might want  

  • to hit in this context”, and, “Which one is  the closest to the player's aim direction?” 

  • In theory, this ensures that any target  the game selects will always make  

  • incremental progress to clearing the room. Weapons which require line of sight will  

  • only snap to targets that have line of sight. Weapons won't snap to enemies that are off screen,  

  • that are currently invulnerable, or that are  outside of the range of the player's attack

  • For each potential target, the game uses the  current gameplay context to make its best estimate  

  • of whether that target is valid for aiming or not. For the targets which pass, the game selects  

  • the target whose direction is closest to the  player's input direction, and whose distance  

  • to the player and arc distance to the player's  input direction falls below some threshold

  • In Death's Door, the aim assist is  formulated in the exact opposite order

  • It first asks, “What are the hittable objects  closest to the player's aim direction”,  

  • then, “Which one is the player  most likely to want to hit?” 

  • This reversal is interesting because it allows  for multiple layers of context sensitivity

  • For example, puzzle elements have  higher priority than nearby enemies,  

  • which in turn have higher  priority than faraway enemies

  • Because both of these games snap the  player input directly onto a target,  

  • any mis-snap can lead to an instantly  frustrating experience for the player

  • This is mainly an issue when the player has a very  specific target in mind, because the incongruity  

  • between player intention and avatar action  leads to a very noticeably jarring experience

  • A common failure mode is when the player  intends to shoot at a target which is  

  • slightly out of range, but the aim assist  erroneously corrects the player aim to a  

  • completely different in-range target. This incongruity between intention and  

  • action can be especially frustrating if aiming at  the wrong target doesn't make any tactical sense,  

  • such as prioritizing a low-threat  enemy instead of a high-threat enemy,  

  • using a high-power attack on a weak enemyor using an AOE attack on a lone enemy

  • Aim assist is something thatwant to incorporate in my own game,  

  • and through some early prototypingtesting, and subsequent reflection,  

  • I've come up with a few ideas that I've  consolidated into what I think is a pretty  

  • flexible framework for thinking about the problem  and potentially addressing some of these issues

  • Let's represent player input  direction and avatar orientation  

  • as the x and y axes on a 2D phase graph. The map between player input and avatar  

  • orientation is a function, f(x)

  • When f is the identity function f(x)=x

  • player input is mapped to avatar  orientation directly without modification

  • The avatar points in exactly the direction of  input, as is the case when there's no aim assist

  • When f(x) is not equal to x,  it describes the transformation  

  • from the player input space to  the avatar orientation space

  • Now let's consider a potential  target near the player

  • The target has a particular angular position  and size from the player avatar's perspective 

  • The angular position changes as the  target moves around the player avatar

  • The angular size changes with distance, and is  also correlated with the target's physical size

  • When f(x)=x, the range of avatar  orientations that are on target  

  • is equal to the range of controller  inputs that are on target

  • However, if we select f(x) to widen the target  region, a greater range of controller inputs  

  • get mapped to on-target avatar orientations.

  • In more mathematical language, we can call this the preimage

  • of the target region B under the function f, and we can think of it as being  

  • the virtual size of the target from the  perspective of the player's controller

  • Even for a physically small target, we can  choose to maintain a large virtual target,  

  • making it easier for the player to input an  angle that points to somewhere on the target

  • In this way, we can imagine constructing  f(x) to always maintain a minimum preimage  

  • size of each target, no matter  their actual size or distance

  • This allows us to set a skill floor for how  hard we want it to be to aim at targets

  • Another way of analyzing this  function is by looking at its slope

  • Regions of low slope represent  regions where the function makes  

  • it easier to aim by decreasing sensitivity.

  • They must be balanced by regions of high slope,

  • to ensure the total domain and range fill up the full 360 degrees.

  • Beyond this constraint, we can make many  different decisions for the design of f(x)  

  • depending on our goal as a gameplay designer. For example, we may like to smooth the curve to  

  • reduce the jarring effect of a sudden change  of sensitivity as we aim toward a target

  • Perhaps for a precision weapon, we  want the player to feel a change in  

  • sensitivity at the center of the  target no matter how large it is,  

  • so we design f to always flatten  the slope at the center of targets

  • Perhaps we want to limit the extent  to which we enlarge the virtual size  

  • of targets on higher difficulty settings. Or to only provide the feeling of locking  

  • on target without actually affecting the  difficulty of hitting targets at all

  • Maybe we decide that for predictability, we want  to apply the same correction to all targets no  

  • matter how big or small they are. And in each case, we can globally  

  • decide how much assistance to provide by  blending it with the identity function

  • As a point of comparison, the sort of direct  snapping aim assist approach we've previously seen  

  • looks something like this in our visualization. We see that this indeed produces large virtual  

  • targets, which any useful aim  assist function should do

  • However, the direct snapping approach also  produces large output discontinuities,  

  • and sets of avatar orientations whose  preimage is completely empty, representing  

  • directions that are impossible to aim at. Consequently, any aim assist system using  

  • the snapping method must be highly  robust in its target identification,  

  • as any failure of the system results  in catastrophic gameplay consequences

  • The generalization of aim assist to include  these smoother functions thus represents a  

  • relaxing of the responsibility  of the aim assist algorithm,  

  • while still addressing the fundamental problem  of compensating for imperfect player inputs

  • In our search for the perfect function  that satisfies our game design goals,  

  • we may like to enforce a couple more restrictions. As previously mentioned we would like the domain  

  • and range of the function to be all  the angles between 0 and 360 degrees,  

  • with a circular topology. We might like to enforce  

  • that the slope of the function is never  negative, or larger than some maximum

  • We might also like to limit the magnitude  of the correction to some reasonable value

  • Beyond these simple constraints, we may  have other constraints we'd like to enforce

  • It might make sense to specify that perfect aim  on target should never result in any correction

  • And maybe we'd like to explicitly specify the sensitivity of f at each target depending on the target.

  • It may seem like a pointless complication to define all of these things mathematically, but there is some advantage  

  • in at least thinking a little bit about it. In my opinion, mathematics is the universal  

  • language of abstraction, and so if we  understand the mathematics of our problem,  

  • we can often find a wealth of solutions beyond  our immediate application that we can make use of

  • For example, these last two constraints  look a lot like specifying function values  

  • and derivatives at a set of points, which isproblem solvable by cubic spline interpolation

  • However, we'd quickly realize that cubic  splines don't guarantee monotonicity,  

  • which violates our second constraint

  • A quick search leads us to monotone cubic  interpolation, which handily solves our problem

  • There are still many open problems  that I haven't addressed by this point,  

  • but that deserve some thought when  implementing such a system in a game

  • How do we handle it when targets occlude one  another, if the projectile requires line of sight

  • How do we handle moving targets if the  projectile has a finite travel speed

  • How should we handle targets moving in and  out of range or line of sight during aiming

  • Should the aim assist correction magnitude  

  • change depending on how fast the  player's analog input changes

  • Does there need to be any sort of balancing  between mouse and controller input

  • I personally think these types of  questions map very naturally to the  

  • paradigm of thinking about aim assist asfunction from player input to avatar output

  • The production burden of this video has  already greatly exceeded my expectation though,  

  • so I'll have to leave further  exploration of the subject to you

  • If you're a game designer who's worked  on the aim assist problem before,  

  • I'd be interested to hear your thoughts  and experiences, things I've missed,  

  • things that did or didn't work out.

  • In any case, as always, thanks for watching.

Something that's been on my mind recently is the  implementation of aim assist in fixed-perspective  

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Designing a Better Aim Assist for 2D Games

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    jeff shih に公開 2022 年 10 月 08 日
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