An essay introducing examples and vocabulary for describing non-player controlled forces that affect player movement in real-time games.
Gravity
Gravity is constant and omnipresent. It’s the reason you are stuck on the ground while reading this article. In video games, with real time movement, gravity is also mostly present. It serves as a grounding (heh) force that introduces a challenge into movement. Vertical traversal is suddenly not as easy as it would be without gravity. Yet it is not only a hindrance to our movement ability, it is also a tool. It is a way to move down, a way to traverse space that is outside of our control – at least in comparison to moving left, right, forwards or backwards.
Gravity is a constant, affective force.
Constant force: A force that is ALWAYS applied, regardless of any factors.
Affective force: A force moving the player’s position, indifferent to the goals and desired movement of the player.
Role of Movement
Moving things across the screen is a way of creating gameplay. I discuss this extensively in my paper on momentum; TLDR: different ways of affecting player movement create different gameplay.
Having interesting affective forces creates movement systems that are diverse. Imagine a Mario, where gravity does not exist. Imagine a Mario where there is a wind pushing you forwards. Imagine a Mario where you bounce instead of jump. These are not classic Mario anymore, these are different games.
Against Gravity
It makes sense that we replicate an abstraction of gravity. We are used to this invisible sensation of being pulled down, of falling when stepping off a ledge. It is familiar even in its stranger abstractions, like the quadrupled gravity of Mario. The apple doesn’t fall far from the tree, so we tend to make games that feature abstract simulations of earth’s gravity. The simulations we create, however, are not affected by the gravitational pull of our home planet. They can simulate whatever we want. While unfamiliar, this offers us a chance to not fall, but to fly, when we step off a ledge, to speed up by taking turns or to slow down by hitting a rock.
Not just gravity
The following is a very incomplete, non-physics-respecting and non-academic list of forces that are not gravity. It features forces that are easily or not so easily replicated in our world.
No affective forces
Having no affective forces is an affective force. You jump, but you stay where you were. You push into a direction, but keep floating in that direction without stopping — à la the classic game Asteroids.
Turn Speed
In almost all racing games, it takes time to turn your avatar. It also affects the direction of velocity. The slower the turn speed, the more precise and calculative we need to be with the turns.
Wind
A force of irregular strength pushing you in its direction. Very similar to gravity except, you know, it does not pull you down. We can stop wind by hiding behind a wall, similar to how we “stop” gravity by standing on ground. Lastly, wind can be a variable force.
Variable force: A force, the strength of which can change based on some condition. For example, wind gets stronger, or weaker, over time.
Magnetic Pull/Push
A magnetic force will push, or pull, depending on some avatar state (polarity, state, whatever you want). In addition, it will (can) also be affected by the distance between the objects. The closer they are, the stronger the force.
Ice
Ice is slippery. Think ice puzzles in pokemon: step on it, and there is no more friction. Think no gravity zones: you step into them and your jump becomes a slippery ride along the Y-axis. Ice is traditionally conditional, being tied to an area in the game world. It is a conditional no-gravity/friction switch that is dependent on the player’s position.
Conditional force: A force only applied when certain conditions are met. Eg. standing in a specified location, performing a sequence of moves etc.
Collisions → One way platforms
Collision is an affective force, a very simple one. It pushes back with the same amount of applied force, to the point where the original force (movement) is cancelled out. A one-way platform makes collision conditional, depending on the approaching direction of the player.
Conveyor belt/Boost pad
Another conditional force based on the location of the player. It applies a constant (while standing on it) or an impulsive (upon entering) force. It can work well to reinforce a direction (boosts in racing games) or to encourage the player to explore different paths (“waterfall” zones in Flywrench).
Impulsive force: A force applied in a single moment (frame), usually in a high amount. Examples include a jump in a platformer, a boost pad in Kart racing games.
Explosion push
An impulsive force the direction of which is varied depending on the relative position of the avatar and the source. Its varied direction can create moments that rely on the player’s skill to pre-position. Based on the player’s position they can make beneficial use of the push – a classic example being the grenade jump.
Epilogue
I’m sure you already thought of an affective force that I missed in this rundown — and that’s the beauty of affective forces. There’s an infinite amount of them, especially when we consider that gravity is fake – at least in games. Our projects are never 1-1 simulations of earth’s gravity. Lose the shackles of 9.81 m/s2, make a game with cool movement.
