Camera Positioning

As part of the game, the user should try to avoid crashing into walls. This should be humanly possible to do. The ship travels at 4 units/s of velocity and from the center of the screen to the top edge is 0.5 of a unit. This means that it takes 1/8th of a second for the map to change completely. Human response time is a bit slower - about 1/4 to 1/5 of a second. A fun racing game should be on the limits of this as that is where a user is pushing the edge of what they can do. To achieve this we need to have about one unit of distance between the edge of the screen and the players ship. There are two options:

  1. Zoom out the camera. This could cause loss of visibility as the players ship becomes small
  2. Place the ship off-center away from the ships center of motion. This could be confusing when the player makes a sudden motion.

I think a combination of both will work best, using the players velocity to move the center position of the camera and to zoom out when the player is moving fast.

So let's create a rough outline of a system for camera positioning:


#![allow(unused)]
fn main() {
pub struct Camera {
    position: Vec2,
    zoom: f32,
    target_position: Vec2,
    target_velocity: Vec2,
}

impl Camera {
    pub fn new() -> Self {
        Self {
            position: (0.0, 0.0),
            zoom: 1.0,
            target_position: (0.0, 0.0),
            target_velocity: (0.0, 0.0),
        }
    }
    
    pub fn reset(&mut self) {
        self.position = (0.0, 0.0);
        self.zoom = 10.0; // Start zoomed out so there is a nice "zoom" animation at the game start
        self.target_position = (0.0, 0.0);
        self.target_velocity = (0.0, 0.0);
    }
    
    
    /// Set information about the entity the camera is tracking
    pub fn set_target_information(&mut self, pos: &Vec2, vel: &Vec2) {
        self.target_position.0 = pos.0;
        self.target_position.1 = pos.1;
        
        self.target_velocity.0 = vel.0;
        self.target_velocity.1 = vel.1;
    }
    
    ///
    pub fn get_camera_matrix(&self, base_resolution: f32) -> [f32; 9] {
        Transform2d::new(
            self.position.0,
            self.position.1,
            0.0,
            1.0 / base_resolution * self.zoom,
        ).to_mat3_array()
    }
    
    /// Update the position of the camera, moving it towards the target
    /// position.
    pub fn update(&mut self, dt: f32) {
        // Do something fancy in here to position the camera
        self.position.0 = self.target_position.0;
        self.position.1 = self.target_position.1;
    }
}
}

What do we put in the update function?

Well, the ideal position is slightly ahead of the player, so:


#![allow(unused)]
fn main() {
    let ideal_position = (
            self.target_position.0 + self.target_velocity.0 * PREDICT_FACTOR,
            self.target_position.1 + self.target_velocity.1 * PREDICT_FACTOR,
        );
}

And the ideal zoom level is to zoom out the faster the player moves:


#![allow(unused)]
fn main() {
let velocity = length(self.target_velocity);
let ideal_zoom = 1.0 + velocity * ZOOM_FACTOR;
}

To avoid the camera position moving wildly when the player changes the ships direction, the camera should move smoothly towards the ideal position, so let's use a proportional controller:


#![allow(unused)]
fn main() {
let zoom_err = self.zoom - ideal_zoom;
let pos_err = (
    self.position.0 - ideal_position.0,
    self.position.1 - ideal_position.1,
);

self.zoom -= zoom_err * dt / SMOOTHING;

self.position.0 -= pos_err.0 * dt / SMOOTHING;
self.position.1 -= pos_err.1 * dt / SMOOTHING;
}

Now it's a case of fiddling constants to make it play nicely. You can do some math to calculate constants to achieve exactly 1 unit of space ahead of the player, but the end goal is for it to "feel nice" rather than be precise. In the end, I found some nice constants were:


#![allow(unused)]
fn main() {
const PREDICT_FACTOR: f32 = 0.6;
const ZOOM_FACTOR: f32 = 0.125;
const SMOOTHING: f32 = 0.4;
}

The result is:

Now compare it to the previous page. The game is exactly the same, but you can probably fly around without crashing now!