Enemy Racers

FLying by yourself around a map is pretty boring, it's time to make the other players fly!

Lets just wire it in quickly with a function:


#![allow(unused)]
fn main() {
use super::ship::Ship;

pub fn calc_ai_control(ship: &mut Ship, skill: f32) {
    ship.linear_thrust = 1.0;
}
}

And putting it in the core gameloop:


#![allow(unused)]
fn main() {
for ship in self.ship_entities[1..].iter_mut() {
    calc_ai_control(ship, 1.0, &self.map);
}
}

Yes, this does mean there's no state in an AI player - but lets face it, when you're driving around a map, you don't really consider what you just did. Another limitation is that we don't know where the other ships are. Due to the simplicity of this game, I don't think this will be a problem.

We have a function on our map that we wrote when finding the start positions, it's called get_track_direction and returns the direction the track is facing for a given polar coordinate around the track. Similarly, using the function get_track_radius which also takes in a polar angle, we can figure out where the full polar coordinates of where the ship would be if it were on the centerline of the track.

About here I noticed a bug that's been present for quite a while. I thought the glsl function for atan was atan(x, y) and the rust atan function was x.atan(y). But actually it's atan(y, x) and y.atan(x). Because I've used it consistently wrong, pretty much the only effect is that my track polar coordinates are out by 90 degrees. Whoops.

I haven't gone through and fixed the previous pages because functionally there's no difference, but here's the diff:

diff src/swoop_camera_positioning/src/map.rs src/swoop_enemy_racers/src/map.rs
34c34
<         let angle = position.0.atan2(position.1);
---
>         let angle = position.1.atan2(position.0);
59c59
<         const ANGLE: f32 = 0.0;
---
>         const ANGLE: f32 = std::f32::consts::PI / 2.0;
72c72
<         let delta_radius = radius_here - radius_a_bit_further;
---
>         let delta_radius = radius_a_bit_further - radius_here;
84c84
<             -angle - extra_angle
---
>             angle - extra_angle - std::f32::consts::PI / 2.0
86c86
<             -angle + extra_angle + std::f32::consts::PI
---
>             angle + extra_angle + std::f32::consts::PI / 2.0


diff src/swoop_camera_positioning/src/resources/map.frag src/swoop_enemy_racers/src/resources/map.frag
24c24
<     float angle = atan(position.x, position.y);
---
>     float angle = atan(position.y, position.x);

Right, that out of the way, we can animate our AI's flying around the map by writing directly to their position/rotations:


#![allow(unused)]
fn main() {
    let polar_position = PolarCoordinate::from_cartesian((ship.position.x, ship.position.y));

    let track_angle_here = map.get_track_direction(polar_position.angle);
    let track_radius_here = map.track_radius(polar_position.angle);

    let track_polar_here = PolarCoordinate {
        angle: polar_position.angle,
        radius: track_radius_here
    };
    let track_centerline_here = track_polar_here.to_cartesian();

    ship.position.x = track_centerline_here.0;
    ship.position.y = track_centerline_here.1;
    ship.position.rot = track_angle_here;
    ship.linear_thrust = 0.1;
}

However, directly writing the position to the centerline of the track is a bit of a cheat. We should control the AI with the same controls the player has: the linear_thrust and angular_thrust variables.

So we have to find a way to convert "ideal" position into a set of control inputs. We can divide this problem into two "rules" for the AI:

  1. Face in the direction of the track
  2. Try to stay in the center of the track

We can compute the difference from facing along the track, and apply it as a steering input:


#![allow(unused)]
fn main() {
    let angular_error = wrap_angle(track_angle_here - ship.position.rot);
    ship.angular_thrust = f32::max(f32::min(angular_error, 1.0), -1.0);
}

This fulfills rule 1, but the AI tends to wallslide. So lets compute a radius error and apply that:


#![allow(unused)]
fn main() {
    let polar_position = PolarCoordinate::from_cartesian((ship.position.x, ship.position.y));

    let track_angle_here = map.get_track_direction(polar_position.angle);
    let track_radius_here = map.track_radius(polar_position.angle);
    
    let mut steering = 0.0;
    let mut thrust = 0.0;
    
    let radius_error = track_radius_here - polar_position.radius;
    let radius_steering_input = f32::max(f32::min(radius_error, PI / 2.0), -PI / 2.0);
    
    let mut target_angle = 0.0;
    target_angle += track_angle_here; // Face direction of track
    target_angle += radius_steering_input;  // Fly towards track center
    
    let angular_error = wrap_angle(target_angle - ship.position.rot);
    steering += angular_error;
   
    thrust += 1.0;

    ship.angular_thrust = f32::max(f32::min(steering, 1.0), -1.0);
    ship.linear_thrust = f32::max(f32::min(thrust, 1.0), -1.0);
}

Note that the addition of the radius_error is done by offsetting the angle we want the ship to fly in rather than direclty influencing the steering input. (It's worth noting that the angular error is a kind-of feed-forward for the radius error)

The ships now fly around, but still hit the walls lots. This is because the ships can't see ahead of them. They are flying looking at how far they are from the walls NOW rather than looking ahead and giving the ship time to turn. We can fix this by instead of using the ship's current position to drive the control system, we can use a simple prediction of the ships position to drive it:


#![allow(unused)]
fn main() {
    let future_position = (
        ship.position.x + ship.velocity.x * LOOKAHEAD_TIME,
        ship.position.y + ship.velocity.y * LOOKAHEAD_TIME,
    );
    
    let polar_position = PolarCoordinate::from_cartesian(future_position);

    let track_angle_here = map.get_track_direction(polar_position.angle);
    let track_radius_here = map.track_radius(polar_position.angle);
    
    let mut steering = 0.0;
    let mut thrust = 0.0;
    
    let radius_error = track_radius_here - polar_position.radius;
    let radius_steering_input = f32::max(f32::min(radius_error, PI / 2.0), -PI / 2.0);
    
    let mut target_angle = 0.0;
    target_angle += track_angle_here; // Face direction of track
    target_angle += radius_steering_input;  // Fly towards track center
    
    let angular_error = wrap_angle(target_angle - ship.position.rot);
    steering += angular_error;
   
    thrust += 1.0;

    ship.angular_thrust = f32::max(f32::min(steering, 1.0), -1.0);
    ship.linear_thrust = f32::max(f32::min(thrust, 1.0), -1.0);
}

With a lookahead time of 0.5 seconds, the ship navigates the map on a pretty nice racing line - turning in close on corners etc. However, now the ship can only see where it is 0.5 seconds ahead, and doesn't know where it is now, so when the course is really twisty/turny, it can still hit the wall. So let's compute the steering input for both now and for the future:


#![allow(unused)]
fn main() {
pub fn calc_ai_control(ship: &mut Ship, _skill: f32, map: &Map) {
    let mut steering = 0.0;
    let mut thrust = 0.0;
    
    steering += calc_steering_input(&ship, &map, 1.0) * 0.15;
    steering += calc_steering_input(&ship, &map, 0.5) * 0.45;
    steering += calc_steering_input(&ship, &map, 0.2) * 0.4;
   
    thrust += 1.0;

    ship.angular_thrust = f32::max(f32::min(steering, 1.0), -1.0);
    ship.linear_thrust = f32::max(f32::min(thrust, 1.0), -1.0);
}


fn calc_steering_input(ship: &Ship, map: &Map, lookahead_time: f32) -> f32 {
    let polar_position = PolarCoordinate::from_cartesian(
        predict_position(ship, lookahead_time)
    );

    let track_angle_here = map.get_track_direction(polar_position.angle);
    let track_radius_here = map.track_radius(polar_position.angle);
    
    let mut steering = 0.0;
    
    let radius_error = track_radius_here - polar_position.radius;
    let radius_steering_input = f32::max(f32::min(radius_error, PI / 2.0), -PI / 2.0);
    
    let mut target_angle = 0.0;
    target_angle += track_angle_here; // Face direction of track
    target_angle += radius_steering_input;  // Fly towards track center
    
    let angular_error = wrap_angle(target_angle - ship.position.rot);
    steering += angular_error;
   
    steering
}

fn predict_position(ship: &Ship, time: f32) -> Vec2 {
    (
        ship.position.x + ship.velocity.x * time,
        ship.position.y + ship.velocity.y * time,
    )
}
}

We now have an AI that can fly inhumanly well. It nails every corner, and as a result it's no fun to race against (and all the AI's are the same). Time to add some imperfections. Let's use the skill parameter to vary the ships and reduce their lookahead.


#![allow(unused)]
fn main() {
let num_ships = self.ship_entities.len() - 2;
for (id, ship) in self.ship_entities[1..].iter_mut().enumerate() {
    let skill = id as f32 / num_ships as f32;
    calc_ai_control(ship, skill, &self.map);
}

<< snip >>

pub fn calc_ai_control(ship: &mut Ship, skill: f32, map: &Map) {
    let mut steering = 0.0;
    let mut thrust = 0.0;
    
    let lookahead_mul = skill;
    
    steering += calc_steering_input(&ship, &map, 1.0 * lookahead_mul) * 0.15;
    steering += calc_steering_input(&ship, &map, 0.5 * lookahead_mul) * 0.45;
    steering += calc_steering_input(&ship, &map, 0.2 * lookahead_mul) * 0.4;
   
    thrust += 1.0;

    ship.angular_thrust = f32::max(f32::min(steering, 1.0), -1.0);
    ship.linear_thrust = f32::max(f32::min(thrust, 1.0), -1.0);
}
}

And there we have it:

I can always beat yellow (no lookahead), often beat yellow (0.5s max lookahead) and am not ever close to purple (1s lookahead). I suppose this makes sense because the players camera only gives about 0.3-0.5 seconds of lookahead, so while the AI flies using the same ship limitations as a human, it can see more of the map...