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use approx::relative_eq;
use bevy::math::prelude::*;

const PI: f32 = std::f32::consts::PI;

/// A (yaw, pitch) pair representing a direction.
#[derive(Clone, Copy, Debug, Default)]
pub struct LookAngles {
    // The fields are protected to keep them in an allowable range for the camera transform.
    yaw: f32,
    pitch: f32,
}

impl LookAngles {
    pub fn from_vector(v: Vec3) -> Self {
        let mut p = Self::default();
        p.set_direction(v);

        p
    }

    pub fn unit_vector(self) -> Vec3 {
        unit_vector_from_yaw_and_pitch(self.yaw, self.pitch)
    }

    pub fn set_direction(&mut self, v: Vec3) {
        let (yaw, pitch) = yaw_and_pitch_from_vector(v);
        self.set_yaw(yaw);
        self.set_pitch(pitch);
    }

    pub fn set_yaw(&mut self, yaw: f32) {
        self.yaw = yaw % (2.0 * PI);
    }

    pub fn get_yaw(&self) -> f32 {
        self.yaw
    }

    pub fn add_yaw(&mut self, delta: f32) {
        self.set_yaw(self.get_yaw() + delta);
    }

    pub fn set_pitch(&mut self, pitch: f32) {
        // Things can get weird if we are parallel to the UP vector.
        let up_eps = 0.01;
        self.pitch = pitch.min(PI / 2.0 - up_eps).max(-PI / 2.0 + up_eps);
    }

    pub fn get_pitch(&self) -> f32 {
        self.pitch
    }

    pub fn add_pitch(&mut self, delta: f32) {
        self.set_pitch(self.get_pitch() + delta);
    }

    pub fn assert_not_looking_up(&self) {
        let is_looking_up = relative_eq!(self.unit_vector().dot(Vec3::Y).abs(), 1.0);

        assert!(
            !is_looking_up,
            "Your camera transform is fucked up. Your look direction {} is probably bad.",
            self.unit_vector(),
        );
    }
}

/// Returns pitch and yaw angles that rotates z unit vector to v. The yaw is applied first to z about the y axis to get z'. Then
/// the pitch is applied about some axis orthogonal to z' in the XZ plane to get v.
fn yaw_and_pitch_from_vector(v: Vec3) -> (f32, f32) {
    debug_assert_ne!(v, Vec3::ZERO);

    let y = Vec3::Y;
    let z = Vec3::Z;

    let v_xz = Vec3::new(v.x, 0.0, v.z);

    if v_xz == Vec3::ZERO {
        if v.dot(y) > 0.0 {
            return (0.0, PI / 2.0);
        } else {
            return (0.0, -PI / 2.0);
        }
    }

    let mut yaw = v_xz.angle_between(z);
    if v.x < 0.0 {
        yaw *= -1.0;
    }

    let mut pitch = v_xz.angle_between(v);
    if v.y < 0.0 {
        pitch *= -1.0;
    }

    (yaw, pitch)
}

fn unit_vector_from_yaw_and_pitch(yaw: f32, pitch: f32) -> Vec3 {
    let ray = Mat3::from_rotation_y(yaw) * Vec3::Z;
    let pitch_axis = ray.cross(Vec3::Y);

    Mat3::from_axis_angle(pitch_axis, pitch) * ray
}

// ████████╗███████╗███████╗████████╗
// ╚══██╔══╝██╔════╝██╔════╝╚══██╔══╝
//    ██║   █████╗  ███████╗   ██║
//    ██║   ██╔══╝  ╚════██║   ██║
//    ██║   ███████╗███████║   ██║
//    ╚═╝   ╚══════╝╚══════╝   ╚═╝

#[cfg(test)]
mod tests {
    use super::*;

    use approx::assert_relative_eq;

    const PI: f32 = std::f32::consts::PI;

    #[test]
    fn test_yaw_and_pitch_identity() {
        let v = Vec3::new(0.0, 0.0, 1.0);
        let (yaw, pitch) = yaw_and_pitch_from_vector(v);

        assert_relative_eq!(yaw, 0.0);
        assert_relative_eq!(pitch, 0.0);
    }

    #[test]
    fn test_yaw_only() {
        let (yaw, pitch) = yaw_and_pitch_from_vector(Vec3::new(1.0, 0.0, 0.0));
        assert_relative_eq!(yaw, PI / 2.0);
        assert_relative_eq!(pitch, 0.0);

        let (yaw, pitch) = yaw_and_pitch_from_vector(Vec3::new(-1.0, 0.0, 0.0));
        assert_relative_eq!(yaw, -PI / 2.0);
        assert_relative_eq!(pitch, 0.0);
    }

    #[test]
    fn test_pitch_only() {
        let (yaw, pitch) = yaw_and_pitch_from_vector(Vec3::new(0.0, 1.0, 0.0));
        assert_relative_eq!(yaw, 0.0);
        assert_relative_eq!(pitch, PI / 2.0);

        let (yaw, pitch) = yaw_and_pitch_from_vector(Vec3::new(0.0, -1.0, 0.0));
        assert_relative_eq!(yaw, 0.0);
        assert_relative_eq!(pitch, -PI / 2.0);
    }

    #[test]
    fn test_yaw_and_pitch() {
        let (yaw, pitch) = yaw_and_pitch_from_vector(Vec3::new(0.5f32.sqrt(), 1.0, 0.5f32.sqrt()));
        assert_relative_eq!(yaw, PI / 4.0, epsilon = 1e-6f32);
        assert_relative_eq!(pitch, PI / 4.0);

        let (yaw, pitch) =
            yaw_and_pitch_from_vector(Vec3::new(-0.5f32.sqrt(), -1.0, 0.5f32.sqrt()));
        assert_relative_eq!(yaw, -PI / 4.0, epsilon = 1e-6f32);
        assert_relative_eq!(pitch, -PI / 4.0);
    }
}