outfly/src/nature.rs

// This stuff here, this stuff is messy. Nobody wants to deal with this,
// nobody cares how it works, but I guess we need it as an ingredient for
// the universe *sigh* so here we go.

pub const OXYGEN_USE_KG_PER_S: f32 = 1e-5;
pub const OXY_S: f32 = OXYGEN_USE_KG_PER_S;
pub const OXY_M: f32 = OXYGEN_USE_KG_PER_S * 60.0;
pub const OXY_H: f32 = OXYGEN_USE_KG_PER_S * 60.0 * 60.0;
pub const OXY_D: f32 = OXYGEN_USE_KG_PER_S * 60.0 * 60.0 * 24.0;
pub const PARSEC2METER: f64 = 3.0857e16;

pub fn star_color_index_to_rgb(color_index: f32) -> (f32, f32, f32) {
    let temperature = 4600.0 * ((1.0 / (0.92 * color_index + 1.7)) + (1.0 / (0.92 * color_index + 0.62)));

    let (red, green, blue) = if temperature <= 6600.0 {
        let red = 255.0;
        let green = 99.4708025861 * (temperature / 100.0).ln() - 161.1195681661;
        let blue = if temperature <= 2000.0 {
            0.0
        } else {
            138.5177312231 * ((temperature / 100.0) - 10.0).ln() - 305.0447927307
        };
        (red, green, blue)
    } else {
        let red = 329.698727446 * ((temperature / 100.0 - 60.0).powf(-0.1332047592));
        let green = 288.1221695283 * ((temperature / 100.0 - 60.0).powf(-0.0755148492));
        let blue = 255.0;
        (red, green, blue)
    };

    let clamp = |x: f32| -> f32 { (x / 255.0).max(0.0).min(1.0) };

    return (clamp(red), clamp(green), clamp(blue))
}

fn smooth_edge(start: f32, end: f32, value: f32) -> f32 {
    let x: f32 = (value - start) / (end - start);
    return 4.0 * x * x * (1.0 - x * x);
}

pub fn ring_density(radius: f32) -> f32 {
    // NOTE: Keep this in sync with assets/shaders/jupiters_rings.wgsl
    // Input: distance to center of jupiter in million meters
    // Output: relative brightness of the ring
    let halo_inner: f32 = 92.0;
    let halo_outer: f32 = 122.5;
    let main_inner: f32 = 122.5;
    let main_outer: f32 = 129.0;
    let amalthea_inner: f32 = 129.0;
    let amalthea_outer: f32 = 182.0;
    let thebe_inner: f32 = 129.0;
    let thebe_outer: f32 = 229.0;
    let metis_notch_center: f32 = 128.0;
    let metis_notch_width: f32 = 0.6;

    let halo_brightness: f32 = 0.75;
    let main_brightness: f32 = 1.0;
    let almathea_brightness: f32 = 0.5;
    let thebe_brightness: f32 = 0.5;

    let mut density: f32 = 0.0;

    if radius >= halo_inner && radius <= halo_outer {
        density = halo_brightness * smooth_edge(halo_inner, halo_outer, radius);
    } else if radius >= main_inner && radius <= main_outer {
        let mut metis_notch_effect: f32 = 1.0;
        if radius > metis_notch_center - metis_notch_width * 0.5 && radius < metis_notch_center + metis_notch_width * 0.5 {
            metis_notch_effect = 0.5 * (1.0 - smooth_edge(metis_notch_center - metis_notch_width * 0.5, metis_notch_center + metis_notch_width * 0.5, radius));
        }
        density = main_brightness * metis_notch_effect * smooth_edge(main_inner, main_outer, radius);
    } else {
        if radius >= amalthea_inner && radius <= amalthea_outer {
            density = almathea_brightness * smooth_edge(amalthea_inner, amalthea_outer, radius);
        }
        if radius >= thebe_inner && radius <= thebe_outer {
            density += thebe_brightness * smooth_edge(thebe_inner, thebe_outer, radius);
        }
    }

    return density;
}