Function optical_depth 

pub fn optical_depth(
    start: Vec3,
    dir: Vec3,
    atm: &AtmosphereParams,
    n_steps: usize,
) -> Vec2

Compute the optical depth (column density) from start to the atmosphere boundary, and also toward the sun for shadow computation.

Returns Vec2(rayleigh_depth, mie_depth).

Parameters

• start — starting position in world space (planet-centred, metres)
• dir — ray direction (normalised)
• atm — atmosphere parameters
• n_steps — number of integration steps