#include "sky_common.hlsli"

#define TRANSMITTANCE_STEPS 32

float4 main(in float2 uv : Position) : SV_Target0
{
    float sun_cos_theta = uv.x * 2.0 - 1.0;
    float3 sun_dir = float3(-sqrt(1.0 - sun_cos_theta*sun_cos_theta), 0.0, sun_cos_theta);

    float distance_to_earth_center = lerp(EARTH_RADIUS, ATMOSPHERE_RADIUS, uv.y);
    float3 ray_origin = float3(0.0, 0.0, distance_to_earth_center);

    float t_d = ray_sphere_intersection(ray_origin, sun_dir, ATMOSPHERE_RADIUS);
    float dt = t_d / float(TRANSMITTANCE_STEPS);

    float4 result = 0.0;

    for (int i = 0; i < TRANSMITTANCE_STEPS; ++i) {
        float t = (float(i) + 0.5) * dt;
        float3 x_t = ray_origin + sun_dir * t;

        float altitude = length(x_t) - EARTH_RADIUS;

        float4 aerosol_absorption, aerosol_scattering;
        float4 molecular_absorption, molecular_scattering;
        float4 fog_scattering;
        float4 extinction;
        get_atmosphere_collision_coefficients(
            altitude,
            aerosol_absorption, aerosol_scattering,
            molecular_absorption, molecular_scattering,
            fog_scattering,
            extinction);

        result += extinction * dt;
    }

    float4 transmittance = exp(-result);
    return transmittance;
}