float interpolateByDistance(vec4 nearFarScalar, float distance)
{
    float startDistance = nearFarScalar.x;
    float startValue = nearFarScalar.y;
    float endDistance = nearFarScalar.z;
    float endValue = nearFarScalar.w;
    float t = clamp((distance - startDistance) / (endDistance - startDistance), 0.0, 1.0);
    return mix(startValue, endValue, t);
}

vec3 getLightDirection(vec3 positionWC)
{
    float lightEnum = u_radiiAndDynamicAtmosphereColor.z;
    vec3 lightDirection =
        positionWC * float(lightEnum == 0.0) +
        czm_lightDirectionWC * float(lightEnum == 1.0) +
        czm_sunDirectionWC * float(lightEnum == 2.0);
    return normalize(lightDirection);
}

void computeAtmosphereScattering(vec3 positionWC, vec3 lightDirection, out vec3 rayleighColor, out vec3 mieColor, out float opacity, out float underTranslucentGlobe)
{
    float ellipsoidRadiiDifference = czm_ellipsoidRadii.x - czm_ellipsoidRadii.z;

    // Adjustment to the atmosphere radius applied based on the camera height.
    float distanceAdjustMin = czm_ellipsoidRadii.x / 4.0;
    float distanceAdjustMax = czm_ellipsoidRadii.x;
    float distanceAdjustModifier = ellipsoidRadiiDifference / 2.0;
    float distanceAdjust = distanceAdjustModifier * clamp((czm_eyeHeight - distanceAdjustMin) / (distanceAdjustMax - distanceAdjustMin), 0.0, 1.0);

    // Since atmosphere scattering assumes the atmosphere is a spherical shell, we compute an inner radius of the atmosphere best fit 
    // for the position on the ellipsoid.
    float radiusAdjust = (ellipsoidRadiiDifference / 4.0) + distanceAdjust;
    float atmosphereInnerRadius = (length(czm_viewerPositionWC) - czm_eyeHeight) - radiusAdjust;

    // Setup the primary ray: from the camera position to the vertex position.
    vec3 cameraToPositionWC = positionWC - czm_viewerPositionWC;
    vec3 cameraToPositionWCDirection = normalize(cameraToPositionWC);
    czm_ray primaryRay = czm_ray(czm_viewerPositionWC, cameraToPositionWCDirection);

    underTranslucentGlobe = 0.0;

    // Brighten the sky atmosphere under the Earth's atmosphere when translucency is enabled.
    #if defined(GLOBE_TRANSLUCENT)

        // Check for intersection with the inner radius of the atmopshere.
        czm_raySegment primaryRayEarthIntersect = czm_raySphereIntersectionInterval(primaryRay, vec3(0.0), atmosphereInnerRadius + radiusAdjust);
        if (primaryRayEarthIntersect.start > 0.0 && primaryRayEarthIntersect.stop > 0.0) {
            
            // Compute position on globe.
            vec3 direction = normalize(positionWC);
            czm_ray ellipsoidRay = czm_ray(positionWC, -direction);
            czm_raySegment ellipsoidIntersection = czm_rayEllipsoidIntersectionInterval(ellipsoidRay, vec3(0.0), czm_ellipsoidInverseRadii);
            vec3 onEarth = positionWC - (direction * ellipsoidIntersection.start);

            // Control the color using the camera angle.
            float angle = dot(normalize(czm_viewerPositionWC), normalize(onEarth));

            // Control the opacity using the distance from Earth.
            opacity = interpolateByDistance(vec4(0.0, 1.0, czm_ellipsoidRadii.x, 0.0), length(czm_viewerPositionWC - onEarth));
            vec3 horizonColor = vec3(0.1, 0.2, 0.3);
            vec3 nearColor = vec3(0.0);

            rayleighColor = mix(nearColor, horizonColor, exp(-angle) * opacity);
            
            // Set the traslucent flag to avoid alpha adjustment in computeFinalColor funciton.
            underTranslucentGlobe = 1.0;
            return;
        }
    #endif

    computeScattering(
        primaryRay,
        length(cameraToPositionWC),
        lightDirection,
        atmosphereInnerRadius,
        rayleighColor,
        mieColor,
        opacity
    );

    // Alter the opacity based on how close the viewer is to the ground.
    // (0.0 = At edge of atmosphere, 1.0 = On ground)
    float cameraHeight = czm_eyeHeight + atmosphereInnerRadius;
    float atmosphereOuterRadius = atmosphereInnerRadius + ATMOSPHERE_THICKNESS;
    opacity = clamp((atmosphereOuterRadius - cameraHeight) / (atmosphereOuterRadius - atmosphereInnerRadius), 0.0, 1.0);

    // Alter alpha based on time of day (0.0 = night , 1.0 = day)
    float nightAlpha = (u_radiiAndDynamicAtmosphereColor.z != 0.0) ? clamp(dot(normalize(positionWC), lightDirection), 0.0, 1.0) : 1.0;
    opacity *= pow(nightAlpha, 0.5);
}