uniform vec3 u_radii;
uniform vec3 u_oneOverEllipsoidRadiiSquared;

varying vec3 v_positionEC;

vec4 computeEllipsoidColor(czm_ray ray, float intersection, float side)
{
    vec3 positionEC = czm_pointAlongRay(ray, intersection);
    vec3 positionMC = (czm_inverseModelView * vec4(positionEC, 1.0)).xyz;
    vec3 geodeticNormal = normalize(czm_geodeticSurfaceNormal(positionMC, vec3(0.0), u_oneOverEllipsoidRadiiSquared));
    vec3 sphericalNormal = normalize(positionMC / u_radii);
    vec3 normalMC = geodeticNormal * side;              // normalized surface normal (always facing the viewer) in model coordinates
    vec3 normalEC = normalize(czm_normal * normalMC);   // normalized surface normal in eye coordiantes

    vec2 st = czm_ellipsoidWgs84TextureCoordinates(sphericalNormal);
    vec3 positionToEyeEC = -positionEC;

    czm_materialInput materialInput;
    materialInput.s = st.s;
    materialInput.st = st;
    materialInput.str = (positionMC + u_radii) / u_radii;
    materialInput.normalEC = normalEC;
    materialInput.tangentToEyeMatrix = czm_eastNorthUpToEyeCoordinates(positionMC, normalEC);
    materialInput.positionToEyeEC = positionToEyeEC;
    czm_material material = czm_getMaterial(materialInput);

    return czm_phong(normalize(positionToEyeEC), material);
}

void main()
{
    czm_ellipsoid ellipsoid = czm_ellipsoidNew(czm_modelView[3].xyz, u_radii);
    vec3 direction = normalize(v_positionEC);
    czm_ray ray = czm_ray(vec3(0.0), direction);
    czm_raySegment intersection = czm_rayEllipsoidIntersectionInterval(ray, ellipsoid);

    if (czm_isEmpty(intersection))
    {
        discard;
    }

    // If the viewer is outside, compute outsideFaceColor, with normals facing outward.
    vec4 outsideFaceColor = (intersection.start != 0.0) ? computeEllipsoidColor(ray, intersection.start, 1.0) : vec4(0.0);

    // If the viewer either is inside or can see inside, compute insideFaceColor, with normals facing inward.
    vec4 insideFaceColor = (outsideFaceColor.a < 1.0) ? computeEllipsoidColor(ray, intersection.stop, -1.0) : vec4(0.0);

    gl_FragColor = mix(insideFaceColor, outsideFaceColor, outsideFaceColor.a);
    gl_FragColor.a = 1.0 - (1.0 - insideFaceColor.a) * (1.0 - outsideFaceColor.a);
}