// -*-C++-*-
#version 120

// Duplicated from generic-ALS-base.vert, but in the form of
// library functions that can be used by other shader cpde

// Shader that uses OpenGL state values to do per-pixel lighting
//
// The only light used is gl_LightSource[0], which is assumed to be
// directional.
//
// Diffuse colors come from the gl_Color, ambient from the material. This is
// equivalent to osg::Material::DIFFUSE.
// Haze part added by Thorsten Renk, Oct. 2011


#define MODE_OFF 0
#define MODE_DIFFUSE 1
#define MODE_AMBIENT_AND_DIFFUSE 2

// The constant term of the lighting equation that doesn't depend on
// the surface normal is passed in gl_{Front,Back}Color. The alpha
// component is set to 1 for front, 0 for back in order to work around
// bugs with gl_FrontFacing in the fragment shader.
varying vec4 diffuse_term;
varying vec3 normal;
varying vec3 relPos;
varying vec4 ecPosition;

varying float yprime_alt;
varying float mie_angle;

uniform int colorMode;
uniform float hazeLayerAltitude;
uniform float terminator;
uniform float terrain_alt;
uniform float avisibility;
uniform float visibility;
uniform float overcast;
uniform float ground_scattering;
uniform float moonlight;

// This is the value used in the skydome scattering shader - use the same here for consistency?
const float EarthRadius = 5800000.0;
const float terminator_width = 200000.0;


float earthShade;

float light_func (in float x, in float a, in float b, in float c, in float d, in float e)
{
    // use the asymptotics to shorten computations
    if (x < -15.0) {
        return 0.0;
    }

    return e / pow((1.0 + a * exp(-b * (x-c)) ),(1.0/d));
}

void render_ALS_base(in vec3 position)
{
    vec4 light_diffuse;
    vec4 light_ambient;
    vec3 shadedFogColor = vec3(0.55, 0.67, 0.88);
    vec3 moonLightColor = vec3 (0.095, 0.095, 0.15) * moonlight;

    //float yprime_alt;
    float yprime;
    float lightArg;
    float intensity;
    float vertex_alt;
    float scattering;

    // this code is copied from default.vert
    ecPosition = gl_ModelViewMatrix * vec4(position, gl_Vertex.w);
    // Project to world space
    gl_Position  = gl_ModelViewProjectionMatrix * vec4(position, gl_Vertex.w);
    gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
    gl_TexCoord[2] = gl_TextureMatrix[2] * gl_MultiTexCoord2;

    vec4 ambient_color, diffuse_color;
    if (colorMode == MODE_DIFFUSE) {
        diffuse_color = gl_Color;
        ambient_color = gl_FrontMaterial.ambient;
    } else if (colorMode == MODE_AMBIENT_AND_DIFFUSE) {
        diffuse_color = gl_Color;
        ambient_color = gl_Color;
    } else {
        diffuse_color = gl_FrontMaterial.diffuse;
        ambient_color = gl_FrontMaterial.ambient;
    }

    // here start computations for the haze layer
    // we need several geometrical quantities

    // first current altitude of eye position in model space
    vec4 ep = gl_ModelViewMatrixInverse * vec4(0.0,0.0,0.0,1.0);

    // and relative position to vector
    relPos = position.xyz - ep.xyz;

    // unfortunately, we need the distance in the vertex shader, although the more accurate version
    // is later computed in the fragment shader again
    float dist = length(relPos);

    // altitude of the vertex in question, somehow zero leads to artefacts, so ensure it is at least 100m
    vertex_alt = max(position.z,100.0);
    scattering = ground_scattering + (1.0 - ground_scattering) * smoothstep(hazeLayerAltitude -100.0, hazeLayerAltitude + 100.0, vertex_alt);


    // branch dependent on daytime

    if (terminator < 1000000.0) // the full, sunrise and sunset computation
    {
        // establish coordinates relative to sun position
        vec3 lightFull = (gl_ModelViewMatrixInverse * gl_LightSource[0].position).xyz;
        vec3 lightHorizon = normalize(vec3(lightFull.x,lightFull.y, 0.0));

        // yprime is the distance of the vertex into sun direction
        yprime = -dot(relPos, lightHorizon);

        // this gets an altitude correction, higher terrain gets to see the sun earlier
        yprime_alt = yprime - sqrt(2.0 * EarthRadius * vertex_alt);

        // two times terminator width governs how quickly light fades into shadow
        // now the light-dimming factor
        earthShade = 0.6 * (1.0 - smoothstep(-terminator_width+ terminator, terminator_width + terminator, yprime_alt)) + 0.4;

        // parametrized version of the Flightgear ground lighting function
        lightArg = (terminator-yprime_alt)/100000.0;

        // directional scattering for low sun
        if (lightArg < 10.0) {
            mie_angle = (0.5 *  dot(normalize(relPos), normalize(lightFull)) ) + 0.5;
        } else {
            mie_angle = 1.0;
        }

       light_diffuse.b = light_func(lightArg, 1.330e-05, 0.264, 3.827, 1.08e-05, 1.0);
       light_diffuse.g = light_func(lightArg, 3.931e-06, 0.264, 3.827, 7.93e-06, 1.0);
       light_diffuse.r = light_func(lightArg, 8.305e-06, 0.161, 3.827, 3.04e-05, 1.0);
       light_diffuse.a = 1.0;
       light_diffuse = light_diffuse * scattering;

       light_ambient.r = light_func(lightArg, 0.236, 0.253, 1.073, 0.572, 0.33);
       light_ambient.g = light_ambient.r * 0.4/0.33;
       light_ambient.b = light_ambient.r * 0.5/0.33;
       light_ambient.a = 1.0;

        // correct ambient light intensity and hue before sunrise
        if (earthShade < 0.5) {
            //light_ambient = light_ambient * (0.7 + 0.3 * smoothstep(0.2, 0.5, earthShade));
            intensity = length(light_ambient.xyz);

            light_ambient.rgb = intensity * normalize(mix(light_ambient.rgb,  shadedFogColor, 1.0 -smoothstep(0.4, 0.8,earthShade) ));
            light_ambient.rgb = light_ambient.rgb +   moonLightColor *  (1.0 - smoothstep(0.4, 0.5, earthShade));

            intensity = length(light_diffuse.xyz);
            light_diffuse.rgb = intensity * normalize(mix(light_diffuse.rgb,  shadedFogColor, 1.0 -smoothstep(0.4, 0.7,earthShade) ));
        }


        // the haze gets the light at the altitude of the haze top if the vertex in view is below
        // but the light at the vertex if the vertex is above
        vertex_alt = max(vertex_alt,hazeLayerAltitude);

        if (vertex_alt > hazeLayerAltitude) {
            if (dist > 0.8 * avisibility) {
                vertex_alt = mix(vertex_alt, hazeLayerAltitude, smoothstep(0.8*avisibility, avisibility, dist));
                yprime_alt = yprime -sqrt(2.0 * EarthRadius * vertex_alt);
            }
        } else {
            vertex_alt = hazeLayerAltitude;
            yprime_alt = yprime -sqrt(2.0 * EarthRadius * vertex_alt);
        }
    }
    else // the faster, full-day version without lightfields
    {
        //vertex_alt = max(gl_Vertex.z,100.0);

        earthShade = 1.0;
        mie_angle = 1.0;

        if (terminator > 3000000.0)
        {
            light_diffuse = vec4 (1.0, 1.0, 1.0, 0.0);
            light_ambient = vec4 (0.33, 0.4, 0.5, 0.0);
        } else {
            lightArg = (terminator/100000.0 - 10.0)/20.0;
            light_diffuse.b = 0.78  + lightArg * 0.21;
            light_diffuse.g = 0.907 + lightArg * 0.091;
            light_diffuse.r = 0.904 + lightArg * 0.092;
            light_diffuse.a = 1.0;

            light_ambient.r = 0.316 + lightArg * 0.016;
            light_ambient.g = light_ambient.r * 0.4/0.33;
            light_ambient.b = light_ambient.r * 0.5/0.33;
            light_ambient.a = 1.0;
        }

        light_diffuse = light_diffuse * scattering;
        yprime_alt = -sqrt(2.0 * EarthRadius * hazeLayerAltitude);
    }

    // default lighting based on texture and material using the light we have just computed
    diffuse_term = diffuse_color* light_diffuse;
    vec4 constant_term = gl_FrontMaterial.emission + ambient_color *
        (gl_LightModel.ambient +  light_ambient);

    // Super hack: if diffuse material alpha is less than 1, assume a
    // transparency animation is at work
    if (gl_FrontMaterial.diffuse.a < 1.0)
        diffuse_term.a = gl_FrontMaterial.diffuse.a;
    else
        diffuse_term.a = gl_Color.a;

    // Another hack for supporting two-sided lighting without using
    // gl_FrontFacing in the fragment shader.
    gl_FrontColor.rgb = constant_term.rgb;  gl_FrontColor.a = 1.0;
    gl_BackColor.rgb = constant_term.rgb; gl_BackColor.a = 0.0;
}