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

// written by Thorsten Renk, Oct 2011, based on default.frag
// Ambient term comes in gl_Color.rgb.
varying vec4 diffuse_term;
varying vec3 normal;
varying vec3 relPos;


uniform sampler2D texture;


varying float yprime_alt;
varying float mie_angle;


uniform float visibility;
uniform float avisibility;
uniform float scattering;
uniform float terminator;
uniform float terrain_alt; 
uniform float hazeLayerAltitude;
uniform float overcast;
uniform float eye_alt;
uniform float cloud_self_shading;

const float EarthRadius = 5800000.0;
const float terminator_width = 200000.0;

float alt;
float eShade;

float fog_func (in float targ, in float alt);
vec3 get_hazeColor(in float light_arg);
vec3 filter_combined (in vec3 color) ;

float getShadowing();

float luminance(vec3 color)
{
    return dot(vec3(0.212671, 0.715160, 0.072169), color);
}



void main()
{

  vec3 shadedFogColor = vec3(0.55, 0.67, 0.88);
// this is taken from default.frag
    vec3 n;
    float NdotL, NdotHV, fogFactor;
    vec4 color = gl_Color;
    vec3 lightDir = gl_LightSource[0].position.xyz;
    vec3 halfVector = gl_LightSource[0].halfVector.xyz;
    vec4 texel;
    vec4 fragColor;
    vec4 specular = vec4(0.0);
    float intensity;

    float effective_scattering = min(scattering, cloud_self_shading);


    vec4 light_specular = gl_LightSource[0].specular;

    // If gl_Color.a == 0, this is a back-facing polygon and the
    // normal should be reversed.
    n = (2.0 * gl_Color.a - 1.0) * normal;
    n = normalize(n);

    NdotL = dot(n, lightDir);
    if (NdotL > 0.0) {
        float shadowmap = getShadowing();
        color += diffuse_term * NdotL * shadowmap;
        NdotHV = max(dot(n, halfVector), 0.0);
        if (gl_FrontMaterial.shininess > 0.0)
            specular.rgb = (gl_FrontMaterial.specular.rgb
                            * light_specular.rgb
                            * pow(NdotHV, gl_FrontMaterial.shininess)
                            * shadowmap);
    }
    color.a = diffuse_term.a;
    // This shouldn't be necessary, but our lighting becomes very
    // saturated. Clamping the color before modulating by the texture
    // is closer to what the OpenGL fixed function pipeline does.
    color = clamp(color, 0.0, 1.0);
    texel = texture2D(texture, gl_TexCoord[0].st);
    fragColor = color * texel + specular;



// here comes the terrain haze model


float delta_z = hazeLayerAltitude - eye_alt;
float dist = length(relPos);

float mvisibility = min(visibility,avisibility);

if (dist > 0.04 * mvisibility) 
{

alt = eye_alt;


float transmission;
float vAltitude;
float delta_zv;
float H;
float distance_in_layer;
float transmission_arg;

// angle with horizon
float ct = dot(vec3(0.0, 0.0, 1.0), relPos)/dist;


// we solve the geometry what part of the light path is attenuated normally and what is through the haze layer

if (delta_z > 0.0) // we're inside the layer
	{
	if (ct < 0.0) // we look down 
		{
		distance_in_layer = dist;
		vAltitude = min(distance_in_layer,mvisibility) * ct;
  		delta_zv = delta_z - vAltitude;
		}
	else 	// we may look through upper layer edge
		{
		H = dist * ct;
		if (H > delta_z) {distance_in_layer = dist/H * delta_z;}
		else {distance_in_layer = dist;}
		vAltitude = min(distance_in_layer,visibility) * ct;
  		delta_zv = delta_z - vAltitude;	
		}
	}
  else // we see the layer from above, delta_z < 0.0
	{	
	H = dist * -ct;
	if (H  < (-delta_z)) // we don't see into the layer at all, aloft visibility is the only fading
		{
		distance_in_layer = 0.0;
		delta_zv = 0.0;
		}		
	else
		{
		vAltitude = H + delta_z;
		distance_in_layer = vAltitude/H * dist; 
		vAltitude = min(distance_in_layer,visibility) * (-ct);
		delta_zv = vAltitude;
		} 
	}
	

// ground haze cannot be thinner than aloft visibility in the model,
// so we need to use aloft visibility otherwise


transmission_arg = (dist-distance_in_layer)/avisibility;


float eqColorFactor;



if (visibility < avisibility)
	{
	transmission_arg = transmission_arg + (distance_in_layer/visibility);
	// this combines the Weber-Fechner intensity
	eqColorFactor = 1.0 - 0.1 * delta_zv/visibility - (1.0 -effective_scattering);

	}
else 
	{
	transmission_arg = transmission_arg + (distance_in_layer/avisibility);
	// this combines the Weber-Fechner intensity
	eqColorFactor = 1.0 - 0.1 * delta_zv/avisibility - (1.0 -effective_scattering);
	}



transmission =  fog_func(transmission_arg, alt);

// there's always residual intensity, we should never be driven to zero
if (eqColorFactor < 0.2) {eqColorFactor = 0.2;}


float lightArg = (terminator-yprime_alt)/100000.0;
vec3 hazeColor = get_hazeColor(lightArg);



// now dim the light for haze
eShade = 1.0 - 0.9 * smoothstep(-terminator_width+ terminator, terminator_width + terminator, yprime_alt);

// Mie-like factor

if (lightArg < 10.0)
	{intensity = length(hazeColor);
	float mie_magnitude = 0.5 * smoothstep(350000.0, 150000.0, terminator-sqrt(2.0 * EarthRadius * terrain_alt));
	hazeColor = intensity * ((1.0 - mie_magnitude) + mie_magnitude * mie_angle) * normalize(mix(hazeColor,  vec3 (0.5, 0.58, 0.65), mie_magnitude * (0.5 - 0.5 * mie_angle)) ); 
	}

// high altitude desaturation of the haze color

intensity = length(hazeColor);
hazeColor = intensity * normalize (mix(hazeColor, intensity * vec3 (1.0,1.0,1.0), 0.7* smoothstep(5000.0, 50000.0, alt)));

// blue hue of haze

hazeColor.x = hazeColor.x * 0.83;
hazeColor.y = hazeColor.y * 0.9; 


// additional blue in indirect light
float fade_out = max(0.65 - 0.3 *overcast, 0.45);
intensity = length(hazeColor);
hazeColor = intensity * normalize(mix(hazeColor,  1.5* shadedFogColor, 1.0 -smoothstep(0.25, fade_out,eShade) )); 

// change haze color to blue hue for strong fogging
//intensity = length(hazeColor);
hazeColor = intensity * normalize(mix(hazeColor,  shadedFogColor, (1.0-smoothstep(0.5,0.9,eqColorFactor)))); 


// reduce haze intensity when looking at shaded surfaces, only in terminator region

float shadow = mix( min(1.0 + dot(normal,lightDir),1.0), 1.0, 1.0-smoothstep(0.1, 0.4, transmission));
hazeColor = mix(shadow * hazeColor, hazeColor, 0.3 + 0.7* smoothstep(250000.0, 400000.0, terminator));




// don't let the light fade out too rapidly
lightArg = (terminator + 200000.0)/100000.0;
float minLightIntensity = min(0.2,0.16 * lightArg + 0.5);
vec3 minLight = minLightIntensity * vec3 (0.2, 0.3, 0.4);
hazeColor *= eqColorFactor * eShade;
hazeColor.rgb = max(hazeColor.rgb, minLight.rgb);

// determine the right mix of transmission and haze

fragColor.rgb = mix(hazeColor, fragColor.rgb,transmission);
}

fragColor.rgb = filter_combined(fragColor.rgb);

gl_FragColor = fragColor;

}