// -*-C++-*-

// 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;
varying vec4 rawPos;


//varying vec3 hazeColor;
//varying float fogCoord;

uniform sampler2D texture;
uniform sampler3D NoiseTex;
uniform sampler2D snow_texture;

//varying float ct;
//varying float delta_z;
//varying float alt;

varying float earthShade;
//varying float yprime;
//varying float vertex_alt;
varying float yprime_alt;
varying float mie_angle;
varying float steepness;


uniform float visibility;
uniform float avisibility;
uniform float scattering;
//uniform float ground_scattering;
uniform float terminator;
uniform float terrain_alt; 
uniform float hazeLayerAltitude;
uniform float overcast;
//uniform float altitude;
uniform float eye_alt;
uniform float mysnowlevel;
uniform float dust_cover_factor;
uniform float fogstructure;

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

float alt;

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


float light_func (in float x, in float a, in float b, in float c, in float d, in float e)
{
x = x - 0.5;

// use the asymptotics to shorten computations
if (x > 30.0) {return e;}
if (x < -15.0) {return 0.0;}

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

// this determines how light is attenuated in the distance
// physically this should be exp(-arg) but for technical reasons we use a sharper cutoff
// for distance > visibility

float fog_func (in float targ)
{


float fade_mix;

// for large altitude > 30 km, we switch to some component of quadratic distance fading to
// create the illusion of improved visibility range

targ = 1.25 * targ; // need to sync with the distance to which terrain is drawn


if (alt < 30000.0)
	{return exp(-targ - targ * targ * targ * targ);}
else if (alt < 50000.0)
	{
	fade_mix = (alt - 30000.0)/20000.0;
	return fade_mix * exp(-targ*targ - pow(targ,4.0)) + (1.0 - fade_mix) * exp(-targ - pow(targ,4.0));	
	}
else 
	{
	return exp(- targ * targ - pow(targ,4.0));
	}

}

void main()
{

// 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 snow_texel;
    vec4 fragColor;
    vec4 specular = vec4(0.0);
    float intensity;



    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) {
        color += diffuse_term * NdotL;
        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));
    }
    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);
    snow_texel = texture2D(snow_texture, gl_TexCoord[0].st);


// this is the snow and dust generating part, ger some noise vectors
vec4 noisevec   = texture3D(NoiseTex, (rawPos.xyz)*0.003); // small scale noise
//vec4 nvL   = texture3D(NoiseTex, (rawPos.xyz)*0.00066);
vec4 nvL   = texture3D(NoiseTex, (rawPos.xyz)*0.0001); // large scale noise
vec4 nvR   = texture3D(NoiseTex, (rawPos.xyz)*0.00003); // really large scale noise

//float ns=0.06;
  //  ns += nvL[0]*0.4;
    //ns += nvL[1]*0.6;
    //ns += nvL[2]*2.0;
    //ns += nvL[3]*4.0;
    //ns += noisevec[0]*0.1;
    //ns += noisevec[1]*0.4;

    //ns += noisevec[2]*0.8;
    //ns += noisevec[3]*2.1;

   // gradient effect for snow


// mix dust
    vec4 dust_color = vec4 (0.76, 0.71, 0.56, 1.0);
    //dust_color.rgb = dust_color.rgb * nvL[1];

    texel = mix(texel, dust_color, clamp(0.5 * dust_cover_factor + 3.0 * dust_cover_factor * nvL[1],0.0, 1.0) );


   float snow_alpha = smoothstep(0.7, 0.8, abs(steepness));

   //vec4 snow_texel =   clamp(ns+nvL[2]*4.1+vec4(0.1, 0.1, nvL[2]*2.2, 1.0), 0.7, 1.0);  
	//snow_texel.a = snow_alpha * snow_texel.a;

    

    // mix snow
    texel = mix(texel, snow_texel, smoothstep(mysnowlevel, mysnowlevel+200.0, snow_alpha * (relPos.z + eye_alt)+ (noisevec[1] * abs(noisevec[1])+ nvL[1])*1500.0));

// gradient
    //fragColor = mix(vec4(ns-0.30, ns-0.29, ns-0.37, 1.0), fragColor, smoothstep(0.0, 0.40,  steepness));// +nvL[2]*1.3));

    fragColor = color * texel + specular;

// here comes the terrain haze model


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


if (dist > 40.0)
{

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,min(visibility, avisibility)) * 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;

//float scattering = ground_scattering + (1.0 - ground_scattering) * smoothstep(hazeLayerAltitude -100.0, hazeLayerAltitude + 100.0, relPos.z + eye_alt);


if (visibility < avisibility)
	{
	transmission_arg = transmission_arg + (distance_in_layer/(1.0 * visibility + 0.8 * visibility * fogstructure * (( 0.4 * nvL[1] + 0.6 * nvR[1]) -0.1) ));
	//transmission_arg = transmission_arg + (distance_in_layer/visibility);
	// this combines the Weber-Fechner intensity
	eqColorFactor = 1.0 - 0.1 * delta_zv/visibility - (1.0 -scattering);

	}
else 
	{
	transmission_arg = transmission_arg + (distance_in_layer/(1.0 * avisibility + 0.8 * avisibility * fogstructure * (( 0.4 * nvL[1] + 0.6 * nvR[1]) -0.1) ));
	//transmission_arg = transmission_arg + (distance_in_layer/avisibility);
	// this combines the Weber-Fechner intensity
	eqColorFactor = 1.0 - 0.1 * delta_zv/avisibility - (1.0 -scattering);
	}



transmission =  fog_func(transmission_arg);

// 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;

hazeColor.b = light_func(lightArg, 1.330e-05, 0.264, 2.527, 1.08e-05, 1.0);
hazeColor.g = light_func(lightArg, 3.931e-06, 0.264, 3.827, 7.93e-06, 1.0);
hazeColor.r = light_func(lightArg, 8.305e-06, 0.161, 3.827, 3.04e-05, 1.0);


// now dim the light for haze
earthShade = 0.9 * smoothstep(terminator_width+ terminator, -terminator_width + terminator, yprime_alt) + 0.1;

// Mie-like factor

if (lightArg < 5.0)
	{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* vec3 (0.45, 0.6, 0.8), 1.0 -smoothstep(0.25, fade_out,earthShade) )); 

// change haze color to blue hue for strong fogging
//intensity = length(hazeColor);
hazeColor = intensity * normalize(mix(hazeColor,  2.0 * vec3 (0.55, 0.6, 0.8), (1.0-smoothstep(0.3,0.8,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));

// randomness

//hazeColor.rgb = hazeColor.rgb + 0.2 * hazeColor.rgb * nvL[1];

// determine the right mix of transmission and haze

//fragColor.xyz = transmission * fragColor.xyz + (1.0-transmission)  * eqColorFactor * hazeColor * earthShade;


//fragColor.rgb = mix(fragColor.rgb, vec3 (1.0, 1.0, 1.0), overcast );


fragColor.xyz = mix(eqColorFactor * hazeColor * earthShade, fragColor.xyz,transmission);


gl_FragColor = fragColor;


}
else // if dist < 40.0 no fogging at all 
{
gl_FragColor = fragColor;
}


//gl_FragColor.rgb = 5.0 * nvL[1] * vec3 (1.0, 1.0, 1.0);

}