// -*-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 vec3 rawPos;
//varying vec3 ecViewdir;


uniform sampler2D texture;
uniform sampler3D NoiseTex;
uniform sampler2D snow_texture;
uniform sampler2D detail_texture;
uniform sampler2D mix_texture;

varying float yprime_alt;
varying float mie_angle;
varying float steepness;


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 mysnowlevel;
uniform float dust_cover_factor;
uniform float wetness;
uniform float fogstructure;
uniform int quality_level;
uniform int tquality_level;

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

float alt;
float eShade;



float rand2D(in vec2 co){
    return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453);
}

float cosine_interpolate(in float a, in float b, in float x)
{
	float ft = x * 3.1415927;
	float f = (1.0 - cos(ft)) * .5;

	return  a*(1.0-f) + b*f;
}

float simple_interpolate(in float a, in float b, in float x)
{
return a + smoothstep(0.0,1.0,x) * (b-a);
//return mix(a,b,x); 
}

float interpolatedNoise2D(in float x, in float y)
{
      float integer_x    = x - fract(x);
      float fractional_x = x - integer_x;

      float integer_y    = y - fract(y);
      float fractional_y = y - integer_y;

      float v1 = rand2D(vec2(integer_x, integer_y));
      float v2 = rand2D(vec2(integer_x+1.0, integer_y));
      float v3 = rand2D(vec2(integer_x, integer_y+1.0));
      float v4 = rand2D(vec2(integer_x+1.0, integer_y +1.0));

      float i1 = simple_interpolate(v1 , v2 , fractional_x);
      float i2 = simple_interpolate(v3 , v4 , fractional_x);

      return simple_interpolate(i1 , i2 , fractional_y);
}


float Noise2D(in vec2 coord, in float wavelength)
{
return interpolatedNoise2D(coord.x/wavelength, coord.y/wavelength);

}



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 * smoothstep(0.04,0.06,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()
{



float dist = length(relPos);

// 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;
    //vec3 halfVector = normalize(normalize(lightDir) + normalize(ecViewdir));
    vec4 texel;
    vec4 snow_texel;
    vec4 detail_texel;
    vec4 mix_texel;
    vec4 fragColor;
    vec4 specular = vec4(0.0);
    float intensity;
    

// get noise at different wavelengths

// used:	5m, 5m gradient, 10m, 10m gradient: heightmap of the closeup terrain, 10m also snow
//		50m: detail texel
//		250m: detail texel
//		500m: distortion and overlay
// 		1500m: overlay, detail, dust, fog
//		2000m: overlay, detail, snow, fog

float noise_10m; 
float noise_5m;  
noise_10m = Noise2D(rawPos.xy, 10.0);
noise_5m = Noise2D(rawPos.xy ,5.0);

float noisegrad_10m;
float noisegrad_5m;

float noise_50m; 
float noise_250m;
float noise_500m = Noise2D(rawPos.xy, 500.0);
float noise_1500m = Noise2D(rawPos.xy, 1500.0);
float noise_2000m = Noise2D(rawPos.xy, 2000.0);





//


// get the texels

    texel = texture2D(texture, gl_TexCoord[0].st);

    float distortion_factor = 1.0;
    vec2 stprime;
    int flag = 1;
    int mix_flag = 1;

    if (quality_level > 3)
	{
	snow_texel = texture2D(snow_texture, gl_TexCoord[0].st);
	}

    if (tquality_level > 2)
	{
	mix_texel = texture2D(mix_texture, gl_TexCoord[0].st * 1.3);
	if (mix_texel.a <0.1) {mix_flag = 0;}
 	}


    if (tquality_level > 3)  
	{
	stprime = vec2 (0.86*gl_TexCoord[0].s + 0.5*gl_TexCoord[0].t, 0.5*gl_TexCoord[0].s - 0.86*gl_TexCoord[0].t);
    	//distortion_factor = 0.9375 + (1.0 * nvL[2]);
	distortion_factor = 0.97 + 0.06 * noise_500m;
	stprime = stprime * distortion_factor * 15.0;
	if (quality_level > 4)
		{
		stprime = stprime + normalize(relPos).xy * 0.02 * (noise_10m + 0.5 * noise_5m - 0.75);
		}
    	detail_texel = texture2D(detail_texture, stprime);
	if (detail_texel.a <0.1) {flag = 0;}
	}


// texture preparation according to detail level

// mix in hires texture patches

float dist_fact; 
float nSum;
float mix_factor;

if (tquality_level > 2)
   {
   // first the second texture overlay
   
   
   if (mix_flag == 1)
	{
	nSum = nSum + 0.2 * (2.0 * noise_2000m + 2.0 * noise_1500m + noise_500m);
        nSum = nSum + 0.2 * (1.0 -smoothstep(0.9,0.95, abs(steepness)));
	mix_factor = smoothstep(0.5, 0.54, nSum);
        texel = mix(texel, mix_texel, mix_factor);

	}
   
   // then the detail texture overlay	
  }

if (tquality_level > 3)
     {	
   if (dist < 40000.0)
   	{
	if (flag == 1)
		{
		noise_50m = Noise2D(rawPos.xy, 50.0);
		noise_250m  = Noise2D(rawPos.xy, 250.0); 
		dist_fact =  0.1 * smoothstep(15000.0,40000.0, dist) - 0.03 * (1.0 - smoothstep(500.0,5000.0, dist));
		nSum = ((1.0 -noise_2000m) + noise_1500m + 2.0 * noise_250m  +noise_50m)/5.0;
        	nSum = nSum - 0.03 * (1.0 -smoothstep(0.9,0.95, abs(steepness)));		
		mix_factor = smoothstep(0.47, 0.54, nSum - dist_fact);
		if (mix_factor > 0.8) {mix_factor = 0.8;}
		texel =  mix(texel, detail_texel,mix_factor);				
		}
	}
   }


vec4 dust_color;
float snow_alpha;

if (quality_level > 3)
	{
	// mix dust
    	dust_color = vec4 (0.76, 0.71, 0.56, 1.0);

    	texel = mix(texel, dust_color, clamp(0.5 * dust_cover_factor + 3.0 * dust_cover_factor * (((noise_1500m - 0.5) * 0.125)+0.125 ),0.0, 1.0) );

    	// mix snow
   	snow_alpha = smoothstep(0.75, 0.85, abs(steepness));
	texel = mix(texel, snow_texel, smoothstep(mysnowlevel, mysnowlevel+200.0, snow_alpha * (relPos.z + eye_alt)+ (noise_2000m + 0.1 * noise_10m -0.55) *400.0));
	}



// get distribution of water when terrain is wet

float water_threshold1;
float water_threshold2;
float water_factor =0.0;


if ((dist < 5000.0)&& (quality_level > 3) && (wetness>0.0))
		{
		water_threshold1 = 1.0-0.5* wetness;
		water_threshold2 = 1.0 - 0.3 * wetness;
		water_factor = smoothstep(water_threshold1, water_threshold2 ,   (0.3 * (2.0 * (1.0-noise_10m) + (1.0 -noise_5m)) *   (1.0 - smoothstep(2000.0, 5000.0, dist))) - 5.0 * (1.0 -steepness));
	}

// darken wet terrain

    texel.rgb = texel.rgb * (1.0 - 0.6 * wetness);


// light computations


    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 ((tquality_level > 3) && (mix_flag ==1)&& (dist < 2000.0) && (quality_level > 4)) 
	{
	noisegrad_10m = (noise_10m - Noise2D(rawPos.xy+ 0.05 * normalize(lightDir.xy),10.0))/0.05;
	noisegrad_5m = (noise_5m - Noise2D(rawPos.xy+ 0.05 * normalize(lightDir.xy),5.0))/0.05;
	NdotL = NdotL + 1.0 * (noisegrad_10m + 0.5* noisegrad_5m) * mix_factor/0.8 *  (1.0 - smoothstep(1000.0, 2000.0, dist));
	}
    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 + (water_factor * vec3 (1.0, 1.0, 1.0)))
                            * light_specular.rgb
                            * pow(NdotHV, gl_FrontMaterial.shininess + (20.0 * water_factor)));
    }
    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);




    fragColor = color * texel + specular;

// here comes the terrain haze model


float delta_z = hazeLayerAltitude - eye_alt;

if (dist > max(40.0, 0.04 * min(visibility,avisibility))) 
//if ((gl_FragCoord.y > ylimit) || (gl_FragCoord.x < zlimit1) || (gl_FragCoord.x > zlimit2))
//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;



if (visibility < avisibility)
	{
	if (quality_level > 3)
		{
		transmission_arg = transmission_arg + (distance_in_layer/(1.0 * visibility + 1.0 * visibility * fogstructure * 0.06 * (noise_1500m + noise_2000m -1.0) ));

		}
	else
		{
		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 
	{
	if (quality_level > 3)
		{
		transmission_arg = transmission_arg + (distance_in_layer/(1.0 * avisibility + 1.0 * avisibility * fogstructure * 0.06 * (noise_1500m + noise_2000m  - 1.0) ));
		}
	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 -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
eShade = 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,eShade) )); 

// change haze color to blue hue for strong fogging
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));




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


gl_FragColor = fragColor;


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



}