vector SunPos = {1.0, 1.0, 1.0, 1.0 };

float time_of_day;

vector Zenith  = { 0.00, 0.34, 0.71, 0.00 };
vector Horizon = { 0.70, 0.40, 0.20, 0.00 };

float4x4 MatWVP 	: ViewProjection;
float4x4 MatViewInv	: ViewInverse;
float4x4 MatWorld	: World;

float3 LightDir	= {1.0f, 1.0f, 1.0f };

sampler TextureOne = sampler_state
{
   MipFilter = LINEAR;
   MinFilter = LINEAR;
   MagFilter = LINEAR;
   ADDRESSU = WRAP;
   ADDRESSV = WRAP;
   ADDRESSW = WRAP;
};

sampler TextureTwo = sampler_state
{
   MipFilter = LINEAR;
   MinFilter = LINEAR;
   MagFilter = LINEAR;
   ADDRESSU = WRAP;
   ADDRESSV = WRAP;
   ADDRESSW = WRAP;
};

sampler TextureThree = sampler_state
{
   MipFilter = LINEAR;
   MinFilter = LINEAR;
   MagFilter = LINEAR;
   ADDRESSU = WRAP;
   ADDRESSV = WRAP;
   ADDRESSW = WRAP;
};

struct VS_input
{
	float4 Position : POSITION0;
	float2 tex1 : TEXCOORD0;
};

struct VS_output
{
	float4 Position : POSITION0;
   	float2 tex1 : TEXCOORD0;
	float3 Vertex : TEXCOORD1;
};

struct PS_output
{
   float4 diffuse : COLOR0;
};

VS_output vs_main( VS_input input )
{
   VS_output output;

   output.Vertex = input.Position;

   output.Position = mul( float4(input.Position.xyz , 1.0f) , MatWVP);
   
   output.tex1 = input.tex1;

   return( output );
}


PS_output ps_main(in VS_output input)
{
   	PS_output output = (PS_output)0;
   	
	float3 norm = normalize(input.Vertex).xyz;
	float3 Sun = normalize(SunPos.xyz);
	float  SunDot = dot(Sun, norm);

    	float2 cycle = {fmod(SunPos.w, 100.0), 0};
    	float fact_clouds = saturate(SunPos.y * 4.0) * 0.3;
   	fact_clouds *= time_of_day;
	
	// first texture: clouds

    	float4 clouds1 = tex2D(TextureOne, input.tex1 * 2.0 + cycle);   
	clouds1.rgb *= fact_clouds;
   	
    	// second texture: stars
    
    	float3 stars = tex2D(TextureTwo, input.tex1 * 4.0  - cycle);
	stars *= 1 - saturate(time_of_day * 1.5);
    
   	// third texture: more clouds

   	float4 clouds2 = tex2D(TextureThree, input.tex1 + cycle * 1.5 );    
	clouds2.rgb *= fact_clouds;
	
	// brighten up the clouds near the sun

	vector rays = 0.7f * pow( max(0.0, SunDot), 60.0f );
	rays *= clouds2.a;

	// use alpha from the clouds to hide stars behind clouds
	
	float clouds_alpha = max(clouds1.a, clouds2.a);

	stars *= 1 - clouds_alpha * time_of_day;
    
    	// moon and sun

	vector light = 0.8f * pow( max(0.0001, SunDot), 360.0f );
	//vector moon  = 0.3f * pow( max(0.0, dot(-Sun, norm)), 60.0f );

	// atmosphere
	
	float Curve = 0.25f * (2.0f - SunPos.y);

	vector sky = Zenith * (1-Curve) + Horizon * Curve;
	sky = sky * (1+SunDot) + Zenith * -SunDot;
	sky *= (1.0-clouds_alpha*(1.0-time_of_day));
	sky *= saturate(SunPos.y+2.0);
			
	// combine everything

    	output.diffuse.rgb = sky + clouds1 + stars + clouds2 + light + rays; // + moon 
	output.diffuse.a = 1.0f;

	return output;

} 


technique sky
{
   pass Pass_0
   {
      VertexShader = compile vs_2_0 vs_main();
      PixelShader = compile ps_2_0 ps_main();

   }
}