struct input{
  float4 position : WPOS;
  float4 color : COLOR;
};

struct output{
  float4 color : COLOR;
};

output FragmentProgram(input IN,
            const uniform samplerRECT PlAtDepth,
            const uniform samplerRECT Planet,
            const uniform samplerRECT ShadowFront,
            const uniform samplerRECT ShadowBack,
            const uniform float4 ZParam)
{
/*
z_buffer_value = (1<<N) * ( a + b / z )
  Where:
     N = number of bits of Z precision
     a = zFar / ( zFar - zNear )
     b = zFar * zNear / ( zNear - zFar )
     z = distance from the eye to the object
*/
    output OUT;

    float zplanet = texRECT(Planet, IN.position.xy);

    float zback = texRECT(PlAtDepth, IN.position.xy);
    float zshb = min(max(texRECT(ShadowBack, IN.position.xy), IN.position.z), zback);
    float zshf = min(max(texRECT(ShadowFront,IN.position.xy), IN.position.z), zback);
    
    // z-buffer linearization
    float4 z = float4(zback, zshb, zshf, IN.position.z);
    z = ZParam.y / (ZParam.x);
    
    //old// float2 dz = float2( max(z.x-z.y, 0.0) + max(min(z.x, z.z)-z.w, 0.0), z.y-z.z );
    
    //float2 dz = float2( z.x-z.y + z.z-z.w, z.y-z.z ); // outside shadow
    float2 dz = float2( z.x-z.z, z.z-z.w ); // inside shadow

    dz = dz / (ZParam.w - ZParam.z);
    
    // atmosphere above planet
    if(zplanet!=1.0){
        //dz = pow(dz.x,0.5);
        dz *= 2.0;
    }
  
    OUT.color.a = IN.color.a;// * dz.x;
    OUT.color.rgb = 3.0*(IN.color.rgb * (dz.x + 0.2 * dz.y));
    
    
       
    return OUT;
}