//--------------------------------------------------------------
// Fragment Shader: Attribute 2D Transfer Function
//--------------------------------------------------------------
uniform sampler3D sam3Tex0;  // Sampler for Transfer Function
uniform sampler3D sam3Tex1;  // Sampler for Value Image
uniform sampler3D sam3Tex2;  // Sampler for Gradient Image
uniform sampler3D sam3Tex3;  // Sampler for Tags
uniform float fSamplingRate; // Current sampling rate
uniform float fNormalOffset; // Offset for normal calculataions

float adjustAlphaForSamplingRate( float a )
{
    return 1.0 - pow( 1.0 - a,  1.0 / fSamplingRate );
}

vec4 computeNormal( )
{
    vec3 pos3Tex1X1 = vec3( gl_TexCoord[1] ); pos3Tex1X1.x -= fNormalOffset;
    vec3 pos3Tex1X2 = vec3( gl_TexCoord[1] ); pos3Tex1X2.x += fNormalOffset;
    vec3 pos3Tex1Y1 = vec3( gl_TexCoord[1] ); pos3Tex1Y1.y -= fNormalOffset;
    vec3 pos3Tex1Y2 = vec3( gl_TexCoord[1] ); pos3Tex1Y2.y += fNormalOffset;
    vec3 pos3Tex1Z1 = vec3( gl_TexCoord[1] ); pos3Tex1Z1.z -= fNormalOffset;
    vec3 pos3Tex1Z2 = vec3( gl_TexCoord[1] ); pos3Tex1Z2.z += fNormalOffset;
    vec4 val4Tex1X1 = texture3D( sam3Tex1, pos3Tex1X1 );
    vec4 val4Tex1X2 = texture3D( sam3Tex1, pos3Tex1X2 );
    vec4 val4Tex1Y1 = texture3D( sam3Tex1, pos3Tex1Y1 );
    vec4 val4Tex1Y2 = texture3D( sam3Tex1, pos3Tex1Y2 );
    vec4 val4Tex1Z1 = texture3D( sam3Tex1, pos3Tex1Z1 );
    vec4 val4Tex1Z2 = texture3D( sam3Tex1, pos3Tex1Z2 );
    float fDiffTex1X = 0.5*val4Tex1X1.a - 0.5*val4Tex1X2.a;
    float fDiffTex1Y = 0.5*val4Tex1Y1.a - 0.5*val4Tex1Y2.a;
    float fDiffTex1Z = 0.5*val4Tex1Z1.a - 0.5*val4Tex1Z2.a;
    return vec4( fDiffTex1X, fDiffTex1Y, fDiffTex1Z, 0.0 );
}

void main()
{
    // Interpolate images
    float value  = texture3D( sam3Tex1, gl_TexCoord[1] ).a;
    float grad   = texture3D( sam3Tex2, gl_TexCoord[2] ).a;
    float attrib = texture3D( sam3Tex3, gl_TexCoord[3] ).a;

    // Interpolate transfer function
    const float fNumLayers = 2.0;
    vec3 pos3TfAll  = vec3( value, 1.0-grad, 0.0/(fNumLayers-1.0) );
    vec3 pos3TfTag1 = vec3( attrib, 1.0-grad, 1.0/(fNumLayers-1.0) );
    //vec3 pos3TfTag2 = vec3( value, 1.0-grad, 2.0/(fNumLayers-1.0) );
    //vec3 pos3TfTag3 = vec3( value, 1.0-grad, 3.0/(fNumLayers-1.0) );
    //vec3 pos3TfTag4 = vec3( value, 1.0-grad, 4.0/(fNumLayers-1.0) );
    vec4 val4TfAll  = texture3D( sam3Tex0, pos3TfAll );
    vec4 val4TfTag1 = texture3D( sam3Tex0, pos3TfTag1 );
    //vec4 val4TfTag2 = texture3D( sam3Tex0, pos3TfTag2 );
    //vec4 val4TfTag3 = texture3D( sam3Tex0, pos3TfTag3 );
    //vec4 val4TfTag4 = texture3D( sam3Tex0, pos3TfTag4 );

    // Adjust alpha for sampling rate
    val4TfAll.a  = adjustAlphaForSamplingRate( val4TfAll.a );
    val4TfTag1.a = adjustAlphaForSamplingRate( val4TfTag1.a );
    //val4TfTag2.a = adjustAlphaForSamplingRate( val4TfTag2.a );
    //val4TfTag3.a = adjustAlphaForSamplingRate( val4TfTag3.a );
    //val4TfTag4.a = adjustAlphaForSamplingRate( val4TfTag4.a );

    // Mix tags
    vec4 val4Mix = val4TfAll;
    if (attrib > 0.0 && val4TfTag1.a > 0.0)
        val4Mix = val4TfTag1;
    //val4Mix = mix( val4Mix, val4TfTag1, 1.0-val4TfTag1.a );
    //if (attrib > 0.0) val4Mix = val4TfTag1;
    //else if (tags > XXX) vec4Mix = vec4TfTag2;
    //else if (tags > XXX) vec4Mix = vec4TfTag3;
    //else if (tags > XXX) vec4Mix = vec4TfTag4;

    // Compute the normal
    vec4 vec4N = computeNormal( );
    vec4N = normalize( vec4N );

    // Get lighting parameters
    gl_LightSourceParameters light0 = gl_LightSource[0];
    float fExp = gl_FrontMaterial.shininess;

    // Compute light vector
    vec4 vec4L = vec4( 0.0, 0.0, -0.5, 0.0 );
    vec4L = vec4L * gl_ModelViewMatrix;
    vec4L = vec4( vec4L.x, vec4L.y, -vec4L.z, 0.0 );
    
    // Compute lighting model values
    float fNdotL = max( dot(vec4N, vec4L), 0.0 );
    vec4 vec4R =  (2.0 * fNdotL * vec4N) - vec4L;
    vec4R = normalize( vec4R );
    float fNdotR = max( dot(vec4N, vec4R), 0.0 );

    // Apply to value
    gl_FragColor  = ( val4Mix * light0.ambient );
    gl_FragColor += ( val4Mix * light0.diffuse  * fNdotL );
    gl_FragColor += ( val4Mix * light0.specular * pow(fNdotR,fExp) );
}