/* -*- mode: c++; c-basic-offset: 4; indent-tabs-mode: nil -*- */
/*
 * Copyright (c) 2004-2013  HUBzero Foundation, LLC
 *
 * Authors:
 *   Wei Qiao <qiaow@purdue.edu>
 *   Leif Delgass <ldelgass@purdue.edu>
 */

#include "common.cg"

//#define LIGHT_TWO_SIDE

/*
 * render one volume 
 *
 * material:
 *   x: ambient coefficient
 *   y: diffuse coefficient
 *   z: specular level/coefficient
 *   w: specular exponent
 * renderParams:
 *   x: sample distance ratio (0 = single slice)
 *   y: isosurface flag (opacity scaling on/off)
 *   z: opacity scale
 *   w: two-sided lighting
 */
PixelOut main(v2f IN,
              uniform sampler3D volume,
              uniform sampler1D tf,
              uniform float4 material,
              uniform float4 renderParams)
{
    PixelOut OUT;
    float4 sample = tex3D(volume, IN.TexCoord.xyz);

    //sample the transfer function texture
    float4 color = tex1D(tf, sample.x);

    if (renderParams.x == 0.0) {
        //If single slice render, only flat shading, completely opaque.
        color.w = 1;
    } else {
        //regular volume rendering, we do PHONG SHADING 
        //lighting parameters
        float3 normal;
        float diffuse, specular;
        if (dot(sample.yzw, sample.yzw) < 5.0e-5) {
            diffuse = 0;
            specular = 0;
        } else {
            normal = normalize(sample.yzw * 2.0 - 1.0);
            float3 light_vector = normalize(IN.Light);
            float3 eye_vector = normalize(IN.EyeVector);
            float3 half_vector = normalize(eye_vector+light_vector);

            //lighting computation
#ifndef LIGHT_TWO_SIDE
            float normal_dot_light = dot(normal, light_vector);
            float normal_dot_half = dot(normal, half_vector);
            if (renderParams.w > 0.5) {
                normal_dot_light = abs(normal_dot_light);
                normal_dot_half = abs(normal_dot_half);
            } else {
                normal_dot_light = max(normal_dot_light, 0);
                normal_dot_half = max(normal_dot_half, 0);
            }
#else
            float normal_dot_light = abs(dot(normal, light_vector));
            float normal_dot_half = abs(dot(normal, half_vector));
#endif
            diffuse = normal_dot_light * material.y;
            if (diffuse > 1.0e-6) 
                specular = pow(normal_dot_half, material.w) * material.z;
            else
                specular = 0.0;
        }
        float ambient = material.x;

        color.xyz = color.xyz * (ambient + diffuse) + float3(specular);

        if (renderParams.y < 0.5) {
            // Opacity scaling: equivalent to scaling transfer function
            color.w = color.w * renderParams.z;
            // Apply opacity correction for ratio of current sample distance
            // to data set sample distance
            color.w = (1.0 - pow((1.0 - color.w), renderParams.x));
        }
    }

    OUT.Color = min(max(color, 0.0), 1.0);
    return OUT;
}