/* -*- mode: c++; c-basic-offset: 4; indent-tabs-mode: nil -*- */
#include "common.cg"

PixelOut main(v2f IN, 
              uniform sampler1D tf,
              uniform sampler3D vol_grid_s, 
              uniform float4 cell_size, 
              uniform float4x4 ModelViewInv, 
              uniform float4 renderParameters)
{
    PixelOut OUT;

    float tf_coord_s, tf_coord_p, tf_coord_d, tf_coord_ss;

    float4 twice_cell_size = cell_size*2.0;
    float4 tex_coord = mul(ModelViewInv, IN.TexCoord);
    float4 tex_coord_a = tex_coord - float4(twice_cell_size.x, twice_cell_size.y, 0, 0);
    float4 tex_coord_b = tex_coord - float4(twice_cell_size.x, 0, twice_cell_size.z, 0);
    float4 tex_coord_c = tex_coord - float4(0, twice_cell_size.y, twice_cell_size.z, 0); 

    float4 voxel_corner = f4tex3D(vol_grid_s, tex_coord);
    float4 voxel_face_center_a = f4tex3D(vol_grid_s, tex_coord_a);
    float4 voxel_face_center_b = f4tex3D(vol_grid_s, tex_coord_b);
    float4 voxel_face_center_c = f4tex3D(vol_grid_s, tex_coord_c);  
    float sample = (voxel_corner.x + voxel_face_center_a.y + voxel_face_center_b.z + voxel_face_center_c.w)*0.25;

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

    if (renderParameters.x < 0.5) { 
        //If single slice render, only flat shading, completely opaque.
        color.w = 1;
    } else {
        //regular volume rendering, we do FLAT SHADING 
        //since all 4 components of the volume texture has been used to store orbital data,
        //there is no room to store normal vector. If we really need PHONG shading we have to pass more volumes to
        //the shader. We might add this later.

        //opacity is modulated by the number of total slices
        //to avoid very opaque volume when number of slices is high
        color.w = renderParameters.y * color.w / renderParameters.x; 
    }

    OUT.Color = color;

    return OUT;
}