// Copyright (C) 2006 Robert Geist
// All rights reserved.
#include "xrgba.h"

uniform sampler2DRect cloud;
uniform sampler2DRect photon;
uniform sampler2DRect omega;
uniform sampler2DRect directions;

#define DIRECTIONS 19

void main(void)
{
float fEDGE = float(EDGE);
float fDIR = float(DIRECTIONS); 
float i, j, k, m;
float row, col, from_m, scatter;
vec4 cloud_density, nbr_incoming_color, scattered_in;
vec3 from_node, xyzdir;
int n;
vec4 color;
float goober;

i = mod(gl_TexCoord[0].t,fEDGE);
j = floor(gl_TexCoord[0].s/fDIR);
k = floor(gl_TexCoord[0].t/fEDGE);
m = mod(gl_TexCoord[0].s,fDIR);

// access incoming photon density in direction m from neighbor
xyzdir = texture2DRect(directions,vec2(m,0.5));
from_node = vec3(i,j,k) - xyzdir;

// build photon texture index;
// note this is always called with 1 < k < EDGE/4 -1
row = from_node.z*fEDGE + from_node.x;
col = from_node.y*fDIR + m;

// grab 4 photon values in parallel
color = texture2DRect(photon,vec2(col,row));

// that's it, unless the neighbor had a non-zero cloud density;
// in that case, he might have scattered some our way (m) from
// other directions
cloud_density = texture2DRect(cloud,vec2(from_node.y,row));

// collect the density scattered our way by the neighbor
scattered_in = vec4(0.0,0.0,0.0,0.0);
for(n=0;n<DIRECTIONS;n++){
	from_m = float(n) + 0.5;
	col = from_node.y*fDIR + from_m;
	nbr_incoming_color = texture2DRect(photon,vec2(col,row));
	scatter = texture2DRect(omega,vec2(from_m,m));
	scattered_in += (scatter*nbr_incoming_color);
	}
color += (scattered_in*cloud_density);
gl_FragColor = color; 
}