// Copyright (C) 2006 Robert Geist
// All rights reserved.
// xrgba_rs.frag ... always called with k == EDGE/4 -1;
// we want to update only rgb layers, a layer remains fixed
// if direction xyzdir yields k == EDGE/4, we need shift the lookup: 
// r(E/4) -> g(0), g(E/4) -> b(0), b(E/4) -> a(0)
#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, row, col, from_m, scatter, alpha;
vec4 color, cloud_density, nbr_incoming_color, scattered_in; 
vec3 from_node, xyzdir, pcolor;
int n;

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);

// we need to look up this color to get the alpha value;
// it's coming from the bottom layer lower boundary, 
// and it shouldn't change
from_node = vec3(i,j,k);
row = from_node.z*fEDGE + from_node.x;
col = from_node.y*fDIR + m;
color = texture2DRect(photon,vec2(col,row));
alpha = color.a;

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

if(from_node.z > (fEDGE/4.0 - 1.0)){
	// color shift
	from_node.z = 0.0;
	row = from_node.z*fEDGE + from_node.x;
	col = from_node.y*fDIR + m;
	color = texture2DRect(photon,vec2(col,row));

	// udate .rgb but read from .gba
	pcolor  = color.gba;
	cloud_density = texture2DRect(cloud,vec2(from_node.y,row));
	
	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;
		}
	pcolor += (scattered_in.gba)*(cloud_density.gba);
	color = vec4(pcolor,alpha);
	}
else {
	row = from_node.z*fEDGE + from_node.x;
	col = from_node.y*fDIR + m;
	color = texture2DRect(photon,vec2(col,row));

	// only the rgb layers change
	pcolor = color.rgb;
	
	cloud_density = texture2DRect(cloud,vec2(from_node.y,row));
	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;
		}
	pcolor += (scattered_in.rgb)*(cloud_density.rgb);
	color = vec4(pcolor,alpha);
	}
gl_FragColor = color; 
}