diff -Prub nethack-3.4.3/include/config.h nethack-3.4.3_LOS/include/config.h
--- nethack-3.4.3/include/config.h	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/include/config.h	2004-09-08 20:03:59.000000000 +0200
@@ -350,6 +350,10 @@
 
 /*#define GOLDOBJ */	/* Gold is kept on obj chains - Helge Hafting */
 /*#define AUTOPICKUP_EXCEPTIONS */ /* exceptions to autopickup */
+#define LINEOFSIGHT	/* Show line of sight */
+#ifdef LINEOFSIGHT
+# define LINEOFSIGHT_COLOUR	/* Allow selection of the colour used for shading */
+#endif
 
 /* End of Section 5 */
 
diff -Prub nethack-3.4.3/include/decl.h nethack-3.4.3_LOS/include/decl.h
--- nethack-3.4.3/include/decl.h	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/include/decl.h	2004-09-07 16:37:24.000000000 +0200
@@ -326,6 +326,9 @@
 /* Vision */
 E NEARDATA boolean vision_full_recalc;	/* TRUE if need vision recalc */
 E NEARDATA char **viz_array;		/* could see/in sight row pointers */
+#ifdef LINEOFSIGHT
+E NEARDATA char **viz_array_old;	/* could see/in sight row pointers previous */
+#endif
 
 /* Window system stuff */
 E NEARDATA winid WIN_MESSAGE, WIN_STATUS;
diff -Prub nethack-3.4.3/include/flag.h nethack-3.4.3_LOS/include/flag.h
--- nethack-3.4.3/include/flag.h	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/include/flag.h	2004-09-08 20:05:32.000000000 +0200
@@ -220,6 +220,9 @@
  * Window capability support.
  */
 	boolean wc_color;		/* use color graphics                  */
+#ifdef LINEOFSIGHT
+	boolean wc_lineofsight;		/* show line of sight                  */
+#endif	
 	boolean wc_hilite_pet;		/* hilight pets                        */
 	boolean wc_ascii_map;		/* show map using traditional ascii    */
 	boolean wc_tiled_map;		/* show map using tiles                */
@@ -271,6 +274,9 @@
 	boolean  showrace;	/* show hero glyph by race rather than by role */
 	boolean  travelcmd;	/* allow travel command */
 	int	 runmode;	/* update screen display during run moves */
+#ifdef LINEOFSIGHT_COLOUR
+	int	 los_colour;	/* colour to use for shaded areas when lineofsight is on */
+#endif	
 #ifdef AUTOPICKUP_EXCEPTIONS
 	struct autopickup_exception *autopickup_exceptions[2];
 #define AP_LEAVE 0
diff -Prub nethack-3.4.3/include/vision.h nethack-3.4.3_LOS/include/vision.h
--- nethack-3.4.3/include/vision.h	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/include/vision.h	2004-09-08 19:05:08.000000000 +0200
@@ -29,6 +29,10 @@
  */
 #define cansee(x,y)	(viz_array[y][x] & IN_SIGHT)
 #define couldsee(x,y)	(viz_array[y][x] & COULD_SEE)
+#ifdef LINEOFSIGHT
+# define couldsee_old(x,y)	(viz_array_old[y][x] & COULD_SEE)
+# define cansee_old(x,y)	(viz_array_old[y][x] & IN_SIGHT)
+#endif
 #define templit(x,y)	(viz_array[y][x] & TEMP_LIT)
 
 /*
diff -Prub nethack-3.4.3/src/decl.c nethack-3.4.3_LOS/src/decl.c
--- nethack-3.4.3/src/decl.c	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/src/decl.c	2004-09-07 16:39:03.000000000 +0200
@@ -254,6 +254,9 @@
 /* Vision */
 NEARDATA boolean vision_full_recalc = 0;
 NEARDATA char	 **viz_array = 0;/* used in cansee() and couldsee() macros */
+#ifdef LINEOFSIGHT
+NEARDATA char	 **viz_array_old = 0;/* used in cansee() and couldsee() macros */
+#endif
 
 /* Global windowing data, defined here for multi-window-system support */
 NEARDATA winid WIN_MESSAGE = WIN_ERR, WIN_STATUS = WIN_ERR;
diff -Prub nethack-3.4.3/src/display.c nethack-3.4.3_LOS/src/display.c
--- nethack-3.4.3/src/display.c	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/src/display.c	2004-09-08 19:06:20.000000000 +0200
@@ -1266,7 +1266,11 @@
 	return;
     }
 
+#ifdef LINEOFSIGHT
+    if ((gbuf[y][x].glyph != glyph) || ((glyph!=cmap_to_glyph(S_stone)) && ((couldsee(x,y)!=couldsee_old(x,y)) || (cansee(x,y)!=cansee_old(x,y))))) {
+#else
     if (gbuf[y][x].glyph != glyph) {
+#endif
 	gbuf[y][x].glyph = glyph;
 	gbuf[y][x].new   = 1;
 	if (gbuf_start[y] > x) gbuf_start[y] = x;
diff -Prub nethack-3.4.3/src/mapglyph.c nethack-3.4.3_LOS/src/mapglyph.c
--- nethack-3.4.3/src/mapglyph.c	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/src/mapglyph.c	2004-09-08 20:42:20.000000000 +0200
@@ -125,9 +125,24 @@
 #ifdef TEXTCOLOR
 	    /* provide a visible difference if normal and lit corridor
 	     * use the same symbol */
+# ifdef LINEOFSIGHT
+	    if (iflags.use_color) {
+		if (iflags.wc_lineofsight) {
+		    if (!couldsee(x,y) || !cansee(x,y) || (offset == S_corr))
+#  ifdef LINEOFSIGHT_COLOUR
+			color = iflags.los_colour;
+#  else		    
+			color = CLR_BLUE;
+#  endif			
+		}
+		else if (offset == S_litcorr && ch == showsyms[S_corr])
+		    color = CLR_WHITE;
+	    }
+# else
 	    if (iflags.use_color &&
 		offset == S_litcorr && ch == showsyms[S_corr])
 		color = CLR_WHITE;
+# endif
 	    else
 #endif
 	    cmap_color(offset);
diff -Prub nethack-3.4.3/src/options.c nethack-3.4.3_LOS/src/options.c
--- nethack-3.4.3/src/options.c	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/src/options.c	2004-09-08 21:27:20.000000000 +0200
@@ -74,6 +74,9 @@
 # else	/* systems that support multiple terminals, many monochrome */
 	{"color",         &iflags.wc_color, FALSE, SET_IN_GAME},	/*WC*/
 # endif
+#ifdef LINEOFSIGHT
+	{"lineofsight", &iflags.wc_lineofsight, TRUE, SET_IN_GAME},
+#endif
 	{"confirm",&flags.confirm, TRUE, SET_IN_GAME},
 #if defined(TERMLIB) && !defined(MAC_GRAPHICS_ENV)
 	{"DECgraphics", &iflags.DECgraphics, FALSE, SET_IN_GAME},
@@ -301,6 +304,10 @@
 						PL_CSIZ, DISP_IN_GAME },
 	{ "runmode", "display frequency when `running' or `travelling'",
 						sizeof "teleport", SET_IN_GAME },
+#ifdef LINEOFSIGHT_COLOUR
+	{ "los_colour", "colour of the shaded area when loneofsight is on",
+						sizeof "bright magenta", SET_IN_GAME },
+#endif						
 	{ "scores",   "the parts of the score list you wish to see",
 						32, SET_IN_GAME },
 	{ "scroll_amount", "amount to scroll map when scroll_margin is reached",
@@ -507,6 +514,9 @@
 	flags.end_top = 3;
 	flags.end_around = 2;
 	iflags.runmode = RUN_LEAP;
+#ifdef LINEOFSIGHT_COLOUR
+	iflags.los_colour = CLR_BLUE;
+#endif
 	iflags.msg_history = 20;
 #ifdef TTY_GRAPHICS
 	iflags.prevmsg_window = 's';
@@ -964,6 +974,13 @@
 	}
 }
 
+#ifdef LINEOFSIGHT_COLOUR
+static NEARDATA const char *lineofsight_colours[] = {
+	"black", "red", "green", "brown", "blue", "magenta", "cyan", "grey",
+	"no,colour", "orange", "bright green", "yellow", "bright blue", 
+	"bright magenta", "bright cyan", "white"
+};
+#endif
 void
 parseoptions(opts, tinitial, tfrom_file)
 register char *opts;
@@ -974,6 +991,9 @@
 	boolean negated;
 	int i;
 	const char *fullname;
+#ifdef LINEOFSIGHT_COLOUR
+	int los_colour;
+#endif
 
 	initial = tinitial;
 	from_file = tfrom_file;
@@ -1133,6 +1153,26 @@
 		return;
 	}
 
+#ifdef LINEOFSIGHT_COLOUR
+	fullname = "los_colour";
+	if (match_optname(opts, fullname, 4, TRUE)) {
+	    if (negated) {
+		iflags.los_colour = CLR_BLUE;
+	    } else if ((op = string_for_opt(opts, FALSE)) != 0) {
+		if (!strncmpi(op, "gray", strlen(op)))
+		    iflags.los_colour = CLR_GRAY;
+		else
+		    for (los_colour=0; los_colour < CLR_MAX; los_colour++) {
+			if (!strncmpi(op, lineofsight_colours[los_colour], strlen(op))) {
+			    iflags.los_colour = los_colour;
+			    break;
+			}
+			if (los_colour == CLR_MAX-1) badoption(opts);
+		    }
+		}
+	    return;
+	}
+#endif
 	fullname = "msghistory";
 	if (match_optname(opts, fullname, 3, TRUE)) {
 		op = string_for_env_opt(fullname, opts, negated);
@@ -2240,6 +2280,11 @@
 # endif
 			}
 #endif
+#ifdef LINEOFSIGHT
+			else if ((boolopt[i].addr) == &iflags.wc_lineofsight) {
+			    need_redraw = TRUE;
+			}
+#endif
 
 			return;
 		}
@@ -2669,6 +2714,28 @@
 	}
 	destroy_nhwindow(tmpwin);
 	retval = TRUE;
+#ifdef LINEOFSIGHT_COLOUR
+    } else if (!strcmp("los_colour", optname)) {
+	const char *mode_name;
+	menu_item *mode_pick = (menu_item *)0;
+	tmpwin = create_nhwindow(NHW_MENU);
+	start_menu(tmpwin);
+	for (i = 0; i < SIZE(lineofsight_colours); i++) {
+		mode_name = lineofsight_colours[i];
+		any.a_int = i + 1;
+		if (i != NO_COLOR)
+		    add_menu(tmpwin, NO_GLYPH, &any, 0, 0,
+			    ATR_NONE, mode_name, MENU_UNSELECTED);
+	}
+	end_menu(tmpwin, "Select colour for LOS shading:");
+	if (select_menu(tmpwin, PICK_ONE, &mode_pick) > 0) {
+		iflags.los_colour = mode_pick->item.a_int - 1;
+		free((genericptr_t)mode_pick);
+	}
+	destroy_nhwindow(tmpwin);
+	need_redraw = TRUE;
+	retval = TRUE;
+#endif
     } 
 #ifdef TTY_GRAPHICS
       else if (!strcmp("msg_window", optname)) {
@@ -3060,6 +3127,10 @@
 		Sprintf(buf, "%s", rolestring(flags.initrole, roles, name.m));
 	else if (!strcmp(optname, "runmode"))
 		Sprintf(buf, "%s", runmodes[iflags.runmode]);
+#ifdef LINEOFSIGHT_COLOUR
+	else if (!strcmp(optname, "los_colour"))
+		Sprintf(buf, "%s", lineofsight_colours[iflags.los_colour]);
+#endif		
 	else if (!strcmp(optname, "scores")) {
 		Sprintf(buf, "%d top/%d around%s", flags.end_top,
 				flags.end_around, flags.end_own ? "/own" : "");
diff -Prub nethack-3.4.3/src/vision.c nethack-3.4.3_LOS/src/vision.c
--- nethack-3.4.3/src/vision.c	2003-12-08 01:39:13.000000000 +0200
+++ nethack-3.4.3_LOS/src/vision.c	2004-09-07 18:00:49.000000000 +0200
@@ -130,7 +130,11 @@
     }
 
     /* Start out with cs0 as our current array */
+#ifdef LINEOFSIGHT
+    viz_array_old = viz_array = cs_rows0;
+#else
     viz_array = cs_rows0;
+#endif
     viz_rmin  = cs_rmin0;
     viz_rmax  = cs_rmax0;
 
@@ -199,7 +203,11 @@
     register struct rm    *lev;
 
     /* Start out with cs0 as our current array */
+#ifdef LINEOFSIGHT
+    viz_array_old = viz_array = cs_rows0;
+#else
     viz_array = cs_rows0;
+#endif
     viz_rmin  = cs_rmin0;
     viz_rmax  = cs_rmax0;
 
@@ -492,7 +500,9 @@
 vision_recalc(control)
     int control;
 {
+#ifndef LINEOFSIGHT
     char **temp_array;	/* points to the old vision array */
+#endif
     char **next_array;	/* points to the new vision array */
     char *next_row;	/* row pointer for the new array */
     char *old_row;	/* row pointer for the old array */
@@ -548,11 +558,19 @@
 	 * anything, so we only need update positions we used to be able
 	 * to see.
 	 */
+#ifdef LINEOFSIGHT
+	viz_array_old = viz_array;	/* set viz_array so newsym() will work */
+#else
 	temp_array = viz_array;	/* set viz_array so newsym() will work */
+#endif
 	viz_array = next_array;
 
 	for (row = 0; row < ROWNO; row++) {
+#ifdef LINEOFSIGHT
+	    old_row = viz_array_old[row];
+#else
 	    old_row = temp_array[row];
+#endif
 
 	    /* Find the min and max positions on the row. */
 	    start = min(viz_rmin[row], next_rmin[row]);
@@ -676,7 +694,11 @@
     /*
      * Make the viz_array the new array so that cansee() will work correctly.
      */
+#ifdef LINEOFSIGHT
+    viz_array_old = viz_array;
+#else
     temp_array = viz_array;
+#endif
     viz_array = next_array;
 
     /*
@@ -698,7 +720,11 @@
     colbump[u.ux] = colbump[u.ux+1] = 1;
     for (row = 0; row < ROWNO; row++) {
 	dy = u.uy - row;                dy = sign(dy);
+#ifdef LINEOFSIGHT
+	next_row = next_array[row];     old_row = viz_array_old[row];
+#else
 	next_row = next_array[row];     old_row = temp_array[row];
+#endif
 
 	/* Find the min and max positions on the row. */
 	start = min(viz_rmin[row], next_rmin[row]);