Sightlines
Draw the guard's line of sight as dots, and detect the player if they step into it.
The guard catches you by contact — walk into the same cell and it’s over. But real guards don’t need to bump into you. They can see you. The guard now projects a sightline: a line of dots extending in its facing direction until hitting a wall.
The Program
10 BORDER 0: PAPER 0: INK 7: CLS
20 DIM m$(8, 16)
30 FOR i = 1 TO 8
40 READ m$(i)
50 NEXT i
60 GO SUB 800
70 LET r = 6: LET c = 2
80 LET g = 3: LET h = 10
90 LET d$ = "WWWWWWEEEEEE"
100 LET q = 1: LET t = 0: LET s = 15
110 LET f$ = "W"
120 REM game loop
130 PRINT AT r + 2, c + 2; INK 7; BRIGHT 1; "@"
140 PRINT AT g + 2, h + 2; INK 2; "G"
150 GO SUB 700
160 LET t = t + 1
170 LET k$ = INKEY$
180 IF k$ = "" THEN GO TO 220
190 LET u = r: LET z = c
191 IF k$ = "q" THEN LET u = r - 1
192 IF k$ = "a" THEN LET u = r + 1
193 IF k$ = "o" THEN LET z = c - 1
194 IF k$ = "p" THEN LET z = c + 1
195 IF u < 1 OR u > 8 OR z < 1 OR z > 16 THEN GO TO 220
196 IF m$(u)(z TO z) = "#" THEN GO TO 220
197 PRINT AT r + 2, c + 2; " "
198 LET r = u: LET c = z
220 REM move guard
230 IF t < s THEN GO TO 350
240 LET t = 0
245 GO SUB 760
250 PRINT AT g + 2, h + 2; " "
260 LET f$ = d$(q TO q)
270 IF f$ = "N" THEN LET g = g - 1
280 IF f$ = "S" THEN LET g = g + 1
290 IF f$ = "W" THEN LET h = h - 1
300 IF f$ = "E" THEN LET h = h + 1
310 LET q = q + 1: IF q > LEN d$ THEN LET q = 1
350 IF r = g AND c = h THEN GO TO 400
360 GO TO 130
400 REM detected
410 BEEP 0.1, 20: BEEP 0.1, 15
420 BORDER 2: PAUSE 10: BORDER 0
430 PRINT AT 12, 6; INK 2; "DETECTED!"
440 STOP
700 REM === draw sightline ===
710 LET a = g: LET b = h
720 IF f$ = "N" THEN LET a = a - 1
722 IF f$ = "S" THEN LET a = a + 1
724 IF f$ = "W" THEN LET b = b - 1
726 IF f$ = "E" THEN LET b = b + 1
728 IF a < 1 OR a > 8 OR b < 1 OR b > 16 THEN RETURN
730 IF m$(a)(b TO b) = "#" THEN RETURN
732 IF a = r AND b = c THEN GO TO 400
734 PRINT AT a + 2, b + 2; INK 6; "."
736 GO TO 720
760 REM === clear sightline ===
770 LET a = g: LET b = h
772 IF f$ = "N" THEN LET a = a - 1
774 IF f$ = "S" THEN LET a = a + 1
776 IF f$ = "W" THEN LET b = b - 1
778 IF f$ = "E" THEN LET b = b + 1
780 IF a < 1 OR a > 8 OR b < 1 OR b > 16 THEN RETURN
782 IF m$(a)(b TO b) = "#" THEN RETURN
784 PRINT AT a + 2, b + 2; " "
786 GO TO 772
800 REM === draw map ===
810 FOR y = 1 TO 8
820 FOR x = 1 TO 16
830 IF m$(y)(x TO x) = "#" THEN PRINT AT y + 2, x + 2; INK 1; CHR$ 143
840 NEXT x
850 NEXT y
860 RETURN
900 DATA "################"
910 DATA "# #"
920 DATA "# ## #### # #"
930 DATA "# # # #"
940 DATA "# #### # #"
950 DATA "# # # #"
960 DATA "# ## # #"
970 DATA "################"
How It Works
Lines 700-736 draw the sightline. Starting from the guard’s position, the subroutine steps one cell at a time in the guard’s facing direction (f$). At each cell:
- Line 728 checks boundaries — if we’ve gone off the map, stop.
- Line 730 checks walls — if the cell is
#, the wall blocks the view. Stop. - Line 732 checks for the player — if the player is in the sightline, detection triggers immediately.
- Line 734 draws a yellow dot at the cell. The sightline is visible.
Lines 760-786 clear the sightline. Before the guard moves and changes direction, erase all the dots by printing spaces along the old sightline path. This subroutine mirrors the draw subroutine but prints spaces instead of dots.
Line 150 draws the sightline every frame with GO SUB 700.
Line 245 clears the old sightline before the guard moves with GO SUB 760.
Two Subroutines, One Pattern
The draw and clear subroutines use the same loop logic. Both start at the guard’s position and step outward in the facing direction. The only difference is what they print: dots or spaces.
This is why subroutines exist. The sightline logic is complex enough that duplicating it would be error-prone. GO SUB 700 and GO SUB 760 let you call the same pattern from different places.
Detection by Line of Sight
The sightline check on line 732 is elegant: if the stepping loop reaches the player’s cell, GO TO 400 triggers detection. The player doesn’t need to touch the guard — they just need to be visible. This transforms the game from “avoid contact” to “avoid being seen.”
Try This
Longer sightlines. Remove some internal walls from the map data. The sightline extends further in open areas, making the guard more dangerous.
Different dot character. Replace "." with "-" or CHR$ 130. Some characters look more like a torch beam than others.