One Drone
A single enemy on screen — position-based collision detection between bullet and target.
The ship moves. The bullet flies. But there is nothing to shoot at. This unit places a single drone UDG on screen and adds collision detection — when the bullet reaches the same row and column as the drone, it is a hit. The drone is replaced with an explosion UDG in red, and the score increases.
The Program
5 REM === ONE DRONE ===
10 BORDER 0: PAPER 0: INK 7: CLS
15 GO SUB 800
20 LET sc = 0: LET hi = 0: LET li = 3
22 LET wv = 1
30 LET px = 15: LET py = 20
32 LET ba = 0: LET bx = 0: LET by = 0
34 LET ex = 15: LET ey = 5: LET ea = 1
40 GO SUB 600
50 PRINT AT py, px; INK 7; CHR$ 152
55 PRINT AT ey, ex; INK 4; CHR$ 144
60 REM === game loop ===
70 LET k$ = INKEY$
80 IF k$ = "o" OR k$ = "O" THEN IF px > 1 THEN PRINT AT py, px; " ": LET px = px - 1: PRINT AT py, px; INK 7; CHR$ 152
90 IF k$ = "p" OR k$ = "P" THEN IF px < 30 THEN PRINT AT py, px; " ": LET px = px + 1: PRINT AT py, px; INK 7; CHR$ 152
100 IF k$ = " " THEN IF ba = 0 THEN LET bx = px: LET by = py - 1: LET ba = 1: PRINT AT by, bx; INK 7; CHR$ 148
110 IF ba = 0 THEN GO TO 60
120 PRINT AT by, bx; " "
130 LET by = by - 1
140 IF by < 1 THEN LET ba = 0: GO TO 60
150 REM check hit
160 IF ea = 1 AND bx = ex AND by = ey THEN LET ea = 0: LET ba = 0: PRINT AT ey, ex; INK 2; CHR$ 149: LET sc = sc + 10: GO SUB 600: PAUSE 10: PRINT AT ey, ex; " ": GO TO 60
170 PRINT AT by, bx; INK 7; CHR$ 148
180 GO TO 60
600 REM === draw HUD ===
605 PRINT AT 0, 0; INK 7; BRIGHT 1; "SC:"; sc; " HI:"; hi; " LV:"; li; " W"; wv; " "
610 RETURN
800 REM === define UDGs ===
805 FOR i = 0 TO 13
810 FOR j = 0 TO 7
815 READ a
820 POKE USR CHR$ (65 + i) + j, a
825 NEXT j
830 NEXT i
835 RETURN
840 REM UDG A: drone
842 DATA 36, 126, 219, 255, 255, 102, 66, 0
844 REM UDG B: scout
846 DATA 24, 60, 126, 255, 219, 24, 36, 0
848 REM UDG C: tank
850 DATA 126, 255, 255, 255, 255, 255, 126, 0
852 REM UDG D: bomber
854 DATA 66, 231, 255, 126, 60, 90, 36, 0
856 REM UDG E: bullet
858 DATA 16, 56, 16, 16, 0, 0, 0, 0
860 REM UDG F: explosion
862 DATA 36, 153, 66, 129, 66, 153, 36, 0
864 REM UDG G: double shot
866 DATA 84, 170, 84, 170, 84, 170, 84, 0
868 REM UDG H: shield
870 DATA 60, 126, 255, 255, 255, 126, 60, 0
872 REM UDG I: player ship
874 DATA 16, 56, 56, 124, 254, 254, 130, 0
876 REM UDG J: enemy bomb
878 DATA 16, 16, 56, 16, 0, 0, 0, 0
880 REM UDG K: explosion 2
882 DATA 129, 66, 36, 0, 36, 66, 129, 0
884 REM UDG L: shield icon
886 DATA 60, 126, 126, 126, 60, 24, 0, 0
888 REM UDG M: drone frame 2
890 DATA 66, 102, 255, 255, 219, 126, 36, 0
892 REM UDG N: scout frame 2
894 DATA 36, 24, 219, 255, 126, 60, 24, 0
How It Works
Lines 30-35 place a drone at a fixed position. Variables ex and ey store the column and row. The drone is drawn with PRINT AT using CHR$ 144 (UDG A) in green (INK 4).
Lines 160-180 add collision checking to the bullet movement routine. Before drawing the bullet at its new position, the program compares the bullet column and row to the drone column and row. If both match, the bullet has hit the drone.
Lines 185-195 handle the hit. The drone is replaced with the explosion UDG (CHR$ 149) in red (INK 2). The score increases by 10. The drone is marked as destroyed so it is not drawn or checked again.
Position-Based Collision
Collision detection in a character-based game is simple: two objects collide when they occupy the same character cell. If the bullet is at row 5, column 12, and a drone is at row 5, column 12, they overlap. That is a hit.
IF b = ex AND c = ey THEN hitTwo comparisons. No pixel-perfect checks, no bounding boxes, no overlap calculations. Character-cell collision is coarse — two objects must be in exactly the same cell to collide — but for an 8x8 grid game, it works well. Bullets are small, enemies are one cell wide, and near-misses feel fair rather than frustrating.
The Explosion
When a drone is hit, its UDG is replaced with the explosion UDG at the same position. For one frame, the player sees a red starburst where the enemy was. Then the cell is cleared. This is not animation yet — that comes in the next unit. For now, it is a single-frame visual confirmation that the shot connected.
Try This
Move the drone. Instead of a fixed position, have the drone shift one column left or right each loop iteration. Now you have a moving target. Is it harder to hit?
Add scoring feedback. When the drone explodes, briefly print “+10” at the explosion position before clearing it. Use PAUSE 5 to hold the text on screen.