I have written a small demo here that uses a technique similar to what Anthony describes. Well I guess the idea is the same on some level, while coding I noticed that this can indeed be quite tricky to figure out. Using a piece of paper and a pen really helps, and even then you have to turn the music off (well I had to).
The important code is in the main function. It checks if a ship is close enough to the player, if so it is drawn. This is the normal case. If the difference between the two is big however, it could be that it is so big because they are both near the edge of the world, on opposite sides. This means they are in fact *close* to each other as the world wraps around! So, if the difference is *big enough*, the ship is also drawn.
In that last case you have to do a bit more work to compute *where* to draw the ship on the screen ("drawOffset"); my program uses 2 different computations based on whether the ship is to the right or to the left of the player.
To be able to see what is going on I added a radar, just like in Defender (more or less). This is really necessary to check if everything works as intended. ;) The three lines on the radar represent the area you can see; the center line is the player position. Reminds me of Falcon Patrol II too... ti ti ti ti de li tiii, ti de li etc. The player moves to the right automatically.
The important code is in the main function. It checks if a ship is close enough to the player, if so it is drawn. This is the normal case. If the difference between the two is big however, it could be that it is so big because they are both near the edge of the world, on opposite sides. This means they are in fact *close* to each other as the world wraps around! So, if the difference is *big enough*, the ship is also drawn.
In that last case you have to do a bit more work to compute *where* to draw the ship on the screen ("drawOffset"); my program uses 2 different computations based on whether the ship is to the right or to the left of the player.
To be able to see what is going on I added a radar, just like in Defender (more or less). This is really necessary to check if everything works as intended. ;) The three lines on the radar represent the area you can see; the center line is the player position. Reminds me of Falcon Patrol II too... ti ti ti ti de li tiii, ti de li etc. The player moves to the right automatically.
Open in online IDE; ######### ; constants ; ######### Const SCREENWIDTH = 800 Const SCREENHEIGHT = 600 Const WORLDWIDTH = 3500 Const SHIPCOUNT = 20 Const SCROLLSPEED = 2 ; ###################### ; program initialization ; ###################### Graphics(SCREENWIDTH,SCREENHEIGHT) SetBuffer(BackBuffer()) SeedRnd(MilliSecs()) ; ####### ; globals ; ####### Global g_playerX ; ######### ; SHIP type ; ######### Type SHIP Field f_x Field f_y End Type ; ###################### ; radar drawing function ; ###################### Function drawRadar() ; draw radar background Color(0,0,200) Rect(0,0,WORLDWIDTH/10,SCREENHEIGHT/10,True) ; plot ships Color(255,255,255) For ship.SHIP = Each SHIP Plot(ship\f_x/10,ship\f_y/10) Next ; plot line at player position Color(200,200,200) Line(g_playerX/10,0,g_playerX/10,SCREENHEIGHT/10) ; compute left and right screen border position, maybe wrap leftBorder = g_playerX-SCREENWIDTH/2 If (leftBorder < 0) Then leftBorder = leftBorder + WORLDWIDTH End If rightBorder = g_playerX+SCREENWIDTH/2 If (rightBorder > WORLDWIDTH) Then rightBorder = rightBorder - WORLDWIDTH End If ; draw left and right border on radar Color(175,175,175) Line(leftBorder/10,0,leftBorder/10,SCREENHEIGHT/10) Line(rightBorder/10,0,rightBorder/10,SCREENHEIGHT/10) End Function ; ################# ; main program loop ; ################# ; create ships first For i = 1 To SHIPCOUNT ship.SHIP = New SHIP ship\f_x = Rand(0,WORLDWIDTH) ship\f_y = Rand(0,SCREENHEIGHT) Next ; player starts in center of world g_playerX = WORLDWIDTH/2 ; loop until player presses escape While(Not(KeyHit(1))) Cls ; get screen position of player -> always center of screen drawPlayerX = SCREENWIDTH/2 ; draw line representing player Color(0,255,0) Line(drawPlayerX,0,drawPlayerX,SCREENHEIGHT) ; draw ships Color(255,255,255) For ship.SHIP = Each SHIP ; compute offset compared to player position (distance of going from player to ship) offset = ship\f_x - g_playerX ; default is not to draw draw = False ; if absolute difference is smaller than screen width... If (Abs(offset) <= SCREENWIDTH) Then ; draw at offset drawOffset = offset draw = True ; if absolute difference is big enough so that wrapping occurs... Else If (Abs(offset) > (WORLDWIDTH-SCREENWIDTH)) Then ; compute drawing-offset based on player position and ship position If (ship\f_x > g_playerX) Then drawOffset = -g_playerX - (WORLDWIDTH-ship\f_x) Else drawOffset = ship\f_x + (WORLDWIDTH-g_playerX) End If draw = True End If ; finally, draw (if visible) using computed drawing-offset If (draw) Then Oval(drawPlayerX+drawOffset,ship\f_y,5,5,True) End If Next ; draw the radar drawRadar() Flip ; move the player to the right, maybe wrap g_playerX = g_playerX + SCROLLSPEED If (g_playerX > WORLDWIDTH) Then g_playerX = 0 End If Wend Delete Each SHIP End