LJR

For ease, let's just assume the level is one huge bitmap, say 1024x1024, and our the screen is 640x480.

That means the coordinates of the map can range from 0 to (1024-640), 0 to (1024-480) and not have any 'run-off' which extends past the map edges when rendered on screen.

When you center the player to the screen, you'd use something like the following to come up with the top-left coordinates to start blitting them map and the top-left of the screen (remember player coordinates are in mapspace, not screenspace).

map_x = player_x - (640 / 2);
map_y = player_y - (480 / 2);

Easy enough, let's take an example:
player_x = 500;
player_y = 600;
calculates to (using above calcs):
map_x = 180;
map_y = 360;

Ok, now let's say the player is much further left and up:
player_x = 200;
player_y = 100;
calculates to:
map_x = -120;
map_y = -140;

Hmm - that won't work, so now what? Those negative values are actually the offset to which move the player from the center of the screen! So, now throwing in the players screenspace coordinates in there:

// get center of screen coords
screen_x = (640/2);
screen_y = (480/2);

// adjust if at top/left edge of map
if(map_x < 0) {
screen_x += map_x;
map_x = 0;
}

if(map_y < 0) {
screen_y += map_y;
map_y = 0;
}

That's it! Now, how about the right and bottom edges? That's just as easy, since we can check if the map coordinates are greater then (1024-640),(1024-480):

if(map_x > (1024-640)) {
screen_x += (map_x - (1024-640));
map_x = (1024-640);
}

if(map_y > (1024-480)) {
screen_y += (map_y - (1024-480));
map_y = (1024-480);
}

Please excuse me if some of the math is a little off - I'm typing this from memory.

Jim Adams

[This message has been edited by Jim Adams (edited November 12, 1999).]