So if you are working in 800x600, then:
camera_x = 1024; // whatever;
camera_y = 1240; // whatever;
view_x = camera_x - 400; // half of 800
view_y = camera_y - 300; // half of 600
blit(view_x, view_y, 800, 600);
The reason it might be slow is you are only scrolling by one pixel at a time. You can actually go up to 1-32 pixels at a time and still make it 'smooth' enough.
Jim Adams
Just as an aside, on x86 platforms assigning more than 2 variables to be register variables doesn't help, as there are only 4 general purpose registers and you need at least two of them for your arithmetic and store/load operations. It's arguable with the more recent (Pentium and newer) processors using more than one register variable is appropriate.
So, you have a camera that you track the coordinates on. Keep track of the mouse coordinates on screen, and you can add the camera coordinates to it to get world coordinates if they are needed.
Use a timer so that when the mouse reaches the edges of the screen, after 1/2 second (or so many frames), the camera moves in that direction.
Let's say the camera coordinates are:
camerax = 1024; // whatever
cameray = 1234; // whatever
the mouse screen coordinates are:
screenmousex = 535; // whatever
screenmousey = 202; // whatever
Then, since the screen is 800x600, you would render the map with the following coordinates:
topx = camerax - 400; // screenwidth / 2
topy = cameray - 300; // screenheight / 2
And you would get the mouse world coordinates like:
worldmousex = screenmousex + topx;
worldmousey = screenmousey + topy;
Now, you check the mouse screen coordinates against the edges (an 8 pixel edge for example):
if(screenmousex < 8)
left_edge_count++;
else left_edge_count=0;
if(screenmousex > 791)
right_edge_count++;
else right_edge_count=0;
if(screenmousey < 8)
top_edge_count++;
else top_edge_count=0;
if(screenmousey > 591)
bottom_edge_count++;
else bottom_edge_count=0;
Check for scrolling - say if the mouse stays in spot for 15 frames (1/2sec at 30fps):
if(top_edge_count >= 15)
cameray -= camera_scrollspeed;
if(bottom_edge_count >= 15)
cameray += camera_scrollspeed;
if(left_edge_count >= 15)
camerax -= camera_scrollspeed;
if(right_edge_count >= 15)
camerax += camera_scrollspeed;
Make sure to get the mouse world coordinates after scrolling the field to ensure the camera coordinates haven't changing during that last part.
Jim Adams
register int xl = 799;
register int xs = 1;
register int yl = 599;
register int ys = 1;
...blah-blah-code...
do {
x = mouse_x - 0;
y = mouse_y - 0;
if (x > xl){
++xl;
++xs;
}
else if (x < xs){
--xl;
--xs;
}
if (y > yl){
++yl;
++ys;
}
else if (y < ys){
--yl;
--ys;
}
request_scroll(xs, ys);
blit(background, screen, 0, 0, 0, 0, w, h);
} while (!keypressed());
The method i use is quite simple, and makes use of the POINT and RECT structs.
for those not familiar with POINT or RECT, POINT has two ints: x,y; and RECT has four ints: left,top,right,bottom.
any position can be represented by a POINT, any rectangular area can be represented by a RECT.
so, your world may be 1600x1600 pixels, and your resolution of choice is 800x600. (for the sake of discussion, i am assuming that the ENTIRE display is used)
your main character (or camera, or whatever) is represented by a POINT structure that describes a location in worldspace (described by the RECT l=0,t=0,r=1600,b=1600) (This is called an anchor point)
and of course, you want the camera to look at the center of the screen(400,300).
so, a RECT describing the extent of the camera is l=-400,t=-300,r=400,b=300
so, adding x to left and right, and y to top and bottom, you have the RECT in worldspace that describes what is seen in screen space.
but, we dont want our RECT to be outside of worldspace, so we clip our anchor to a RECT that ensures that the RECT will not contain any points not in worldspace.
we take the anchor extent RECT (-400,-300,400,300) and subtract it from the coordinates of worldspace (0,0,1600,1600), to get anchorspace (400,300,1200,1300). now, as long as the anchor is within anchorspace, we can plot whatever is in worldspace into screenspace without fear.
how does this relate to scrolling?
well, when scrolling, all you need to do is move the anchor, and clip it to anchorspace.
[This message has been edited by TANSTAAFL (edited December 16, 1999).]
