From: eglebbk AT phys DOT uva DOT nl (Evert Glebbeek)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Using (page-flipping) to speed up a program
Date: Tue, 04 Jul 2000 18:43:49 GMT
Organization: Physics student, University of Amsterdam
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Newsgroup: comp.os.msdos.djgpp
 From: "23yrold3yrold" <cbarry AT pangea DOT ca>
 On Sun, 2 Jul 2000 10:53:54 -0500

>I found the original program refused to page flip, simply because 640 x 960
>seems too big for it to handle.
How much video memory does your card have? I think Allegro comes with
an `info' program in it's standard distribution that can tell you
this.
BTW, what version of Allegro are you using?
>No prob compiling, big prob running.
There are a few problems with your code; see below.

>set_color_depth(16);
>
>set_gfx_mode(GFX_MODEX, 320, 240, 320, 480);
16 bit colour doesn't normally work in VGA modes, such as mode X, so
you'll need to change to 8 bit colour if you want to do mode X.
Also, your program doesn't check if set_gfx_mode() failed. Checking
the result of a function that can fail is usually a good idea and
helps to make you program more stable. It's also easier to track down
bugs later if you make a habbit of it.
>
>page1 = create_sub_bitmap(screen, 0, 0, SCREEN_W, SCREEN_H);
>page2 = create_sub_bitmap(screen, 0, SCREEN_H, SCREEN_W, SCREEN_H);
>active_page = page2;
While not a real error, I use

page1=screen;
page2=create_video_bitmap(SCREEN_W, SCREEN_H);
...
show_video_bitmap (page1); or show_video_bitmap (page2);

which works just fine. Still, your code should be fine in this regard.
I wrote a couple of routines that make it easy to support multiple
screen update methods (pageflipping/double buffering/dirty
rectangles), but they contain rather a lot of game-specific code that
doesn't make sense when taken out of context.
If you're interested, I can try to pull them out of the program and
make a small demo program to illustrate their use.
>ship = get_rle_sprite(load_bitmap("Ship.bmp",  pal));
You still have the memory leak.
>  // Move the sprite according to the controls
>  if(key[KEY_LEFT])
>    {if(sx > 0)   {sx = sx - 5;} else {sx = 0;};};
>  if(key[KEY_RIGHT])
>    {if(sx < 595) {sx = sx + 5;} else {sx = 595;};};
>  if(key[KEY_UP])
>    {if(sy > 0)   {sy = sy - 5;} else {sy = 0;};};
>  if(key[KEY_DOWN])
>    {if(sy < 425) {sy = sy + 5;} else {sy = 425;};};
Since you set your resolution to 320x240, any coordinates outside that
space are outside the screen. Since RLE sprites can't be clipped, this
could do Bad Things to your program. If at all possible, don't use
hard coded constants like this, but use #defines or make things
relative to the dimensions of the screen.

Finally a warning about page flipping. It's a great way for doing
screen updates, but my own experience shows that it is incompatible
with the Allegro GUI routines. Maybe that's because I'm doing
something illegal by setting `page1=screen;', I'm not sure, but I'd
thought I'd warn you about this anyway.

Hope this helps,

Evert Glebbeek