From: ptwitche AT eznet DOT net (Ryan Twitchell)
Newsgroups: comp.os.msdos.djgpp
Subject: Page flipping
Date: Thu, 16 Apr 1998 17:15:39 GMT
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To: djgpp AT delorie DOT com
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I've looked through some of the examples that came with Allegro, but I
still don't understand how page flipping works. I know that it
involves the virtual screen and switching between different areas of
it, but the code in ex9 has me confused. Specifically the part where
page2 is made into a sub-bitmap of the screen, and it's start y
position and height are the same. Could someone please explain this to
me?

	Ryan Twitchell


(ex9)

   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;

   /* do the animation using page flips... */
   for (c=-32; c<=SCREEN_W+32; c++) {
      clear(active_page);
      circlefill(active_page, c, SCREEN_H/2, 32, 255);
      textout(active_page, font, "Page flipping (mode-X)", 0, 0, 255);

      if (active_page == page1) {
	 scroll_screen(0, 0);
	 active_page = page2;
      }
      else {
	 scroll_screen(0, SCREEN_H);
	 active_page = page1;
      }
   }