Message-ID: <3872A91F.103CE108@sympatico.ca>
From: BRAD MURPHY <bjmurphy AT sympatico DOT ca>
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Subject: Blit Question!
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Date: Wed, 05 Jan 2000 02:20:21 GMT
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

   I am currently learning allegro and I was wondering if anyone could
give me a some help with blit.  I've read the allegro.txt definition,
but some things are still not clicking.  The code that i'm working with
is:

circlefill(dblbuffer, 0, 100, 50, 15); /* draw first time */
vsync();
blit(dblbuffer, screen, 0, 0, 0, 0, SCREEN_W, SCREEN_H);     // blit 1

for (x = 1; x < 320; x++) {
 rest(10);  /* slow it down so we can see it */
 circlefill(dblbuffer, x - 1, 100, 50, 0); /* erase from last place */
 circlefill(dblbuffer, x, 100, 50, 15); /* redraw at new place */
 textout(dblbuffer, font, "Double buffering (mode 13h)", 0, 0, 15);
 vsync();
 blit(dblbuffer, screen, 0, 0, 0, 0, SCREEN_W, SCREEN_H);    // blit 2
    }

  I understand that blit is putting the dblbuffer that is in memory onto
the screen, but what is the difference between the first blit and the
second?  How does the second blit know that it is putting the circle in
a different spot as the first blit.  And why do you put the second blit
right before it loops back to rest(10).  Doesn't this create a pause
where two circles are on the screen before the erase?  It looks to me
that every time through the loop the circle is being painted twice on
the screen and only being erased once.
  Any info would be greatly appreciated!
BRAD MURPHY