From: Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Allegro mode setting problem
Date: Wed, 29 Jan 1997 22:06:53 +0000
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Distribution: world
Message-ID: <1yJu75A9n87yEwNZ@talula.demon.co.uk>
References: <32EBC0D2 DOT 5C7F AT bc DOT sympatico DOT ca>
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To: djgpp AT delorie DOT com
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eblazecka writes:
>I am trying to write a program in C++ using DJGPP 2 and
>Allegro 2.1, but am having problems with the mode-setting
>code. My application needs to switch between the available
>graphics modes, but when I try to do so, I get unusual
>results. If I try to change the graphics mode more than

It's possible that this could be a hardware/driver problem. When you run
the test.exe program, can it change modes successfully?

>  while(!done) {
>    if(keypressed())
>      if((readkey() & 0xFF) == ' ')
>        gfx_mode_select(&card, &w, &h);
>        set_gfx_mode(card, w, h, w, h);
>        textout(screen, font, "This is a test string", 32, 32, 255);
>      }
>      else
>        done = TRUE;
>  }

You should turn the mouse off, by calling show_mouse(NULL), before
changing the graphics mode. The docs state you must do this whenever you
draw to the screen, and when changing mode it is even more important!

It would also be a good idea to set the palette after you select the
graphics mode. Most BIOS's will load a default palette as part of the
mode set, but some don't, and Allegro doesn't do this for you.

Also, you should check the return value from set_gfx_mode(). There is a
very good chance that it will fail, particularly if you are letting the
user enter whatever weird card parameters they like :-)

Nb. a couple of people seem confused about whether you need to
explicitly clear the screen. You do need to do a clear every time you
create a memory bitmap, but not when you set the graphics mode in the
first place, because one of the last things set_gfx_mode() does is to
call clear(screen).

/*
 *  Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
 *  Ghoti: 'gh' as in 'enough', 'o' as in 'women', and 'ti' as in 'nation'.
 */