From: "Tony Friery" <tony AT basoft DOT enta DOT net>
Newsgroups: comp.os.msdos.djgpp
References: <7oiu0a$vqa$1 AT nnrp1 DOT deja DOT com>
Subject: Re: Allegro: scroll_screen problems
Date: Sun, 8 Aug 1999 12:43:58 +0100
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

Hi,

I am relatively new myself to Allegro, but have used this function so
perhaps can share my experiences...


<don_rogers AT my-deja DOT com> wrote in message
news:7oiu0a$vqa$1 AT nnrp1 DOT deja DOT com...
>   Could someone please help me with this problem.
> I am new to Allegro so I decided to keep it
> simple, just draw some shapes on the screen
> (virtual) and then scroll around a bit. So,
>
> set_gfx_mode(GFX_AUTODETECT,800,600,0,0);

As I understood it, the last two 0,0 should really be the size of the
virtual screen, i.e. to make a 1024x4096 virtual, you should use:

set_gfx_mode(GFX_AUTODETECT, 800, 600, 1024, 4096);

> .
> .
> set_pallete(desktop_pallete);
> set_clip(screen,VIRTUAL_W,VIRTUAL_H);
> draw some shapes on 1024x4096 virtual
>         screen
> .
> .
> scroll_screen(50,0);
>
> Everything moved over just as I had expected.
> But when,
>
> '
> '
> scroll_screen(0,50)
>
> shapes on the screen move diagonally instead of
> vertically!?
>

[snipped]

I also believe that the scroll_screen specifies an absolute co-ordinate for
the top-left pixel. so to scroll it upwards, one might use:

int a;

for (a=0; a<600; a++)
{
    scroll_screen(0, a);        /* scroll_screen(a,0); would scroll to the
left instead */
    pseudo_delay(50ms);    /* Substitute your favourite delay function here!
*/
}


Hope this helps.

Best Regards,
Tony Friery