From: Christoph Kukulies <kuku AT gilberto DOT physik DOT rwth-aachen DOT de>
Message-Id: <199703190811.JAA23035@gilberto.physik.rwth-aachen.de>
Subject: Re: allegro - sprites and smudge on screen
In-Reply-To: <qR$ZTcAmxvLzEw5v@talula.demon.co.uk> from Shawn Hargreaves at "Mar 18, 97 08:34:46 pm"
To: Shawn AT talula DOT demon DOT co DOT uk (Shawn Hargreaves)
Date: Wed, 19 Mar 1997 09:11:40 +0100 (MET)
Cc: djgpp AT delorie DOT com
Reply-To: Christoph Kukulies <kuku AT gilberto DOT physik DOT rwth-aachen DOT de>
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> Christoph Kukulies writes:
> >While I'm moving a round the sprite smudge gets spattered over
> >the screen (pieces of the sprite, pixels are set where they not
> >should be etc etc.).
> >
> >When I'm doing a show_mouse(NULL) before I'm doing the drawing the
> >smudge disappears but this is leading to intolerable flickering
> >of the sprite.
> 
> Drawing to the screen while the mouse pointer is being displayed is a
> bad thing for two reasons:

Thanks for the detailed elaboration. I understand so far. But I seem
to recall that I didn't have that sort of problems with GRX - not sure
though at the moment - would to have grab out some code.

> 
> - In order to restore what was under the mouse when it next moves, 
>   Allegro stores a copy of what lies underneath it on the screen. If you 
>   draw something else over the top of the cursor, this buffer will 
>   become invalid. If you then move the mouse, the old, incorrect image 
>   will get restored where it used to be, so it will leave some 'mouse 
>   droppings' behind...
> 
> - The mouse pointer is drawn inside a timer interrupt, which means that 
>   it may get drawn while the main program is in the middle of some other 
>   drawing operation. This wreaks havoc with the SVGA bank switching 
>   mechanism, as the mouse drawing code will change bank, and then return 

Just a naive question (or two):

Could the interrupt routine save the SVGA bank and restore it back? Or
would that be too slow or impossible? Or is my question totally off
target :-) ? Excuse my ignorance.

>   from the interrupt to the other drawing function which fails to 
>   realise that it now needs to switch back to whatever bank it was 
>   previously using. This can result in pixels being written to 
>   completely the wrong part of the screen, but is only a problem in 
>   banked SVGA modes.
> 

[valuable tips... deleted for sake of brevity]
> 
> 
> --
> Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
> Beauty is a French phonetic corruption of a short cloth neck ornament.
> 

--
Christoph P. U. Kukulies kuku AT gil DOT physik DOT rwth-aachen DOT de