OpenEuphoria

Re: Virtual Screens

• Posted by Joe Otto <jotto at NETZERO.NET> Apr 26, 1999
• 564 views

Good explanation...

I'd like to clarify that the contents of video memory are sent to the
monitor by hardware on the video card.  By and large, it's not something
that Windows or the BIOS actively concerns itself with.  The only thing
Windows or the BIOS have to do with this is to set the desired video mode.

Joe

-----Original Message-----
From:   Ralf Nieuwenhuijsen [SMTP:nieuwen at XS4ALL.NL]
Sent:   Sunday, April 25, 1999 3:02 PM
To:     EUPHORIA at LISTSERV.MUOHIO.EDU
Subject:        Re: Virtual Screens

Don't feel sorry, we all need to learn this stuff _somewhere_, and the list
server is a pretty good place for it.
First of all, you should know that what you see on the screen in stored in
the video memory.
The video memory is just a part of memory that works like normal memory, in
which bytes represent either palette values, or RGS
values.
The palette modes use a list of 'color indexes' of which each index is
associated with a RGB-value. (RGB= RedGreenBlue)
The contents of this memory is automatically (by either windows or your
bios) sent to the monitor through the videocard a number
of times per seconds. This happens independently / in-the-background of
your program.

It could thus occur that the video memory is sent to the monitor, during a
drawing operation.
Say you were drawing a line, it could thus occur that we see the line get
drawn.
In cases where we draw multiple layers of graphical data every frame, there
would be lots of flickering and unwanted see-through
effects.
Consider the case of a platform game, where first the background is drawn,
then the 'tiles' and 'objects' and then the 'actors'.

This is when we use a 'virtual screen'. On a virtual screen we do all this
drawing, after which we copy the 'finished' drawing
to the video memory.
Even hear a slight flickering could occur. (with older video cards, that
is). For this a small ASM trick, that 'waits' for a
screenupdate, should help. This offcourse does slow down your program.

A virtual screen is, in other words, just an allocated piece of memory,
that is copied to the video memory at our command.
"At our command" is preciously the obvious benefit of the method.

Now you could wonder why we don't wait for a screen update, and then do all
the drawing directly to the screen.
This is because the drawing of a frame usually takes more time than
interval between screen updates.
And you don't want to lower screen updates, since the higher the number of
updates, the more 'stable' and 'steady' the screen
looks.
Something very important with PC, since we tend to spent a lot more time,
from a much closer position, behind our monitor, than
we do behind a television (with a refresh rate of about 30-50 I believe)

Ralf N.
nieuwen at xs4all.nl
ralf_n at email.com

----- Original Message -----
From: Andrew Gainey
To: EUPHORIA at LISTSERV.MUOHIO.EDU
Sent: zondag 25 april 1999 19:04
Subject: Virtual Screens


Sorry if this is a little "elementary", but could someone explain briefly
 what a virtual screen is and the benefits of using it?
Thanks.

Gandrew

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