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Re: OpenGL question

You could always allocate yourself an OpenGL texture and write to the
pixels before rendering it to the screen.

Checkout the tutorials at

There are a lot of examples of using textures.


----- Original Message -----
From: "Stephen J Baker" <sjbaker@link.com>
To: <linuxgames@sunsite.dk>
Sent: Tuesday, February 11, 2003 12:58 PM
Subject: Re: OpenGL question

> On Tue, 11 Feb 2003 voidstar@tin.it wrote:
> > does anybody knows if and how can I do a thing like
> > this in OpenGL?
> >
> >   unsigned char* pA000 = vga_getgraphmem();
> >   pA000[pos] = color;
> You can't (nor are you ever likely to be able to).
> There are multiple problems with such a thing:
>   1) The graphics card is HEAVILY pipelined during OpenGL
>      operations - when you draw a polygon, it may not appear
>      on the screen until several milliseconds later.  If you
>      could just do raw screen accesses, who knows what partial
>      junk you'd read.
>   2) There is no guarantee that there *are* any pixels laid
>      out simply in memory.  The if you are doing FSAA with the
>      nVidia GeForce cards (for example), they are rending to a
>      multi-sample buffer with up to four RGB samples per pixel.
>      The new GeForceFX (if it ever appears) aparrently adaptively
>      stores either whole pixels or partial sub-pixels depending
>      on whether the pixel lies on the edge of a polygon or not.
>   3) What about windows that are partially overlaid by other
>      windows?
> Basically, this has not been possible since maybe the second generation
> of 3D graphics cards - and it's NEVER likely to be possible again.
> The simplistic idea that one memory-location == one pixel == one
> spot on the display is no longer valid with windowing systems,
> 3D and antialiasing.
> > With svgalib i can do this, but in OpenGL? Is there
> > a way to access the screen like an array (not only 320x200,
> > but also 640x480, etc.)
> No...nor should there be.  Raw screen access died a couple of
> generations of hardware ago - and I didn't shed a tear to see it
> go!
> I would lay very good odds that whatever it is that you are
> trying to do with raw screen access could be done more efficiently
> using the OpenGL API.
> ----
> Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
> Work: sjbaker@link.com           http://www.link.com
> Home: sjbaker1@airmail.net       http://www.sjbaker.org