Re: Why run xserver on the OpenGL API?

John McCutchan <ttb-/[email protected]> wrote:

> On Tue, Nov 25, 2003 at 02:26:54PM -0800, Keith Packard wrote:
> > I thought that was implicit in the plan to write a GL backend for the
> > existing X server rendering infrastructure.  The idea is that all core X
> > and Render acceleration would be done with GL function calls, and that
> > for ease of porting the existing code, a dumb frame buffer could be
> > accessed where necessary.
> 
> Why does an opengl extension need to be written to provide the
> frame buffer access? what do you need that you can't accomplish
> already with glReadPixels () and glDrawPixels ()? 

glReadPixels() and glDrawPixels() are horrendously slow for doing any 
kind of software rendering. Reading from framebuffer memory is a huge 
performance hit, so to expect the X server to read the framebuffer 
memory, do some software rendering and put it back again will cause a 
massive performance hit.

The solution is to allow the X server software rendering engine to draw 
directly to the hardware framebuffer, so long as you provide the 
necessary access functions to ensure the engine is idle before you render 
to it and the software cache is flushed after you render to it.

Regards,

---
Kendall Bennett
Chief Executive Officer
SciTech Software, Inc.
Phone: (530) 894 8400
http://www.scitechsoft.com

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