Re: Why run xserver on the OpenGL API?

--- Keith Packard <[email protected]> wrote:
> The latter is a much stronger statement that encourages/forces application
> developers to migrate to a strange and possibly inappropriate graphics
> model.  Far better is to provide appropriate glue between the API they want
> to use and the graphics hardware, let them use GL where appropriate, and
> also provide support for cairo and xlib.  Cairo is a clean 2D API that can
> live atop either xlib, Render or GL, and exposes a full PDF 1.4 rendering
> model which is as close to 'complete' as we've seen in a 2D API.

My understanding is that Cairo is a retained mode interface and OpenGL is an
immediate mode one. So we are comparing apples and oranges.  Maybe someone can
elighten me, why is Cario being designed instead of just using the SVG API? Is
Cairo going to be integrated with stylesheets like SVG? If not, what good is
this to the browser developers?

> Let the application developer select their API, and just make sure that
> reasonable APIs map as directly to the hardware as feasible.

My goal is to ensure that OpenGL is available on every Linux desktop as soon as
possible. A big reason we don't have Linux games is that the status of OpenGL
support under Linux is pretty chaotic. A pervasive OpenGL API is the key to
getting mainstream apps to use it, but it will take years of deployments before
this happens. This says nothing about using the Cairo API, Cairo is retained
mode which is a different beast,

> So, if you just mean migrating to using GL for 2D rendering, then I think
> that's a bit pushy, and I would rather see us build a system that gets from
> whatever 2D api the developer is using to the hardware as conveniently as
> possible.  For existing applications, we want the X server to map X
> rendering calls to GL function calls, for new applications I'd really
> rather see them written to cairo which can use GL where available, and fall
> back to either Render or the core protocol where necessary.

It it better to write a toolkit to Cairo and then have Cairo convert to GL, or
is it easier to just write the toolkit to use GL? This is assuming that the
toolkits already have xlib support.

Another obvious point is that since GL doesn provide fonts, a font library is
needed. Would this be provided by Cairo? Can I freely intermix GL drawing and
Cairo to the same window?

> Of course, I do hope to see interesting 3D applications developed as well,
> and those would be ill-served with a 2D-centric API. I think it's
> reasonable to assume that systems using 3D applications will have hardware
> capable of supporting full GL; we know that essentially all hardware will
> provide that in the coming years.

We differ here. Becuase of what Microsoft is doing there won't be a distinction
between 2D and 3D in five years. MS is dropping their pure 2D API in favor of a
2D/3D combo API. OpenGL is an equivalent 2D/3D combo API. Their 3D API is always
available even if software rendered.

keithw/brian may be able to comment more on this. Could we define a 2D API out
of the existing GL API and then guarantee that it will be accelerated on 2D
hardware. The rest of the 3D API would still be there via Mesa. This would give
apps concerned about performance on 2D only hardware an API set that is
guaranteed to still be accelerated. 

> In summary:
> 
>  +	Design GL extensions to allow for direct access to the frame 
> 	buffer.
Meaning app specific buffers, not the mainwindow buffer?

>  + 	Write a GL backend for the X server so that environments with 
> 	GL drivers need not also provide a 2D driver.
> 
>  +	Encourage device driver authors to include support for the above
> 	mentioned GL extensions in preference to separate 2D backends
> 	for X.
> 
>  +	Encourage 2D applications to consider using cairo for rendering as
> 	that will allow direct GL drawing while still running on legacy
> 	systems (which seem to hang around forever in the open source world)
> 
+ Make sure that all Linux desktops have the GL API available whether it is
software or hardware rendered. It's not an optional module.

+ Provide a font solution for GL based apps.

> -keith
> 
> 
> 
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=====
Jon Smirl
jonsmirl-/[email protected]

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