Some considerations

[Keith Whitwell]

Marcelo E. Magallon wrote:
> On Wed, Nov 26, 2003 at 01:19:14AM -0800, Keith Packard wrote:
> 
>  > Window buffers which aren't actively updating will be migrated back
>  > to main memory if other applications have more pressing need for
>  > video memory.  But, if everything is updating all the time, you're
>  > gonna need piles of memory to keep everything resident.  Do realize
>  > that even a full-screen image is "small" these days -- with 256meg
>  > video cards, even several 4meg full-screen images are manageable.
> 
>  Well, yes, but you are ignoring windowed OpenGL applications that _use_
>  those 256 MB of RAM (or something close to it), e.g. Maya, which the OP
>  mentioned.  The question is probably how is texture memory going to be
>  used?  Does every application get its own texture?  A pretty smart
>  texture manager is in order.  Since window resizes would mean
>  destroying and allocating new textures, that might wreak havoc with the
>  driver's texture manager.
> 
>  I have been reading the last few weeks worth of posts to this mailing
>  lists, particularly looking for something related to the OpenGL-based
>  renderer, particularly:
> 
>     * How to leverage existing proprietary OpenGL drivers
> 
>     * How will OpenGL applications work
> 
>     * What's the impact on OpenGL applications
> 
>  For the first question, as far as I understand it, there's a regular
>  Xserver using the proprietary driver, and a fullscreen application runs
>  on this.  This application receives all the requests from clients and
>  translates them to an OpenGL command stream to do the actual rendering.
>  The actual implementation might look a bit different, but somewhere
>  there's got to be a client that uses the vendor's OpenGL driver.  Since
>  all the proprietary OpenGL drivers are GLX-based, you need an Xserver.

Initially this is correct, but I see this as a development/transitional 
architecture to be replaced by one in which the proprietry and opensource 
drivers are engineered to exist and operate without a second X server.

>  That has an impact on the second question.  A great deal of effort has
>  been put into implementing direct rendering drivers (and I'm not
>  refering only to those provided by the DRI project).  In plain English,
>  OpenGL applications running locally are direct rendering clients for
>  all situations of interest.  The application sets up a window and event
>  loops with the Xserver but all of the rendering in that window goes
>  straight to the card bypassing the Xserver.
> 
>  Which brings me to the third question: on SGIs, where sync-to-vblank is
>  standard, I can have continously updating OpenGL applications running
>  side-by-side with little trouble.  On Linux using NVIDIA drivers and a
>  2.6 kernel, this gets very jerky (_even_ with sync-to-vblank).
>  Framerates aren't bad, but inconsistent.  In the context of transparent
>  windows, double-buffering gets interesting :-)  

This has much to do with scheduling algorithms - SGI I'm sure has done a lot 
of work to integrate the behaviour of opengl apps and the system scheduler so 
that this appearance of smooth behaviour results.

Additionally, there's a suprising amount of work, including probably hardware 
support you need to get multiple applications running sync-to-vblank 
efficiently.  My (limited) understanding is that the nvidia hardware has much 
of what's required, but who knows if it's been a priority for them at the 
driver level.

If you think about it, a GL app with direct rendering probably looks a lot 
like a batch application to the scheduler.

> Much larger areas of
>  the screen get damaged.  In can imagine the best solution is to render
>  each window to a texture and then rendering a bunch of polygons one on
>  top of the other. 

Who is doing this rendering?  Are you talking about doing the rendering on CPU 
to a buffer which is then handed off to the card as a texture?

> The blending code I posted before is pretty good,
>  but it only beats up blending on the card _if_ the data isn't already
>  available on the card's memory:
> 
>             Operation        | Texture | DrawPixels
>             =======================================
>             Clear            | 1       | 1
>             =======================================
>             Texture upload   | 36      |
>             Texture upload   | 36      |
>             Render quad      | 1       |
>             Render quad      | 1       |
>             ---------------------------------------
>             DrawPixels       |         | 16
>             DrawPixels       |         | 18
>             ---------------------------------------
>             ReadPixels       | 42      | 41
>             ---------------------------------------
>             Total            | 117     | 76
> 
>             (times are ms; images are 1k x 1k RGBA; hardware is a
>             GeForce3)
> 
>  Just ignore the ReadPixels call.
> 
>  Blending a handful of large quads is not _that_ expensive, but
>  uploading large amounts of texture data is.
> 
>  And where does that leave my OpenGL application?  As long as my OpenGL
>  application is a top-level window everything is ok, but when I lower
>  it, I start to get inconsistent results, or did I miss something?

I'm not really sure what you're getting at - an opengl app would be rendering 
to an offscreen buffer and composited along with everything else.

Keith