Re: Re:FW: XFree86 future

On 23 Mar 2003, Owen Taylor wrote:

> > > (For the local case, GTK+ has a hack to keep source pixmaps out of 
> > > video ram ...it uses ShmPixmaps when possible.)
> > 
> >    This causes a big performance limitation long term because that
> > prevents acceleration, even if the driver could have done it.
> 
> Assuming that the drivers keep the current code to copy the data
> from the source pixmap into video ram immediately before compositing,
> I don't think it actually matters:
> 
>  ShmImage => 
>     PutImage to pixmap in video ram => 
>     composite against destination in video ram
> 
>  ShmPixmap =>
>     copy to scratch buffer in video ram =>
>     composite against destination in video ram
> 
> Should be more or less identical. 

  It depends on the usage.  Are these "one-shot" images?
If so it doesn't make much difference, but for reused images
it could have been.

  In the second case the pixmap could have resided in video
ram avoiding the copy had it not been for it being a ShmPixmap.
This makes the whole operation about 10x faster.  The NVIDIA
drivers do this with the new XAA.  


			Mark.