Strange speed differences on composite

[Tomasz Torcz]

On Tue, Feb 14, 2006 at 12:07:59PM -0500, Adam Jackson wrote:
> On Saturday 11 February 2006 14:13, Tomasz Torcz wrote:
> > On Fri, Feb 10, 2006 at 07:00:03AM -0500, Owen Taylor wrote:
> > > On Thu, 2006-02-09 at 18:55 +0100, Tomasz Torcz wrote:
> > > >   The question: how it comes that software-only Xephyr is _faster_ at
> > > > drawing shadows than fully hw accelerated X server with mga driver?
> > >
> > > If you ever have to read data from the frame buffer, that's *slow* -
> > > you lose far more than you gain from any hardware acceleration you
> > > might be getting.
> >
> >   But why read data? Shouldn't it be composed by graphic card in VRAM?
> > Isn't that what acceleration is for?
> 
> "Acceleration" isn't an all-or-nothing thing.  Almost all the drivers we have 
> are accelerated to one degree or another.  The issue is that very few of them 
> accelerate the image composition operations that Render exposes.
> 
> When you do an Over blend in Render, you're computing values for each pixel:

 [ .. cut .. ]

 Thanks for your profound explanation!

> 
> Phrased another way, your assertion above:
> 
> > > > drawing shadows than fully hw accelerated X server with mga driver?
> 
> is that the mga driver is fully hardware accelerated.  It's not, it doesn't 
> accelerate Render in hardware.

  Now, this is something that surprised me. I lived under impression
that mga actually accelerates everything. Few years back I bought this
costly (ati and nvidia cards were 2-3 times cheaper) card _because_ it
had open source driver including 3D part. And it was sufficient to play
quake3. Somehow I thought 3d == full acceleration. I was wrong all those
years :(
  Apparently, now I have to wait until Xgl comes to more compilable
shape. Or until it will be shipped by my distro (Slackware).

-- 
Tomasz Torcz               RIP is irrevelant. Spoofing is futile.
zdzichu at irc.-nie.spam-.pl     Your routes will be aggreggated. -- Alex Yuriev