Strange speed differences on composite

[Dennis Jacobfeuerborn]

Tomasz Torcz wrote:
> 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).
> 

That's actually something that I wondered about recently. How is a user 
supposed to make a decision when he intends to go out and buy a videocard 
for his machine running Xorg? It seems there exists no recommendation from 
Xorg for such a case and given the statements above it doesn't even seem to 
be possible to give one.

Regards,
   Dennis