[mythtv-users] XvMC -> DXVA equivalence?

Daniel Kristjansson danielk at cuymedia.net
Fri Feb 24 02:47:53 UTC 2006


On Fri, 2006-02-24 at 09:55 +0800, Bob wrote:
> Every now and then on the list, this page gets posted
> http://www.elecard.com/technology/dxva.shtml
> in threads about the hardware acceleration capabilities of various GPUs, 
> with the explanation that DXVA is the Ms version of XvMC but that due to 
> poor support from manufacturers, you won't get some cards to work under 
> linux.
> But my nVidia Geforce 2 MX/MX400 while claiming to support DXVA with 
> HWMC doesn't support XvMC, is this just nVidia turning it of in the 
> drivers so I have to buy a new card, or does the MPEG acceleration under 
> windows for this card use a different technology not available under linux?

The GeForce2 is a pretty old card, my guess is that there are levels
of DXVA, just like there are levels of XVideo acceleration.
There is plain XVideo which just does color conversion and scaling.
Then there is basic XvMC as used by the GeForce3 MX and GeForce4 Ti.
Then there is XvMC with IDCT acceleration (GeForce4 MX and later).
Then there is XvMC with VLD acceleration (VIA chipset).

Basic XVideo is pretty much always used by MythTV, but MythTV will
fall back to X11+Shm or plain X11 support if it isn't available.
MythTV only uses XvMC if you request it, mostly because it doesn't
work so well. MC stands for motion compensation (i.e. when MPEG notices
that it can just move a 16x16/8x8 block over a few pixels to generate
the next frame). IDCT stands for inverse discrete cosine transform,
which is how images and image differences are stored in MPEG. It is
basically like a small 2-D FFT, which can be done much more quickly
in specialized silicon than on a general purpose CPU/GPU.

-- Daniel



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