Brian Foddy wrote:
On Fri, 24 Jun 2005, Gregorio Gervasio, Jr. wrote:
Brian Foddy writes:
b> On Thu, 23 Jun 2005, Mudit Wahal wrote:
The HD recording is already in mpeg2. If you get a decent nvidia
graphics card with component output (gt6200, gt6600, gt6800), then
most of the hardware processing can be offloaded to the graphics card.
b> Is this correct? I've been using 5200/5600 series cards for a couple
b> years using XvMC. It works and helps some but I would hardly call
b> it offloaded. Does the 6200 etc cards have more mpeg2 processing
b> in Linux than their predicessors? And how is it activated?
b> With the XvMC lib/api or is there something new?
The newer cards have what nVidia calls PureVideo technology:
http://www.nvidia.com/page/purevideo.html
which looks interesting but there are no drivers nor libraries for
Linux. Maybe someday ...
Same old story, I think for several generations Nvidia cards have had
pretty descent mpeg2 playback capabilities that were never available
in the Linux drivers.
Disclaimers: I'm not a graphics or video expert. This information is
based on my current understanding of the state of the technology pieced
together over several years of watching the industry, so I may be wrong
about some/most/all of the points below, but I'm posting this in hopes
of getting closer to the whole truth. If I'm talking nonsense, please
let me know because my goal is to understand... :)
Most relatively modern GPU's--including the GF 4 and Radeon 8xxx series
(and probably many before that)--provide hardware support for "motion
compensation" and "inverse Discrete Cosine Transform" (iDCT) (and
possibly "Discrete Cosine Transform" (DCT)). Motion compensation and
DCT, respectively, provide a means for removing temporal and spatial
redundancy from video streams and are key components of MPEG
compression. (iDCT is basically decoding the DCT used for encoding).
X Windows provides support for hardware-assisted motion compensation and
iDCT via the X-Video Motion Compensation (XvMC) API. Therefore, using a
card/driver/and application supporting XvMC allows you to offload a hunk
of math from the CPU to the video card, but there's still quite a bit
more to MPEG decoding than just these transforms.
The NVIDIA NV40 core (i.e. GeForce 6800) was the first to provide
"hardware-based" MPEG-1/2/4 encoding/decoding and WMV9H.264 decoding
(H.264 is one format used by both Blu-Ray and HD-DVD). However, the
decoding is actually implemented using the "programmable video
processor" and the GPU's shader pipeline (basically, the NV40 was the
first GPU to provide support for enough instructions in the "program" to
perform this complex an algorithm with shaders). I would assume similar
decoding could be performed on the current generation of ATI GPU's, but
I've never seen anything definitive (although I know the
soon-to-be-released next-gen ATI GPU can do shader-based decoding, as
mentioned below).
The ATI R520 is expected to contain silicon dedicated to H.264 decoding
(which, IMHO, will be the first true "hardware-based" decoding in a
general-purpose GPU) instead of requiring apps to use its programmable
shaders (although the app could instead use the shaders to do the same
job less efficiently). However, even if ATI's Linux drivers provide
support for its hardware H.264 decoding, TTBOMK, there is no standard
X-Windows extension that can be used as an API, so it's likely to
receive lukewarm support from Linux programmers.
If a card provides "hardware decoding" using programmable shaders, it's
the responsibility of the application to "program" the shaders. The
OpenGL 2.0 specification provides support for programmable shaders
through the aptly named OpenGL Shading Language (GLSL), but OpenGL 2.0
is still very new: NVIDIA's 7664 release was the first to add OpenGL
2.0 support (
http://www.nvidia.com/object/linux_display_ia32_1.0-7664.html ).
Therefore, it will take some time for applications to begin to support
video decoding in shaders. (If anyone wants to buy me a 7800GTX, I'd
love to add (try my hand at adding?) GLSL MPEG decoding to Myth. :)
So, basically, when you see that a GPU has hardware MPEG decoding
support, there are a lot of shades of gray in the picture... Many
vendors have been promoting the MC/iDCT capabilities as "hardware MPEG
decoding" for years. Marketing: telling the truth without telling the
whole truth.
Mike
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