On Wed, 7 Jul 2004, Dik Takken wrote:
> On Wed, 7 Jul 2004, Matto Marjanovic wrote:
>
> > Recall that digital video is like drugs: you gotta know your source, or
> > you're gonna get burned...
> I have no idea. The source is a bttv video grabber that outputs 25
I haven't used a bttv device for a couple years (finally got around to
freeing up the PCI slot by pulling the card out) and while it was
a good thing to play with and it did serve as an introduction to
the world of video I'd never use one again.
> frames per second, it looks *really* smooth. So, I guess this grabber card
> is doing the right thing.
As long as you're grabbing at the full frame size (which with
the 4:2:0 format works out to around 13MB/s or ~42GB/hr). Otherwise
there have been reports of the card/drivers replicating the chroma
from the first field rather than what was expected. I seem to recall
there were other problems mentioned (ffmpeg and/or mplayer mailing
lists) as well.
Supposedly the bttv cards can produce Rec.601 format data (10:11 for
NTSC, 59:54 for PAL) but programs like xawtv and so on don't seem
to do that so a resample/rescale operation is needed (y4mscaler to
the rescue ;)).
> Yes, mplayer is said to be extremely messy. But it is the most powerful
> video processing tool available, AFAIK.
It's a very powerful decoder, that is true - if I need to decode
a weird format then MPlayer's the tool I turn to. For decoding
MPEG-1/MPEG-2 then mpeg2dec from the libmpeg2 project is a very good
tool.
> > In my experience, this means that fidelity is not going to
> > be a strong point.
>
> You mean the quality of the output will be a problem? I haven't tried
> using a long filter chain, but the output looks really good to me.
The YUV4MPEG2 header is suspect or wrong sometimes (seems to hit
NTSC folks more than PAL though).
The DV and MPEG-4 decoding is fine - I haven't used the other
formats extensively but the MPEG-2 decoding looked ok. I tend to
use mpeg2dec though for MPEG2 decoding.
> I use mplayer to decode the MJPEG video into high quality yuv4mpeg, using
> postprocessing filters like deblocking and deringing. It really helps
> reducing compression artifacts a lot, if there are any. I haven't compared
I think that's the type of thing Matt was talking about when he
mentioned someone becoming inspired to write a filter/program for
mjpegtools - it could then be used in a pipeline even when mplayer is
not being used.
It'd be interesting to see if the postprocessing makes an overall
difference or if it is the encoding parameters that have the most
important effect.
> thing that I want to experiment with is adding noise. Some old videotapes
> look quite bad (details are gone) and adding some noise to them makes it
> look more crisp again.
Well, for _that_ we have y4mdenoise. It's a bit slow but the
quality improvement is amazing. I've processed a number of VHS tapes
recently with it and I am thinking about redoing the laserdisc
conversions (I saved the raw DV data - cheap disk space ;)). For
VHS material a setting of "-t5" or perhaps "-t6" will do wonders.
> Removing color information may help as well, especially if the color bleeding is
> very bad.
Any number of ways to do that - y4mdenoise has a "black&white" option
which kills the chroma, y4mscaler has a '-S mode=mono' which will do
it, and y4mshift can zap the chroma as well.
Cheers,
Steven Schultz
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