On Wed, 5 Jul 2006, Michael Hanke wrote:
> Hmmm... Please excuse my dumb question. Here are you puzzling me a little bit.
It is not dumb at all! You raise some very interesting points that
people should be aware of.
> My model of thinking with respect to interlacing/deinterlacing is as follows:
> - If the material is tv life (say a football match with very fast motion) then
> I think that is is a good idea to let the material interlaced because of the
> "kammeffekt" (Sorry, I don't know the correct english word).
"temporal resolution" is the appropriate phrase. And 60 (or 50)
fields/sec will give better "motion" than 25 (30) frames/sec.
Of course 60 (or 50 for Europe) _frame_/second progressive would
be fantastic for sporting (high action) events - and in the US some
stations do indeed broadcast 1280x720p HDTV at 60 frames/sec.
> - If the source is a movie, then it depends. If I have recorded with lavrec
> from my bt787, I have 422 subsampling. So there is no loss in the vertical
Is that the same as a Bt878? I knew the chipset was capable of
4:2:2 but I have not heard of anyone actually using 4:2:2. That is
probably because at some point there needs to be a quality
4:2:2 -> 4:2:0 conversion done and that is not trivial to do properly.
> direction. The critical thing happens if I use my Canopus to digitize to dv
> which has 420 subsampling. If I would deinterlace in that case, the vertical
> chroma resolution becomes wrong. Am I wrong here?
You are CORRECT. Deinterlacing 4:2:0 is TRICKY and complicated by
the fact there are 3 variants of 4:2:0
See
http://www.mir.com/DMG/chroma.html
for a good explanation of the 420 layouts.
> BTW. Sometimes I have the feeling that the fields are shifted by one frame in
> the dv capturing. Can this happen?
I would say no.
BUT DV is "bottom field first" while other capture
cards are "top field first" (TFF). I do not know if the the field order
is TFF for the Bt878 but it could be. For MJPEG cards (DC30, etc)
the field order is TFF. For AJA equipment (www.aja.com) everything
is bottom field first.
I am away from the references at the moment BUT I do remember that
DV is defined to be digitized 1 line offset/different from
standard video. For NTSC (525 line video system) the "active" area
being digitized for DV does NOT start at the same line as
"SD" (8bit uncompressed 4:2:2 720x486 video. It's a long
story - recommended reading is Charles Poynton's book "Digital
Video and HDTV Algorithms and Interfaces":
http://www.amazon.com/gp/product/1558607927/sr=8-1/qid=1152590390/ref=pd_bbs_1/103-9617851-7557445?ie=UTF8
> often used commercial VHS tape for my archive. It had an unusual high noise
> level. Playing around with the different denoiser settings my test audience
> had a very strange view: The best impression made the noisy tape because of
> its "sharpness"! When using yuvdenoise (without any tuning) the picture was
> considered blurred, unsharp, unviewable
If I read that correctly you're saying that the NOT-denoised video
was preferred to the 'denoised' version.
Correct?
If so then I have come to the same conclusion. In many cases I will
omit the *denoise processing _or_ using more conservative settings
(with yuvdenoise that means omit/not-use -g, use -t 2,6,6 instead of
-t 4,8,8).
But then I pre-condition the data with the plugins/filters available
with (or added to) FinalCutPro ;)
> This seems to be a psychological question: Better noisy than unsharp.
It depends on the person of course. But yes, excessive denoising
seems to blur / soften the image too much.
> Do you have an idea about a good compromise? It should allow for low bit rate
> encoding, too.
Yes. It is, as you have mentioned, often preferable to not denoise
the data. But that means using many bits to encode the noise - which
means fewer bits for the image.
The solution is to either 1) use higher capacity media (dual layer)
or 2) use "half D1" (that is a misnomer but the term has become common
usage) frame size - for "PAL" that's 352x576 (for "NTSC" it is
352x480) and a high bitrate.
Believe it or not you probably will NOT notice the difference,
ESPECIALLY if you're using a (noisy/old) VHS tape! VHS doesn't have
700+ lines of resolution anyhow - rarely gets much over 240 or so
(with a good tape, good deck, etc). So 352xN is a good choice
for encoding (noisy) VHS tapes.
As it happens "352xN" is also known as "CVD" (or China Video Disc)
and 'y4mscaler' has a preset for CVD. "-O preset=CVD" and y4mscaler
will do "the right thing" when presented with a 720xN frame (crop to
704 and then do a 2->1 downscale).
ANOTHER "denoising" method is to apply a mild bandpass filter - VHS
tapes do not have a lot of resolution so a lowpass filter will
reduce noise without excessive softening of the image.
> In the literature about denoising methods, I found edge sharpening denoising
> methods based on anisotropic diffusion equations. For me as a mathematician
> this approach looks very sound. Does somebody have experiences with such
No, I've no experience with that - but then I'm not a mathematician ;)
It sounds similar to the 'blur' + 'unsharp mask' technique though.
I'll have to try enabling the 'motion compensated' ("high quality")
mode that was recently added to yuvdenoise. It will be interesting to
see the difference (both in speed and quality).
Cheers,
Steven Schultz
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