Hi -
On Sat, 19 Mar 2005, Dik Takken wrote:
> Two questions about making video streams better suited for encoding with
> mpeg2enc:
You've come to the right place :)
> Question 1:
>
> negative impact on image detail (Does it?). Is it always a Good Thing to
> use yuvdenoise as some sort of numerical conditioner before feeding the
> stream to mpeg2enc, no matter the image cleanness? Are there
For conditioning like that I use either 'y4mspatialfilter' (with
relatively high thresholds) or 'yuvmedianfilter' (in its 'fast' mode
with a high weight on the center pixel).
> any cases where using 'yuvdenoise -f' is not advisable?
Which is 'now'. The new yuvdenoise that was checked in yesterday
(Stefan rewrote it completely) does not have '-f'.
As a last step before the encoder perhaps 'y4mdenoise -t 1' would be
a good choice.
> Question 2:
>
> Is there any way to denoise the chroma channel, and have the denoise
> strength depend on the luma channel? It seems that humans are unsensitive
Not at the moment. What can be done is run 'yuvmedianfilter' on
the chroma channels only. Use 'yuvmedianfilter -t 0' to disable
luma processing.
> to subtle chroma changes when the luma is very low (maybe also when it
> is very high?), so you could perform more aggressive filtering in those
> area's, saving bits for encoding.
Which is very close (or identical) to the comment I made the other
day about "desaturating the lows". In dark(er) areas removing some
of the color info does a lot to improve the encoded image. I've found
that desaturating the color info about 50% below 16 IRE (which roughly
corresponds to a Y' of 32 if I've interpreted things correctly) works
well.
Actually the eye is relatively insensitive to color in the first place -
that's why subsampling the chroma so drastically (from 4:4:4 to 4:1:1
or 4:2:0) works and we still get color pictures :)
It's possible with 'y4mspatialfilter' and 'yuvmedianfilter' and now
the new 'yuvdenoise' to specify different thresholds/amounts/etc for
chroma and luma.
In the Canopus ADVC300 (if you're using or thinking of using DV) there
are luma and chroma filtering settings available - I've found that
using weak/mild settings for the luma but strong/aggressive settings for
the chroma to work well. I still filter thru y4mdenoise and others
but doing the initial preprocessing in the hardware at capture time has
resulted in quality I never thought I'd get from junky/old VHS tapes.
Cheers,
Steven Schultz
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