On Wed, 17 Sep 2003, Richard Ellis wrote:
> Has anyone else noticed that the -Q parameter to mpeg2enc v 1.6.1.90
> seems to have much less effect than it did in version 1.6.1? I've
No, I haven't noticed. But then I haven't used -Q since I was
warned of possible artifacts. That was some time ago and I see
now that the code (in ratectl.cc) that pays attention to -Q was
reworked about 7 months ago or so.
If you look at the CVS info:
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/mjpeg/mjpeg_play/mpeg2enc/ratectl.cc
and look at the difference between the 1.4 version and
the previous version (1.3) you'll see the section that was
rewritten back in January 2003. There's a section of code
by the ifdef OLD_QUANTISATION_STEARING that was redone to help
out in scenes with low luma variance - at least that's what the
comment says ;)
There is an (undocumented) -X option that by default is set to 100
(max of 2500). The -Q setting is only, from what I can see, used
if the lum_ariance is less than the 'ctl_boost_var_ceil' (which is
what -X sets).
You might try using -X <num> and see if setting <num> to something
like 200 or 300 allows -Q to have a greater effect.
Ideally -X should be documented of course ;)
> got one pair of encodes (same capture file input to both) that I ran
> yesterday where using -Q 0.0 and -Q 4.0 resulted in exactly the same
> size encoded file. Under 1.6.0, adjusting the -Q factor would result
Do the files compare identical (with 'cmp' for instance)? If so
then -Q had no effect at all. If not then it had a slight effect
on a few macroblocks here and there but not enough to make a file
size difference.
> The reason I'd gotten to use the -Q factor for "fine tuning" is to
Perhaps -N <num> (num =0.0 to 2.0) would work as well. In 1.6.1
and earlier -N was hardwired to what is now called 1.5 but in
the CVS/1.90 version -N takes a parameter. I've found -N 1.5 to
be a bit too "aggressive" in some cases but 1.0 works well and
preserves more of the quality. For fine tuning I've found that
0.5 to 0.9 to be of use (when I cared about the size - for the
last set of 25 or 30 DVDs the requirement was to get at most an
hour of play time so I just set the bitrate up to 8500 and let it
fly ;)).
Cheers,
Steven Schultz
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