On Thu, 28 Oct 2004, Steven M. Schultz wrote:
Ah, that's an important detail. It might be a good thing if the man page would be a bit more clear about this.
Actually it is mentioned in the toplevel (mjpegtools) manpage:
"A quality factor should be chosen that way that the mplex output of Peak bit-rate and average bit-rate differ by about 20-25%\&. If the difference is very small (less than < 10%) it is likely that you will begin to see artifacts in high motion scenes. The most common cause of the average rate being too close (or equal) to the maximum rate is wrong value for the maximal bitrate or a quality factor that is too high."
I read this manpage many times, but there is so much information in it that I didn't remember this particular section. I think it would still be good to add a line to the mpeg2enc manpage to make it clear that the specified bitrate isn't exactly a brick wall. The hint about mplex showing the real peak bitrate would also be most valuable there.
The 20-25% is too conservative and should probably be changed to 15-20%.
Perhaps it's a good idea to have mpeg2enc print a warning when the actual bitrate exceeds 9700 kbit saying that some hardware DVD players may not be able to play the stream perfectly.
Ideally what should be done is get rid of the "-q" concept completely and use a "minimum", "desired average" and "thou shall not exceed" set of bitrates. "desired average" and "absolute maximum" might be enough. The encoder would internally adjust its parameters to meet the specified rates.
Indeed. With the current behavior it's going to be difficult to automate the encoding of video material. It would require the script to encode multiple times and select the best stream by examining mplex's output.
Another caution is that some DVD players have difficulty placing high
bitrate recordable (DVD+R/-R/+RW/-RW) media. I've seen recommendations
in Apple's documentation about not using higher than 8000Kb/s in order
to assure portability of recorded discs.
Is that a "thou shall not exceed" or a "desired average"?
Smoothing the frames a bit is a good idea indeed. I like soft images better than compression artifacts. :)
yuvmedianfilter will do that. yuvdenoise can also be tried. The
cvs version has 'y4mspatialfilter' which performs bandwidth limiting
function that can, with aggressive settings, soften/blur the image.
Numerous ways to process the data - have fun! :)
I think I'd better use Imagemagick to do that. The frames I generate are composed of multiple (partly transparent) layers. Some layers give mpeg2enc a very hard time, like a background layer which has a slowly moving checkers pattern on it. Softening that layer will probably save a lot of bits already. All visible text is also rendered on one layer and could use some softening as well. The other layers can be left untouched this way.
I guess my little (cough..) DVD menu generating script will need some more lines... :)
Cheers!
Dik
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