On Thu, 6 Apr 2006, Christian Ebert wrote:
> What about -H|--keep-hf ? Supposing I just want highest quality?
I think it would be a good idea to define "highest quality" ;) The
same type of request/desire comes up a lot on a couple forums I lurk
in. It's often in the form of a statment that implies the existence
of a magic option or tool that will give a perfect (restored/enhanced
to be what the user *wants*) image even though the data's been
compressed (lossily) between somewhere between 15 and 40 to 1
Uncompressed 10bit 4:2:2 is the highest quality - but it's about
200 megabits/second. Next highest would be downsampled to 8bit
4:2:0 at "only" 124 megabits/second. That is as high as the
quality gets.
Oh, you want MPEG-2 :) :)
-H can actually lower the quality. Remember - there is a hard limit
on the bitrate you can use and -H increases (often dramatically) the
number of bits used to encode an image. IF the number of bits/sec
available is not sufficient then the encoder has NO choice but to
lower the quality (by increasing the amount of lossy compression
performed). Using -H has, the times I've used it, raised the
average bitrate about 30%. IF you're already at the maximum for
a project (2hr movie on a DVD is limited to ~4.7Mb/s) then increasing
the number of bits required will cause the quality to be dropped (by
higher lossy compression) to make those bits available.
If you're using double layer media and have sufficient space then -H
might be usable but many projects are bitrate/size constrained and
are already at those limits - no leeway or headroom left.
Encoders in general have developed to the point where there aren't
too many obviously "bad ones" around. So that leads to the next
paragraph...
This may come as a surprise but the key to "quality" lies LESS with
the 'encoding' than it does with the pre-processing of the data _before_
going to the encoder. It's amazingly labor intense - going scene
by scene to apply different filters depending on the content of the
scene BUT it gives "higher quality" because the encoder is not spending
bits on details/areas that the eye/brain will not see (or can be
distracted away from).
Compression is an art form - there is no magic "use this <feature> for
high quality".
Cheers,
Steven Schultz
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