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Advanced usage
Compressibility
check
The compressibility check (CC) allows you to determine the optimal
Quality Factor (QF) for your movie. Without the CC, tuxrip asks the
user to enter a targeted QF and calculates the movie resolution
according to this QF. The user has to enter an empirical QF value,
usually 0.22 for 1 CD and 0.27 for 2 CD. However, the optimal QF, which
is a compromise between the number of Bits per Pixel and per Frame, and
the video resolution, may be extremely variable, and depends on :
- the
motility of the image movie (action movie vs calm movie)
- the
grain of the image
- the
quality of the encoding of the DVD itself
- the
lightness and colour of the movie.
For
example, a movie like “The Seventh Seal” by Bergman may be encoded with
a QF of 0.18, whereas “Speed” will need 0.26.
The compressibility check allows the user to determine this value, by
encoding a sample (e.g. 5% of the movie, taken at different positions)
at the maximum quality, and determine the maximum QF. The
compressibility is expressed as a percentage defined as:
%comp = ( targeted CQ / max CQ ) * 100
Then
resolution is determined according to this value. Values in the range
of 50-60% are recommended to obtain a good-looking encoding, 40% is
still OK, in particular when using the B-Frames (see the codec options
in the FAQ). Above 60% may be an overkill, under 40% may result in a
lot of compression artifacts.
Tuxrip
can perform this test very easily, using this option :
tuxrip --test-comp
All
steps up to the resolution determination are identical to the normal
mode.
After entering the size of your medium, do a compressibility check
using a targeted QF of 0.22 (the QF used for the CC does not modify the
result of the test significantly). By default, the CC uses 5% of the
film, but you may use a value comprised between 1 and 20%, the higher
the longer the test is.
Tuxrip then returns the maximum QF and calculates the possible
resolutions, with the corresponding %comp. Type your choice for the
resolution you want (choosing a resolution corresponding to 40-60%).
You may also cancel this test and do it again with other encoding
parameters (e.g. you may try 2 CD instead of 1 CD if the %comp values
are too low).
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