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 :
example, a movie like “The Seventh Seal” by Bergman may be encoded with
a QF of 0.18, whereas “Speed” will need 0.26.
motility of the image movie (action movie vs calm movie)
grain of the image
quality of the encoding of the DVD itself
lightness and colour of the movie.
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:
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.
%comp = ( targeted CQ / max CQ ) * 100
can perform this test very easily, using this option :
steps up to the resolution determination are identical to the normal
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).