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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).