[x264-devel] preset veryfast always generates lower bitrate than veryslow. Why and how to fix?

[Alex KAS]

 Hi,

 I can confirm that I consistently get the
-preset veryfast
generating even lower bitrate then
-preset veryslow
 I tried this with on many types of video and every time carefully
verified that all parameters (surely including -crf) stay intact.
 This at least contradicts the idea of presets and the corresponding
explanations about them.
 I cannot believe it is an expected behavior.
 The expected and documented is the intention to maintain -crf while
allow more (slow) or less (fast) compression for the same perceptual
quality.

 I used ffmpeg or directly x264 encoder programs.
 I'm on linux gentoo and the package version is 0.0.20140308 (stable in
gentoo).
 The library in question is
/usr/lib64/libx264.so.142 (via a symlink from /usr/lib64/libx264.so)

 What I describe resembles very much this:
https://trac.ffmpeg.org/ticket/5854
 and this:
https://forum.videolan.org/viewtopic.php?t=135702
 where many technical details present.

 Any ideas around?

 My wish is not only to fix veryfast.
 My wish is to check the integrity of the "preset" notion in all instances.
 It is important at least to check that for any preset the specified
-crf (and any other video/audio quality parameter) is maintained accurately.

 Notice, I encountered this about 6 months ago already presumably using
an older libx264 version but that time was just surprised and attributed
this to some (unknown) peculiar property of my video.
 Now I tried literally few dozens of different scene videos and in all
cases veryfast generates a smaller file.

 Thanks and best regards,
 Alex