Handbrake CQ bitrate

[kodec]

Ok, I have two different huffy files of almost totally similar content, one is 50 mins long, the other 1 hour 50 mins. After conversion to .mp4 with totally same Handbrake settings (and CQ23 for both files) the first, smaller file has final .mp4 bitrate around 2150, the other bigger around 3500 (which I like better). I mean, if the source material is very similar in moving and everything and the settings in Handbrake are 100% same, why such discrepancy in final bitrate, what gives? Is this CQ factor really so impredictable for different lengths I'll just have to adjust it from file to file no matter similarities and same settings?

[sneaker]

Movie duration has no influence on bitrate resulting from CRF ("RF" in HandBrake) encoding. (bitrate is number of bits per unit of time)
The source material must not be as similar as you think.

[kodec]

Yes, that must be it. It's live reporter camera footage, news kind of stuff, but most likely with overall less movement scenes in the smaller video.

[jagabo]

There can easily be a 2x difference in bitrate between different videos encoded at the same crf. Things like grain/noise, motion, details, even brightness effect the resulting bitrate.

[LigH.de]

"Constant quantization" or "constant rate factor" will have the bitrate the quantization produces or they need to achieve the given threshold of quality preservation. Different material with different amount of details or motion will require different bitrates.

[jagabo]

Keep in mind that high compression codecs get a most of their compression by not repeating parts of the picture that don't change from frame to frame -- something along the lines of "repeat the last frame, then add in these changes..." The more pixels change the more bits are required to encode those changes. For example, attached is a 60 second video of a still frame with no noise. Every single frame of the source was identical. At CRF 18 and the slow preset in x264 it encoded at ~34 kbps. The same video with a little added grain (different in each frame) and the same encoding settings increased the bitrate to ~3300 kbps, almost 100 times as much.