FFmpeg reencoding x264 with the same crf multiple times

Question

I have a video, a screen recording: recording00.mp4. I compressed it with FFmpeg:

ffmpeg -i recording00.mp4 -c:v -libx264 -pix_fmt yuv420p -crf 27 -preset veryslow output00.mp4

If I compress output00.mp4 with the same parameters again, what will happen?

Will quality decrease?

Will file size decrease?

I ask this because I create these recordings on different days. I would like them to take minimal disk space, so I encode them right after recording. And when the last of them is finished, I concatenate them with the line:

ffmpeg -f concat -safe 0 -i list.txt -c copy whole.mp4

The problem is that there should be lots of synergies between the files (really similar or identical frames), however, the size of whole.mp4 is about the sum of the outputxx.mp4 files.

I thought of reencoding them with the first command but I am afraid that it will lower the quality because I am not sure if this CRF parameter is a relative value (relative to the input quality) or an absolute value.

I mean if it is relative, then 60% * 60% = 36% at the end, if it is absolute, then even though I reencode it twice, it will be 60%. (The 60% is just a fictional quality measure.) Which one is this CRF?

Answer 1

The quality will decrease

Avoid re-encoding if quality is important.

Will file size decrease?

Probably. Some detail is lost, but encoding artifacts are added.

I am not sure if this CRF parameter is a relative value (relative to the input quality) or an absolute value.

Previous -crf value is not considered. The input is fully decoded into raw video then passed to the encoder.