Hi,
I did another experiment with a random 'coloured noise' video (generated via : ffmpeg) :
http://stackoverflow.com/questions/1...ating-tv-noise
which gave me about 74% compression.
This is all lossy compression. Using the HEVC encoder (x265, default parameters).
For coloured noise video :
Avg QP:41.44,
For grayscale noise video :
Avg QP:35.62
This article says that standard H.264 can achieve good quality video at 100:1 compression for normal video sequences. I've tried many test sequences mainly of fast motion video, and they too were around 98%-99%.
I guess is also depends on the level of entropy - maybe /dev/urandom isn't purely random to give low compression.