Blocking in H.265 Export

[Craig]

Hello

Really hoping for some assistance with the results I'm getting from a H.265 export.  The export sequence works fine - no problem at all.  The issue is the H.265 export generating blocking in a part of a clip that is blue sky and wispy cloud.  The blocking is very noticeable. 

Any first response would suggest compression artefact.  I too believe this to be the case.  That said I am certain it is at the H.265 export point.  The original clip is a time-lapse generated from Raw stills and exported in Prores 4444 6K. Then in Resolve using the appropriate color space a Mystery box clip is applied (taking the clip to HDR) and then re exported in H.265.  Prior to the H.265 export point the only compression has been from Raw to Prores 4444.  Hardly a compression process that will result in blocking.  Further I have checked this clip in STANDARD DYNAMIC RANGE UHD using H.264 compression and the result is perfectly fine - no blocking.  I have also checked the  HDR Prores 4444 export and it too seems to be clean (this check is not completely fool proof as I have only checked on a standard dynamic range monitor so the picture appears over exposed.  None the less no blocking is evident)

So to me the problem lies in the H.265 export.  All of the settings I am using on the Hybrid H.265 export are as per recommendation of Mystery Box.  These can be seen here . . . https://www.mysterybox.us/blog/2016/10/2...vering-hdr

As a basic summary I am using average bitrate 1 pass, 18000 bitrate, sampling i420 and 10 bit bit depth, Level 5.1 / Main10 / Main.  I have tried doubling the bitrate - blocking problem exists.  My knowledge on this compression process is almost non existent.  I have exhausted my understanding of all the possible causes - but as yet I still have not resolved the blocking.  I would have thought given the color space I am working in as well as the 10 bit bit rate there should be no such problems as this.  

Really hoping someone can suggest a solution.

Thanks in advance
CraigPlease, read the 'Infos needed to fix&reproduce bugs,..'-sticky before you post about a problem.

[Selur]

My first suggestion when reading your post is:
Rate control in 'average bitrate (1-pass)' isn't that good.
Do not use 'average bitrate (1-pass)', better use 'constant rate factor (1-pass)'.

That said, the blog entry mentions:

...
Encoding Mode: Average Bitrate (1-Pass)
Target Bitrate: 18,000 - 50,000 kbps
...

while handling, 4k content, so keeping that in mind while using 6k instead of 4k it seems to be obvious, that the lowest bit rate encouraged to use for 4k might be to low for 6k.
-> If you want to stick with 'average bitrate (1-pass)' (against my advice), you will have to use a higher bit rate.

Cu Selur

[Craig]

Much appreciated.  Suggestion led to resolution.
Thanks