Question about the 'color sampling' options in x265 base Main 10 settings

[Lukwtwz]

I've been using Hybrid to export x265 Main 10 HDR videos (for playback on HDR TV's and other hardware players) using ProRes 422 and 422 LT (and occasionally 422 HQ) source videos for quite some time now, and I've been meaning to ask for clarification about the color sampling statement "specifies the input and output color sampling that is used". Since the Main 10 profile is 4:2:0, and the sources are 4:2:2, I've been hoping that the appropriate color subsampling is being performed during these exports, but since Hybrid clearly implies that "...input and output color sampling.." is being set by a single setting, I've been wondering whether maybe Hybrid isn't actually performing any subsampling, but is instead just seeing the 4:2:2 source as 4:2:0 (i.e. If that's even possible, say by just discarding certain chroma info in a way that doesn't qualify as actual subsampling).

I'm sorry if this question comes across as obtuse. It may very well be the case that any Main 10 4:2:0 export that uses a 4:2:2 video as a source simply cannot do so without additional subsampling, but I'm not knowledgeable enough about codecs and chroma subsampling to determine that. 

TLDR;
Does Hybrid treat the video source as 4:2:0 (instead of proper conversion from 4:2:2 to 4:2:0) whenever Main 10 is used, regardless of the actual source chroma sampling, or does it do proper 4:2:2->4:2:0 ? I ask only because "specifies the input and output color sampling that is used" gives me pause; it seems to imply that input and output sampling are necessarily coupled without subsampling being performed during the export.

[Selur]

Does Hybrid treat the video source as 4:2:0 (instead of proper conversion from 4:2:2 to 4:2:0) whenever Main 10 is used, regardless of the actual source chroma sampling, or does it do proper 4:2:2->4:2:0 ?

In the x265 settings you specify the color sampling of for the encoder.
If your source is for example 4:2:2 and you tell Hybrid to use x265 with 4:2:0 it will do a 4:2:2->4:2:0 conversion during the decoding/filtering of the source.
Same goes if your source if 4:2:0 and you tell Hybrid to encode to 4:4:4, Hybrid will convert the 4:2:0 input to 4:4:4 before feeding the encoder.
Some goes for the color precision. If your input is 4:2:0 8bit and your tell Hybrid to use an encoder with 4:2:2 10bit, it will convert to that output format during the decoding/filtering of the source.
Hybrid tries to avoid having encoders do other stuff than encoding.

Hope this clears things up for you.

Cu Selur

[Lukwtwz]

Does Hybrid treat the video source as 4:2:0 (instead of proper conversion from 4:2:2 to 4:2:0) whenever Main 10 is used, regardless of the actual source chroma sampling, or does it do proper 4:2:2->4:2:0 ?

Hope this clears things up for you.

Cu Selur

Very much so. I mistakenly thought that "input" referred to the Hybrid app's source file input rather than to the input of Hybrid's encoder specifically. Although I've been generally satisfied with the Main 10 encode results I've been getting via Hybrid, there have been a few cases where the end result wasn't consistent with other encodes from the same types of sources, and I wanted to rule out Hybrid's involvement in color sampling before troubleshooting factors extraneous to Hybrid. I use Main 422 10-bit a lot as well (for Youtube, now that it accepts that format, albeit with insanely long processing times for videos longer than a few minutes), but my 2016 HDR TV only plays Main 10, so I wanted to get that profile right. Thanks for clearing that up.