[HELP] AV1+NVENC Constant Quality

[tomek398]

I'm encoding 4K file from a GoPro (H.264) to AV1 using NVEnc
Constant Quality. 
Encoder Preset: Quality

Most recent NVidia Studio driver. 

No other changes to the input (same output resolution, frame rate etc....)

 I'm very surprised to see that the size of the output file doesn't really depend on the Quality factor. It looks like that:

CQ16: 109,592MB
CQ18: 109,592MB
CQ20: 109,592MB
CQ22: 109,592MB
CQ24: 108,746MB
CQ26: 108,735MB
CQ28: 108,723MB

Why is it like that? It is very different behaviour to the NVEnc H.265 where the output file size varies a lot when changing CQ factor

[Selur]

It mainly depends on the amount of details and how they are preserved at different quantizers.
Assuming crf scaled linear with file size is wrong. If there are no details, a lower quantizer will retain, lowering the quantizer / crf does not change the output.
(Sometimes quantizer x-y can give the same output, but x-(0.1) or y+(0.1) can be a lot different.)

I tested:
a. an SD clip and got:

Constant Quality	Size MB
1	13.0
5	11.2
10	7.6
15	4.4
20	2.8
25	1.7
30	0.9

b. HD clip and got:

Constant Quality	Size MB
1	58.585
5	58.585
10	58.58
15	58.58
20	58.4
25	36.8
30	18.9

c. 4k clip and got:

Constant Quality	Size MB
1	531.68
5	531.79
10	531.20
15	529.58
20	512.4
25	502.9
30	482.2

With software encoders, you usually get way more diversity in sizes and detail retention, since they usually take more/better psycho visual effects into account.

Cu Selur

[tomek398]

Thanks for replying. I wasn’t sure if this was an issue with the drivers or maybe some incorrect settings. My input is 4K, and I expected to see something similar to H.265, not a linear dependence.