[FFmpeg-user] Why ffmpeg libx265 generate way smaller files than cuda/nvidia based one?

[jatmvp ctf]

Thank you all for your responses!!! :)

Thank you Gabri Shally, and I must say that I was both surprised and
astonished too, about those quality score and size output.
I thought quality goes in pair with size. I got quality 36 (higher is
better, yup?) on libx265 and it looks better when I watch it, and 19
(lower) hevc_nvenc. The size is smaller when achieving higher quality...
Maybe this depends on many "static" scenes (where only little of picture is
having non-zero spatial differentiation of picture) and the encoding
algorithm indeed! Or CPU capabilities (I have quite decent CPU I think..)
If so, I would then stick and always use the CPU version because of this
2-4x penalty of encoding time is nothing in comparison of output size and
quality I get :) Good job and kudos for everyone who was involved in
achieving this in ffmpeg code! :) Not only getting great output (metered
for quality and data density), but also while low latency preparation on
decoding is set up.

It is probably related to some things Clayton Macleod said about encoders
are not standardized as well as decoders are (and as I understood, they do
not have to, so why :) ), the setup of nvidia encoder I not have made maybe
(or not available on Windows platform nowadays - would find  ) with
comments from Mick Finn (would read doc, thanks!) & Harald Reindl, and some
issues about which David Stephen wanted to tell, but the response of the
last FFMPEG User I've mentioned is not yet understood by me well I think,
but still David thanks for your input and understood your emotions between
OSS and closed /proprietary one.

I use the msi manufacturer RTX 3080, supplied from a stable power source
with 3 independent power lane sockets, so should be okey. Should I attach
the version of GPU always here? :)
Sorry for not mentioning it earlier, I read some messages here, but I have
a lot of knowledge to assess :)

I take your point Clayton Macleod, and thanks for elaboration on this. I
think that was also what Harald Reindl wanted to tell me but I was not
knowing about nVidia encoder goal "in mind". Must rethink and would test +
query if have other ideas/results/benchmarks (maybe helpful for someone) to
share.

Also, thanks Harald Reindl for sharing your look on my interpretation and
ease on my (not to-the-point) valid assumptions.
Of course, I agree implementation (code) is highly different as for CUDA
cores and the whole GPU ecosystem processing is done differently than on
CPU + RAM + MB, and they are different in architecture, silicon et al.
Would learn more before asking next things.

btw. I seen many of you run it not with cuda nor dxva2.
Mine available are "Hardware acceleration methods: cuda dxva2 d3d11va"
Is that bad? I cannot find nvenc/nvdec there

Any sources I should read from begin to end you think? Or some ffmpeg code
source part?

Really appreciate any further and presented help! :)

sob., 6 sie 2022 o 13:21 Gabri Shally <gabri.ns at gmail.com> napisał(a):

> on both command, you didn't give any option about bitrate or quality
> target, so the implementation may freely choose those.
>
> if you see output near the end, nvidia give quality of 19 while libx265
> give 36, so the size would be different wildly.
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