[FFmpeg-devel] [PATCH 0/1] ffmpeg: Switch cuvid to generic hwaccel

[Hendrik Leppkes]

On Sat, Jun 17, 2017 at 9:29 PM, Timo Rothenpieler
<timo at rothenpieler.org> wrote:
> Am 17.06.2017 um 00:11 schrieb Philip Langdale:
>>
>> On Fri, 16 Jun 2017 21:53:22 +0200
>> wm4 <nfxjfg at googlemail.com> wrote:
>>
>>> On Fri, 16 Jun 2017 20:31:14 +0200
>>> Timo Rothenpieler <timo at rothenpieler.org> wrote:
>>>
>>>> Am 16.06.2017 um 16:41 schrieb Philip Langdale:
>>>>>
>>>>> This is mechanically simple, but does the fact that additional
>>>>> command line arguments have to be used to get the same results
>>>>> count as a compatibility break? Do we need to fix that before we
>>>>> can actually make this change?
>>>>
>>>>
>>>> I'd consider that as a breaking change, even though things don't
>>>> "break", it results in unexpected behavior that is not obvious.
>>>
>>>
>>> Probably, but it's also slightly saner in general if you consider that
>>> a user could add video filters. Currently, hwaccel filters and normal
>>> filters are completely different beasts, so the user needs to be aware
>>> of the difference, and select the appropriate mode. So depending on
>>> whether you output hw surfaces by default, you'll break hw filters or
>>> sw filters by default. Making sure sw filters work by default seems
>>> like the better choice.
>>>
>>> But yes, maybe we could rename the hwaccel just so that users of the
>>> old hwaccel become aware of the change. You could even keep the old
>>> hwaccel name, and make it somehow override the -hwaccel_output_format
>>> option and print a warning, in case you care enough for compatibility
>>> in this case.
>>
>>
>> We could use it as an opportunity to rename it to 'cuda' which is more
>> accurate.
>>
>> Keeping the old specialised code for 'cuvid' would also ensure
>> compatibility :-)
>
>
> One thing to keep in mind is that cuvid is vastly different from other
> hwaccels.
> While for example for vaapi, specifying -hwaccel vaapi makes the h264
> decoder use vaapi. But for cuvid, the h264_cuvid decoder always uses
> hardware, and the -hwaccel cuvid flag is essentially used to switch its
> output mode to cuda frames.
> With the new approach, there is no apparent difference from using cuvid with
> or without -hwaccel cuvid.
> That situation would seem kinda weird and confusing to me.
>

The situation right now is weird and confusing, specifically because
-hwaccel cuvid is different from all the others.
To rectify that, it should probably be changed to behave just like
-hwaccel vaapi or similar

So basically, -hwaccel cuvid should instruct ffmpeg.c to use the
cuvid-based decoders, without needing to manually specify them on top
of that, and thats all it should do - ie. it behaves just like
-hwaccel vaapi/dxva2, just with a different logic in the background.

- Hendrik