[FFmpeg-devel] [PATCH v4 1/9] lavc/libopenh264enc: Add default qmin/qmax support

[Martin Storsjö]

On Tue, 28 Apr 2020, Fu, Linjie wrote:

>> From: Martin Storsjö <martin at martin.st>
>> Sent: Tuesday, April 28, 2020 14:19
>> To: Fu, Linjie <linjie.fu at intel.com>
>> Cc: FFmpeg development discussions and patches <ffmpeg-
>> devel at ffmpeg.org>
>> Subject: RE: [FFmpeg-devel] [PATCH v4 1/9] lavc/libopenh264enc: Add
>> default qmin/qmax support
>>
>> On Tue, 28 Apr 2020, Fu, Linjie wrote:
>>
>>>> From: Martin Storsjö <martin at martin.st>
>>>> Sent: Tuesday, April 28, 2020 03:27
>>>>
>>>> If qmax/qmin < 0, i.e. wasn't specified by the user, wouldn't it be better
>>>> to not touch param.iMax/MinQp at all (and use the default value of the
>>>> library, which may change between versions), instead of overriding it with
>>>> a value hardcoded here?
>>>>
>>> Okay, this seems more natural if the recommended QP range varies
>> between
>>> versions, though one of my original purposes is to avoid the warning in
>> default
>>> situation for changing the QP inside libopenh264 library.
>>
>> Well in general I'd want to avoid hardcoding opinionated defaults within
>> our own wrapper - I'd like it to behave as close to what upstream
>> intended, so that whatever issues we see with defaults, are the same
>> issues that everyone else sees as well, so any fixes to those defaults
>> upstream also end up for us - so we don't get stuck on whatever we thought
>> was a good default at some point.
>
> Agree, this makes more sense.
>
>> What warnings about changing QP are you referring to?
> Warning:Change QP Range from(0,51) to (12,42)
> https://github.com/cisco/openh264/blob/master/codec/encoder/core/src/encoder_ext.cpp#L375
>
> The main reason is that "0" is not supported well, which is the default 
> value of iMinQp.

Ah, right.

In that case, setting some other default might make sense indeed. 
How/where does OpenH264 set suitable values for this, in order not to get 
the warnings everywhere? Are the values e.g. 12-42 hardcoded everywhere in 
every caller?

// Martin