[FFmpeg-devel] [PATCH v2 1/3] avformat/vpcc: Calculate VP9 level from Luma's Sample rate and Picture size

[James Almer]

On 4/23/2018 8:58 AM, Ronald S. Bultje wrote:
> Hi,
> 
> On Mon, Apr 23, 2018 at 2:10 AM, Karthick J <kjeyapal at akamai.com> wrote:
> 
>> -    int level = par->level == FF_LEVEL_UNKNOWN ? 0 : par->level;
>> +    int level = par->level == FF_LEVEL_UNKNOWN ?
>> +        get_vp9_level(par, frame_rate) : par->level;
> 
> 
> 
> After this change, how will I create a file without a level?
> 
> This patch blurs the line between "unknown", "unspecified", "non-existent"
> and "autodetect". Linguistically, each of these mean something
> fundamentally different. I think it's acceptable to not have too many ways
> of specifying something which in practice comes down to "nope", but you're
> removing the "nope" altogether, which isn't quite right either.

The spec (VP9 in mp4) doesn't allow 0 nor defines an "unknown" value to
be muxed into the vpcc atom. Technically speaking, we have been creating
invalid files all this time by using 0 when the profile is unknown.
See https://www.webmproject.org/vp9/mp4/

I don't think anything really looks at this field, beyond maybe just
making sure it's a valid value if the parser is super pedantic, as it's
not needed to actually start decoding. So an approximation is probably
better than an out of range value like 0.