[FFmpeg-user] Interlaced x264

[Andy Furniss]

PSPunch wrote:
> On 2016/04/08 5:04, Andy Furniss wrote:
>> Why do you need to make interlaced from progressive?
>
> Excuse me to jump in. For example, when encoding for BD I need to
> make the choice between 1080i to retain resolution, or 720p for ease
> of handling. (Although I never had to used FFmpeg for this task)

I guess there are some valid cases.

720p is somewhat higher vertical res than 1080i allowing for two
fields and extra low pass filtering interlaced content undergoes.

>> If you have good reason (and 50/60 fps source) then you also have
>> to be careful around chroma conversions and any scaling filters for
>> which may get auto inserted and break chroma (cause it to bleed
>> between fields).
>
> Is there a technical term for this phenomenon, or any resources I can
>  read further on this? Hopefully one aimed at video operators rather
> than codec designers...

I don't really know the technical term, but mpeg mandates that 420
weaved interlaced and 420 progressive content have different chroma
sub-sampling positions. In the weaved case fields have to be treated
separately.

www.mir.com/DMG/chroma.html

I don't think ffmpeg can handle the positions for top/bottom correctly.

It can treat fields separately when scaling or changing chroma format.

-vf scale=interl=1 should do this, but sometimes depending on other
commands a scaler may be auto inserted which will mess this up.