[FFmpeg-devel] [PATCH] vf_scale: Detecting changes of incoming frame properties and dinamically evaluate width and height expressions

Bodecs Bela bodecsb at vivanet.hu
Tue Jan 19 22:36:13 CET 2016

2016.01.19. 22:23 keltezéssel, Paul B Mahol írta:
> On 1/19/16, Bodecs Bela <bodecsb at vivanet.hu> wrote:
>> 2016.01.19. 21:42 keltezessel, Paul B Mahol irta:
>>> On 1/19/16, wm4 <nfxjfg at googlemail.com> wrote:
>>>> On Tue, 19 Jan 2016 19:00:18 +0100
>>>> Bodecs Bela <bodecsb at vivanet.hu> wrote:
>>>>> 2016.01.19. 16:53 keltezessel, Carl Eugen Hoyos irta:
>>>>>> Bodecs Bela <bodecsb <at> vivanet.hu> writes:
>>>>>>> To retain the current behaviour and not to break any
>>>>>>> earlier app, a new config parameter has been introduced.
>>>>>> Is the current behaviour useful or do you consider it a bug?
>>>>> First I thought that it was a bug, but I borrowed an idea from
>>>>> vf_overlay filter, where there is a very same config parameter causing a
>>>>> very similar behaviour distinction on expression evaluation.
>>>>> I will soon post a patch to vf_overlay where there was really a bug in
>>>>> this behaviour.
>>>> I think it's actually a bug. From what I can tell libavfilter's design
>>>> doesn't actually support mid-stream reconfiguration, although it
>>>> happens to work in some cases?
>>> It appears it's undocumented feature. Filter which changes parameters just
>>> calls
>>> avfilter_config_links().
>>> But not all filter have been tested with this in mind.
>> Working with mpeg-ts streams this feature is very usefull.
>> I have tested vf_scale and vf_overlay, both of them work well now.
>> This is the scenario where I use them:
>> You have an mpeg-ts incoming stream and you transcode it to hls with a
>> scaled down resolution.
>> The incoming stream has constant PAL width and height but the aspect
>> ratio regularly changes between 4/3 and 16/9 in the middle of the stream.
>> But you want to create a 16/9 output with constant width/height
>> regardless of incoming aspect ratio. In the 4/3 case you put black bars
>> to both sides of screen.
>> With my two recent patch - regarding vf_scale and vf_overlay - it is
>> possible.
>> Cascade the vf_scale and the vf_overlay filters.
>> First you create a fix height but variable width output with vf_scale
>> and than you overlay its output onto a fixed sized 16/9 black
>> background, positioned dynamically to the center by vf_overlay.
>> In case of incoming 16/9 a.r., the whole background will be covered by
>> the usefull content and in case of 4/3 a.r. the usefull content only
>> partially will cover the black background.
>> These options were almost ready in these filters.
>> (I know the padding filter would do the same job, but it lacks a dynamic
>> frame-by-frame pad sizing feature)
> Willing to write patch for it?
If you mean the pad filter, yes, I will check it.

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