[FFmpeg-soc] libavfilter: fps ignored in ffmpeg command line

[Vitor Sessak]

Hi

Víctor Paesa wrote:
> Hi,
> 
> On Dec 31, 2007 12:03 PM, Vitor Sessak <vitor1001 at gmail.com> wrote:
>> Víctor Paesa wrote:
>>> On Dec 31, 2007 9:04 AM, Vitor Sessak <vitor1001 at gmail.com> wrote:
>>>> Víctor Paesa wrote:
>>>>> Hi,
>>>>>
>>>>> The filter fps seems ignored in the ffmpeg command line.

[...]

>> If you do
>>
>> ffmpeg -i input.avi -vfilters fps=1 output.avi
>>
>> where input.avi has a rate of 25 fps, you'll have
>>
>> Input -> 25 fps
>> Filtered input -> 1 fps
>> Output -> 25 fps
>>
>> The only problem is that the output rate (set in ffmpeg.c) is equal by
>> default to the unfiltered input rate. The filter does its job, but I
>> agree it would be more reasonable to set the default output rate to the
>> filtered input rate. That would unfortunately have a small problem:
>> imagine for example overlaying a 20 fps and a 30 fps movie together. The
>> filter output will not have a fixed frame rate, so creating and setting
>> a frame_rate field in the AVFilterLink structure will not make a lot of
>> sense. A second problem would be to allow broken filters to set
>> frame_rate to a value not coherent with the interval between timestamps.
> 
> Yeah, I was thinking of patching vf_overlay.c as attached:

I think that maintaining a constant frame rate could be handled by the 
filter framework instead of having to be handled by each filter.

I've thought a bit more about the subject and have the following idea:

1- add a field named suggested_rate to AVFilterLink that would be the 
suggested frame rate of the output.
2- initialize this field by default by the input frame rate.
3- vf_fps will set it to its nominal value and vf_overlay will set it 
either as the highest one, the image rate or the subimage rate
4- in ffmpeg.c, the vf_fps filter will _always_ be inserted in the end 
of the filter chain (it's fast). It'll be given either the framerate 
specified by the -r command line parameter or if none specified by the 
value of suggested_rate in the end of the chain.

What do you think?

-Vitor