[FFmpeg-user] Question about handling / unwanted changes of SAR in ffmpeg

[Uwe Freese]

Am 26.12.18 um 15:08 schrieb Reino Wijnsma:
>>      Stream #0:0: Video: h264 (High), yuv420p(progressive), 688x560 [SAR 64:45 DAR 2752:1575], SAR 172:121 DAR 7396:4235, 50 fps, 50 tbr, 1k tbn, 100 tbc (default)
>>
>> What does SAR (and DAR) mean in the brackets compared to the second SAR 172:121, which is slightly different?
> As far as I know:
>
> 688    x 64/45     = 978,49w
> 978,49 x 1575/2752 = 560h     --> 978x560 @ bitstream level
> 688    x 172/121 =   977,98w
> 977,98 x 4235/7396 = 560h     --> 977x560 @ container level

So when I understand correctly, it could be that somewhere in ffmpeg, 
the 64/45 is rounded to 978, given to another function / class 
calculating the SAR for the container level, and this calculates to 
172/121, which is best matching for 978?

Instead, the better solution would then be to give the SAR to whatever 
calculates the SAR to store at container level not as a double or int, 
but as a "AVRational", which contains the "num" and "den" values (in 
this case 64 and 45)?

Could this be done, or is there a reason not to do that?

Regards,
Uwe