[FFmpeg-devel] -vf copy does transcode rather than copy

Paul B Mahol onemda at gmail.com
Thu Oct 31 20:06:46 CET 2013


On 10/31/13, David Favor <david at davidfavor.com> wrote:
> Michael Bradshaw wrote:
>> On Thu, Oct 31, 2013 at 11:28 AM, David Favor <david at davidfavor.com>
>> wrote:
>>
>>> Maybe I'm missing something.
>>>
>>> The copy filter states...
>>>
>>>
>>> http://ffmpeg.org/ffmpeg-**filters.html#copy<http://ffmpeg.org/ffmpeg-filters.html#copy>
>>>    Copy the input source unchanged to the output. Mainly useful for
>>> testing purposes.
>>>
>>> Test case below shows input video bitrate is 938 kb/s output video
>>> bitrate
>>> is 836 kb/s
>>> and ffmpeg output shows libx264 is transcoding.
>>>
>>
>> If you want to copy the stream without re-encoding it, don't you want
>> -c:v
>> copy instead of -vf copy?
>>
>> The copy filter just copies the input data to the output. But the input
>> is
>> decoded video frames (which it then copies to the output). Note that
>> input
>> and output here are referring to the filter graph, not the input/output
>> files.
>>
>> Data gets decoded -> Goes into filter graph -> "copy" filter just does a
>> straight copy of the data -> Goes out of the filter graph -> Gets encoded
>> by your command line parameters (and here it seems to be using defaults,
>> because you haven't specified any).
>>
>> Someone please correct me if I'm wrong, though.
>
> -c:v copy maintains SAR + DAR info.
>
> -vf copy should allow (to me) a filter chain of something like...
>
>      -vf copy,setdar=dar=0,setsar=sar=0
>
> which should accomplish the following...
>
>      1) copy the video stream (bit for bit) so no transcode
>
>      2) completely remove SAR + DAR to repair broken videos
>
> Bug is this....
>
>      ffmpeg -i in.mp4 -c:a copy -vf copy out.mp4
>
> does a transcode instead of a copy... so it appear that -vf copy simply
> does not work.
>
> A work around for my situation is to override SAR + DAR in video stream
> via setting it in the container... via...
>
>      ffmpeg -i in.mp4 -c:a copy -c:v copy -aspect 16:9 out.mp4
>
> And -vf copy still appears broken and should be fixed as there are other
> situations where video streams should be copied bit for bit.

Nonsense.

>
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