[FFmpeg-user] Combining .webm side by side - frame size changes

[Paul B Mahol]

On 11/24/15, Moritz Barsnick <barsnick at gmx.net> wrote:
> Hi Mike,
>
> On Mon, Nov 23, 2015 at 22:56:08 -0600, Mike Grommet wrote:
>
>> During the playback of the combined file, The right side video appears
>> to zoom in to the top left of itself two times. The originals appear
>> fine when played individually.
>
> Interesting that the latter should happen. I would (personally, not
> suggesting to) inspect each frame of both files with ffprobe.
>
>> I assume it has something to do with these messages from the ffmpeg
>> run:
>> Input stream #0:0 frame changed from size:640x480 fmt:yuv420p to
>> size:480x360 fmt:yuv420p
>> Input stream #0:0 frame changed from size:480x360 fmt:yuv420p to
>> size:320x240 fmt:yuv420p
>
> That seems likely. :)
>
>> I also attempted a different way to merge the files (See Alternative
>> Command), and it has the same issue.
>
> Firstly, you may want to use the vstack filter for this very purpose,
> it saves you a lot of filter syntax:
> https://ffmpeg.org/ffmpeg-filters.html#vstack
> Of course, as it notes, "[all] streams must be of same pixel format and
> of same width."
>
> That said, and considering the weird size jumps in the input: Have you
> considered just inserting a scale filter to force the original size?
>
> Something like
> $ ./ffmpeg -i a.webm -i b.webm -filter_complex "[0:v]scale=640:480[left];
> [1:v]scale=640:480[right]; [left][right]vstack[out] -map [out] -map 0:a -map
> 1:a c.webm

hstack is for side by side. vstack is for top/bottom.

>
>> Additional note:  As an experiment,  If I first convert the original
>> .webm files to .mp4,  combine these files together with a .webm final
>> target, I don't experience this problem.  This might be an option, but
>> it seems to be an awfully hacky solution.
>
> Even more interesting. Are you re-encoding or just remuxing ("-c
> copy")?
>
> Cheers,
> Moritz
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