ffmpeg: Portrait video with black borders gets stretched / cropped

[rebus_x]

So i have a source which is 540x720 if it matters. It was processed with a deshaker, and because of that there are occasional black borders, several pixels wide.

What happens is after encoding these black areas get filled by stretching the actual image. The width of the video stays 540. It seems to not happen with top/bottom fields, only the sides.

I noticed that the video is stretched even at moments when there is no borders, it's like the image just became wider by 4-5 pixels.
The right-hand black border got filled (covered) completely throughout the whole video, I guess that's what this stretch is.

I'd very much like to get rid of this but I don't know what exactly enables it. My command line is

ffmpeg -i input.avi -vf "colormatrix=bt601:bt709" -c:v libx264 -preset medium -crf 17 output.mkv

Any help would be greatly appreciated.

[ProWo]

Upload here a mediainfo report (text mode) of your source video and the encoded video, and a screenshot of both too.

[rebus_x]

Well, what do you know. Turns out this is VLC player's fault and the video is actually not distorted. I haven't yet figured out what setting this is exactly, but I'll post here when I find it.

[jagabo]

You probably have VLC set to stretch the video to fill the screen (or window). Just set it to keep the aspect ratio. While playing, right click within the window and select Video -> Aspect Ratio -> Default.

[rebus_x]

Fixed in VLC by choosing a different video output module apart from Automatic or Direct3D9 (which gives this problem):
Settings - Video - Output ("simple" settings mode)