ffmpeg **doubled** my duration

Question

I'm using ffmpeg to convert a PNG image sequence into a .mov file with alpha channel for use in my NLE.

The PNG sequence was rendered out of DashWare where it was generated in sync with a 4m35s (approx) 1080p50 video clip.

As expected the image sequence consisted of just over 13,750 images.

I used the following command to convert these into the required PNG encoded, uncompressed RGBA (rgb32) .mov file.

ffmpeg -i imageSeq-%06d.png -vcodec png -r 50 -pix_fmt rgb32 -y video.mov

And the file converted OK but it's duration came out at 9m10s - exactly double what it should have been.

By my reckoning 4m35s at 50fps is: ((4x60)+35)x50 which is 13750 frames/images. So why did the duration double in conversion with the above command?

Answer 1

By default, ffmpeg treats image sequence and raw video inputs as having a native rate of 25 fps unless told otherwise. So, the -r 50 duplicates each frame.

Use

ffmpeg -framerate 50 -i imageSeq-%06d.png -vcodec png -pix_fmt rgb32 -y video.mov

You can skip the -r 50 since ffmpeg will use the input's framerate.