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I have video frames in PNG format at 1 FPS and I'm trying to convert them into a video using ffmpeg.

If I do something like this:

ffmpeg -i data/input-%4d.png data/output.mp4

I get a video at 25FPS which is basically a very fast-forward of the input (which is captured at 1FPS).

If I try:

ffmpeg -i data/input-%4d.png -r 1 data/output.mp4

I get something that VLC doesn't want to play :)

Now, if I take the first video (the FF one) and apply a filter to slow it down (e.g. -filter:v 'setpts=24.0*PTS'), I can get it to play like a 1 FPS video, but of course the price is file size. It's generating a bunch of repeated frames I guess.

So, the question is how do I create a video that has exactly 1 FPS and actually plays at that speed? The output format, btw, isn't that important for me.

If you want a one-liner for FFMPEG that generates a video that plays at 1 frame per second, what you want to do is specify framerates for both input and output, like this:

ffmpeg -r 1 -i data/input-%4d.png -pix_fmt yuv420p -r 10 data/output.mp4

The -r 1 means the video will play at 1 of the original images per second.

The -r 10 means the video will play at 10 frames per second.

(The -pix_fmt yuv420p is just there to ensure compatibility with a wide range of playback programs. It is required here, for example, for the video to be playable by Windows Media Player.)

I tested many different output framerates, and 10 seems to be the lowest number you can use that will still produce a video that VLC will play.

Of course, the command above means each original image is being multiplied, but it is a simpler method than the "slow it down" one you mentioned, and depending on the codec it may not produce a video much larger than a true 1-FPS video.

To test this, I just produced a true 1-FPS video, which came out to 2.24 kiB. I then produced a video with the same input images but output at 24 FPS, and it came out to 5.76 kiB. That's just over double the size, and nowhere near 24 times the size. :)

What if you augment your second example slightly as follows:

$ ffmpeg -r 1 -i data/input-%4d.png -c:v libx264 out.mp4

The -r 1 needs to come before the .png files, not after.

From the FFmpeg documentation:

As a general rule, options are applied to the next specified file.
Therefore, order is important, and you can have the same option on the
command line multiple times. Each occurrence is then applied to the
next input or output file.

Use both -framerate and -r

E.g., to have a final video that looks like 1FPS:

ffmpeg -framerate 1 -pattern_type glob -i '*.png' 
 -c:v libx264 -r 30 -pix_fmt yuv420p out.mp4

This is similar to what Converting PNG frames to video at 1 FPS | Unix & Linux Stack Exchange says, but I needed -framerate instead of -r for it to work.

This is mentioned on the wiki at: http://trac.ffmpeg.org/wiki/Slideshow#Framerates

It sets the output framerate to 30, which VLC can handle, and copies each images 30 times, so that the output video appears to be at 1 FPS. See also: Playback issues in VLC with low fps video from images using ffmpeg | Stack Overflow

VLC is then able to play the video normally.

Tested on Ubuntu 16.10, VLC 2.2.4, ffmpeg 3.0.5, in a directory with 10 PNGs.

See also: https://stackoverflow.com/questions/19267443/vlc-freezes-for-low-1-fps-video-created-from-images-with-ffmpeg