Framerate Timelapse

Question

I'm doing a time-lapse film project about a six-month period. Now I write a script to process the thousands of photos. I use ffmpeg. The paths to the photos are in the list.tmp file and i'm using the concat option.

ffmpeg -r 400 -f concat -safe 0 -i list.tmp -video_size hd1080 -vcodec libx264 -preset medium -crf 23 timelapse.mkv

With this command I get a time lapse with 400fps. If I now set the frame rate of the output video to 25fps, the film still looks the same subjective, but is a lot smaller (15mb instead of 60mb).

ffmpeg -r 400 -f concat -safe 0 -i list.tmp -r 25 -video_size hd1080 -vcodec libx264 -preset medium -crf 23 timelapse.mkv

ffmpeg also reports that most images were droped.

frame= 201 fps=3.7 q=-1.0 Lsize= 15647kB time=00:00:07.92 bitrate=16181.8kbits/s dup=0 drop=2989 speed=0.148x

Is it so, that I don't see the difference between 25fps and 400fps because people perceive 25fps as a smooth video?

Sorry for my English.

Answer 1

Even though the first output is 400 fps. Normal graphics adapters or displays can't show 400 fps. Common refresh rates are 60 to 75 Hz, so the max number of frames per second that could have been displayed.

With a timelapse edit, the material naturally features higher stepping of scene changes between consecutive frames, so there's greater leeway to drop output framerate without a significant subjective difference.

ffmpeg also reports that most images were droped.

The input framerate is 400 and output is 25, so ffmpeg will select 25 out of every 400 input frames and drop the rest, in your command.

because people perceive 25fps as a smooth video

I think a century of cinema and video at similar rates should be confirmation.