[FFmpeg-user] Output video size depending on frame rate while creating video from image list

[Andy Furniss]

Stéphane Saffré wrote:
> Hi again,
>
> Sorry for that, the email was accidentally sent...
>
> I am creating a video from a list of images and as the video frame rate is
> not important in my use case, I have experimented with different frame
> rates and noticed that the ouput video size changes a lot depending on the
> chosen frame rate to reach a minimum size limit.
>
> Here is the command I ran for different frame rates:
>
> ffmpeg -framerate 60 -start_number 1 -i %d.jpg -c:v libx264 -r 60 -pix_fmt
>> yuvj420p  out-60fps.mp4
>
>
> Here is some info on different output videos with different frame rate:
>
> fps     video size    bit rate      bit/frame
>> 30      47368102      16841991      561399.7
>> 60      37289465      26516952      441949.2
>> 100     30917271      36642691      366426.91
>> 1000    30917271      366426915     366426.915
>
>
> The bit rate increase as expected with the frame rate but the number of
> bits per frame decrease slowly to reach a certain limit (note that the
> video size is the same for 100 and 1000 fps).
>
> I did notice a small decrease in image quality between lower and higher
> frame rates but I don't have a rational explanation to this. Do you have
> any idea ?

If you don't ask for a bitrate when using libx264 you will get crf which
attempts to target a quality. The decrease in bits per frame will be
down to how libx264 implements this, not ffmpeg. If you want some
different quality with crf or a target abr/cbr bitrate you can add the
relevant commands to do this.