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

[Stéphane Saffré]

Thank you Andy! Indeed, after reading more about CRF, it takes motion into
account to determine the amount of compression. So I guess that with a very
high frame rate it artificially increase the motion speed and then the
compression factor as less details should be perceived by the user.

--
*Stéphane SAFFRÉ*

2016-05-10 1:08 GMT+02:00 Andy Furniss <adf.lists at gmail.com>:

> 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.
>
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