[FFmpeg-user] Cutting out part of a video does not work

Cecil Westerhof Cecil at decebal.nl
Sat Mar 27 09:29:26 EET 2021


Peter White <peter.white at posteo.net> writes:

> On Sat, Mar 27, 2021 at 06:18:59AM +0100, Cecil Westerhof via ffmpeg-user wrote:
>> Peter White <peter.white at posteo.net> writes:
>> 
>> > On Fri, Mar 26, 2021 at 05:28:40PM +0100, Cecil Westerhof via ffmpeg-user wrote:
>> >> Peter White <peter.white at posteo.net> writes:
>> >> 
>> >> > On Fri, Mar 26, 2021 at 09:55:21AM +0100, Cecil Westerhof via ffmpeg-user wrote:
>> >> >> I want to publish a speech I gave during a Zoom meeting. But cutting
>> >> >> it out does not work.
>> >> >> 
>> >> >> When I use:
>> >> >>     ffmpeg -y -i 2021-03-25ToastmastersClubAvond.mp4 -ss 1190 -to 1631
>> >> >> -acodec copy -vcodec copy -async 1 speech.mp4
>> >> >> 
>> >> >> The video starts just a bit to late. But when I use:
>> >> >>     ffmpeg -y -i 2021-03-25ToastmastersClubAvond.mp4 -ss 1185 -to 1631
>> >> >> -acodec copy -vcodec copy -async 1 speech.mp4
>> >> >> 
>> >> >
>> >> > If you can live with further quality loss in the video, you can
>> >> > transcode it, i.e. -c:v libx264.
>> >> >
>> >> 
>> >> I now use:
>> >>     ffmpeg -y -ss 1189 -i 2021-03-25ToastmastersClubAvond.mp4 -to 442
>> >> -acodec copy -vcodec libx264 -crf 8 -async 1 speech.mp4
>> >
>> > CRF 8 seems excessive. Try 16 for a start. From various online sources I
>> > gathered that it is pretty much transparent, as in no noticeable
>> > difference to the original. My own experience shows the same.
>> 
>> So crf is useful? (Other post said not.) I am now running it without
>> crf (and async). When it is finished I will try it with crf 16.
>
> I am pretty certain Carl Eugen also meant that CRF 8 is excessive for
> no visible gain in quality. From the documentation of x264 I remember
> that an decrement of 1 results in a size increase of the video of
> roughly 12.5 %, so when you use 8 instead of 16 that is 1.125 to the
> power of 8 the size of the same video encoded with CRF 16, so roughly
> 2.5 times the size. And you won't notice the difference. Might even try
> higher values for CRF. Since I, personally, are very conscious about not
> losing any quality, I use 16. But even that might be too conservative.
> Consider it a rough ball park figure to get you started.

Before working with the video I narrowed it. (If I do not my processor
gets to hot and the computer shuts down.) There I use crf 23, maybe I
should redefine that to 16.


>> >> This takes about 8 minutes instead of a second. But I have to live
>> >> with that.
>> >
>> > You could try to do this in multiple stages, maybe. Only transcode the
>> > first few seconds up to the next keyframe and then stitch that and the
>> > copied rest together. In theory this should work, but may be not as easy
>> > to achieve. Obviously the codecs, frame rates and resolutions need to
>> > match. I guess codec parameters need to match as well, not sure. The
>> > question is if it is worth the effort.
>> 
>> I was thinking about a variant of this. Create a few seconds of the
>> start and a few seconds of the end until I entered the correct values
>> and then generate the complete file.
>
> Don't worry about the end, because there is no restriction on what kind
> of frame is allowed there. The encoder will simply stop encoding.

I need to. ;-) If it is wrong I need to enter a better one an convert
again.


>> It seems that without crf the video is generated faster. It now only
>> took five minutes. (But maybe my computer was doing less.) It is a lot
>> smaller: 41.5 MB instead of 147.8 MB.
>
> Have a look at the defaults of the x264 encoder. If memory serves, CRF
> is 23 by default and you will most certainly notice artifacts,
> especially since you are transcoding from one lossy format to another,
> because losses propagate.
>
>> Now trying with crf 16. And then comparing the video quality.
>
> You will see a difference between not specifying CRF and 16. I am pretty
> sure about that. ;)

I did not, but that is probably because I already did a conversion
with crf 23.

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof


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