Peter White <peter.wh...@posteo.net> writes: > On Sat, Mar 27, 2021 at 06:18:59AM +0100, Cecil Westerhof via ffmpeg-user > wrote: >> Peter White <peter.wh...@posteo.net> writes: >> >> > On Fri, Mar 26, 2021 at 05:28:40PM +0100, Cecil Westerhof via ffmpeg-user >> > wrote: >> >> Peter White <peter.wh...@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 _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".
