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Render of FFMPEG Copy lower quality than Original Render?

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Thomas Ames
Render of FFMPEG Copy lower quality than Original Render?
on Sep 10, 2016 at 3:38:59 pm

I feel like an FFMPEG forum would be more appropriate, but people here seem to know about it so I'll leave this here.

I have a raw file of computer capture which is 22.7 GB large. There is only a minute long clip of that raw file which I wanted to keep for editing purposes, so in order to save disk space, I looked into tools which destructively slice. I used FFmpeg with the line:

ffmpeg -ss [START TIME] -i [INPUT].mp4 -map 0 -c copy -t [DURATION] [OUTPUT].mp4

I used -map 0 to map all 5 of my channels (1 video, 4 audio) and -c copy to avoid re-encoding.



This resulted in a nice small file only a little over a minute long which contained the footage I wanted. All video and audio streams were supposedly copied, retaining the original quality, but I feel like the FFmpeg footage is worse quality than the original, when watching the rendered videos of each.

Instead of uploading to Youtube and decreasing the quality further, I created a DropBox with the full, non-compressed renders. Each of them are smaller than 180 GB.
https://www.dropbox.com/sh/jg0t4idezyung72/AACOIXvqmieQZPs8FzaOZqzOa?dl=0 (you'll have to download the videos, don't watch them through DropBox's video player)

I don't know how to compare two videos for quality, but I'd like to assure that I'm preserving as much as possible from my original capture.


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László Kovács
Re: Render of FFMPEG Copy lower quality than Original Render?
on Sep 10, 2016 at 4:55:36 pm

Hi,

you did it right.

To my eyes, the 2 files look identical.
And they are, apart from the fact, that the ffmpeg output starts 3 frames later.

I placed both file onto timeline, and synced (that's how I know the 3 frames delay).
Then I substracted first original from ffmpeg, then ffmpeg from riginal.
Both resulted in a pure black screen, so I think they are identical in quality.
I suspect, the worse quality from ffmpeg you think/feel to see is some kind of placebo.

Best regards

László Kovács


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Thomas Ames
Re: Render of FFMPEG Copy lower quality than Original Render?
on Sep 12, 2016 at 2:12:10 pm

Hi László,

Thanks for the response! That sounds like a pretty intricate method of comparison; I really appreciate it.

I did suspect that the lesser quality was just me being nitpicky and expecting that my method was incorrect and you've pretty much assured me of that.

Thanks for your time.

-Tom


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László Kovács
Re: Render of FFMPEG Copy lower quality than Original Render?
on Sep 13, 2016 at 5:01:04 am

I'm glad to help ;)

You're welcome!

Best regards

László Kovács


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