[MPlayer-users] Are mencoder lossless codecs really lossless?

[James Board]

________________________________
 From: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
To: James Board <jpboard2 at yahoo.com>; "MPlayer usage questions, feature requests, bug reports" <mplayer-users at mplayerhq.hu> 
Sent: Tuesday, July 9, 2013 6:08 PM
Subject: Re: [MPlayer-users] Are mencoder lossless codecs really lossless?
 

On 09.07.2013, at 14:52, James Board <jpboard2 at yahoo.com> wrote:
> Okay, so then if I start with some specific color space, compress with the lossless codecs, and then transcode back to that same original colorspace (assuming everything else is the same too: resolution, frame-rate, etc), then the frames should be exactly the same as the original (except for maybe small round-off errors, or artifacts of that nature)?

No "except". Encoding to a lossless codec and decoding again results in exactly the same output.
I am only talking about the video encoder or decoder. They are less than 50% of the code involved if you use a program like mencoder and unless you are very careful that code will not be lossy.

> I know that's what the name implies, but I just wanted to make sure.  The problem I had was that I grabbed a 30 second segment from a video to experiment with.  That segment started at the 10-second mark of the original video and ended at the 40-second mark.  The purpose of that was to have a small fast piece of video merely to experiment with. But somehow it looks like mencoder put the full 42 seconds in the experimental video file, and then when I translated to lossless, and back to the original format, it became the expected 30 seconds.  So I was seeing large reductions of filesize, which made me wonder if these lossless codecs really were.

As I said before: it's not the codec. Also, encoding with a lossy codec does not by itself reduce the size after decoding again, so even if the file size had been identical that would not have meant anything.
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I'm merely trying to figure out whether or not I lose any pixel information when I convert to a 'lossless' codec.  Most people are telling me no, I won't lose any information.  But you seem to be telling be that I will lose information with this comment "unless you are very careful that code will not be lossy."  So, I'm somewhat confused.