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

[James Board]

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)?
 
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.
 


________________________________
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 4:05 AM
Subject: Re: [MPlayer-users] Are mencoder lossless codecs really lossless?


James Board <jpboard2 at yahoo.com> wrote:

>But are the codecs really lossless?
>
>
>
>________________________________
> From: Phil Rhodes <phil_rhodes at rocketmail.com>
>To: James Board <jpboard2 at yahoo.com>; "MPlayer usage questions, feature
>requests, bug reports" <mplayer-users at mplayerhq.hu>
>Sent: Monday, July 8, 2013 12:12 PM
>Subject: Re: [MPlayer-users] Are mencoder lossless codecs really
>lossless?
>
>
>
>Is "raw" YUV 4:2:2, like the input? You may find raw implies RGB, but
>it may default to whatever pixel format comes out of the codec. That'd
>make it bigger, anyway.
>
>You also mention "VY12" by which I assume you mean YV12, which is a
>4:2:0 format with lower chroma resolution. If mencoder is maintaining
>the chroma dimensions, then yes it will be smaller than a 4:2:2 input,
>by about a sixth.
>
>There are also a number of different ways of building AVI files, with
>various types of interleaving and chunk sizes, which could account for
>a reasonably small difference in file sizes.
>
>As regards precision, these sorts of codecs are theoretically capable
>of providing bit-exact results, but real world, practical
>implementations of them may not due to scaling and rounding errors,
>etc. This should not affect the file size as raw output of the same
>resolution and pixel format should be the same size regardless of image
>content.
>
>P
>
>
>________________________________
> From: James Board <jpboard2 at yahoo.com>
>To: "mplayer-users at mplayerhq.hu" <mplayer-users at mplayerhq.hu>
>Sent: Monday, 8 July 2013, 16:33
>Subject: [MPlayer-users] Are mencoder lossless codecs really lossless?
>
>
>My input file is an uncompressed raw YUV 4.2.2 format.  I want to
>compress individual frames with a lossless codec.  My mencoder
>implementation listed three such codecs it the man page:
>    * ljpeg: lossless jpeg
>ffv1: FFmpeg's lossless video codec
>    * ffvhuff: nonstandard 20% smaller HuffYUV using VY12
>I encoded my file using the following:
>    mencoder -ovc lavc -lavcopts ljpeg -oac copy -o out.avi in.avi
>And then I converted it back to raw pixels with
>    mencoder -ovc raw -oac copy -o out2.avi out.avi
>The above two steps should regenerate exactly the
>original file in.avi, right?  I did this with the three lossless codecs
>I listed above.  The problem is that my original input file in.avi is
>bigger than the final output file out2.avi: they should be exactly the
>same, right?
> 
>So, my question is this: are those codecs really lossless?  Bit-exact
>lossless?  Or are they merely pretty good?
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Yes, of course. Why else would they be called that?
But the codec being lossless doesn't necessarily mean all other parts involved in transcoding are lossless.