[MEncoder-users] Mencoder quality settings ...

[Leopold Palomo Avellaneda]

A Divendres 04 Gener 2008, Moritz Barsnick va escriure:
> Hi,
>
> On Thu, Jan 03, 2008 at 20:58:24 +0100, Leopold Palomo Avellaneda wrote:
> > I would like to convert it to using another codec (for example xvid or
> > x264) preserving the _same_ quality (in terms of taking one frame from
> > both and not finding any difference). It's something like I have a bunch
> > of files and I compress it using bz2 or zip. I know that I always will
> > have the original. Some kind of lossless compression.
>
> Let me take a mathematical and logical approach here:
> > I think that it's impossible because when I decode a frame and I
> > recode it it cannot be equal to the original.
>
> It can, if the codec used when recoding is lossless. Not many such
> codecs come to mind, I think snow and hufyuv are candidates. As
> mentioned in the other answers, there _are_ settings for other more
> common codecs making them "nearly" lossless, even possiby at a bitrate
> possibly lower than your original.

OK.

> > But for example, if I have a photo, for example in tiff and I save it
> > to jpeg, I can choose the % of quality, understanding that a 100%
> > quality jpeg is equal to the original tiff using less space.
>
> This is wrong since JPEG has _no_ lossless option available whatsoever.
> JPEG2000 e.g. on the other hand does. I understand what you're trying
> to say though, but the example is wrong. ;-)
>
ummmmmm ok you are right. The correct example should be jpeg2000 :-)

> > In my case I don't care about the disk space totally but yes to have a
> > conversion to another more efficient codec.
>
> Your problem is you want both lossless and more efficient than MPEG2.
> At the same time your "original" is already MPEG2 and thereby lossy. If
> you had captured from that "home video device" to something less lossy
> or non lossless, your next (lossy) recoding step would have the chance
> of being closer to this "absolute" original.
>
> > So, if I don't want to look in to the bitrates, etc, there's some way to
> > convert one stream in one format to another preserving the _same_ image
> > quality but less space?
>
> Again, in the chain:
> real original -> MPEG2 -> next format
> the next format will statistically always be worse (further from the
> real original) unless it uses a lossless codec.
> If you replace that chain (which you might not be able to) with
> real original -> new format
> you have the chance of using less space and even of higher quality.

NO, my real original is mpeg2. A DVB-T signal is codec with mpeg2. My video 
device is a simple device that that the video stream and saves is to a hd. I 
came from that.

>
> Now, apart from that:
> I totally get your point. I sometimes also think something along this
> line:
>
> "Okay, I have this video here which is already half-bad, and I must
> recode it to a different format. With what settings can I get a
> satisfactory low bitrate result, considering that my original format
> was already quite bad."
>
> Not quite the same as your issue, but along the same lines IMO.

yes .....


best regards,

Leo


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