[MEncoder-users] CABAC and NoCABAC

[Reimar Döffinger]

On Tue, Apr 05, 2011 at 02:36:03PM +0200, Grozdan wrote:
> On Tue, Apr 5, 2011 at 1:57 PM, Stjepan Brbot <stjepan.brbot at gmail.com> wrote:
> > On Tue, 2011-04-05 at 12:57 +0200, Grozdan wrote:
> >> On Tue, Apr 5, 2011 at 3:48 AM, Stjepan Brbot <stjepan.brbot at gmail.com> wrote:
> >> > I encoded the same movie twice; first time I had CABAC entropy coding
> >> > turned on and second time turned off. Although that resulted in
> >> > different file sizes, since entropy coding is lossless coding, I
> >> > expected the same PSNR for both encoded movies but psnr parameter
> >> > reported different figures!!! How's that possible?
> >>
> >> Wait, so you're complaining of having different PSNR but are comparing
> >> two different coding schemes yet expect the exact same results? Are
> >> you serious?
> >
> > No, you're wrong I'm talking about the same coding scheme. When video
> > compression coding is done then quantization coefficients, motion
> > vectors and reference indicies are coded in entropy coding using some of
> > lossless algorithms. This process of entropy coding is lossless and
> > taking different algorithms should affect only performance and
> > compression efficiency but all algorithms must keep the same entropy and
> > that should result in exactly the same input data for decoder.
> 
> Sorry, I misread...
> 
> what kind of options did you use? The differences between cabac and
> cavlc, afaik, are that both use different RDO and cabac has trellis
> turned on (although cavlc recently gained a trellis mode but it's not
> the same as the cabac one). So if only entropy coding would differ and
> wouldn't touch rdo/analysis/etc (which they do), both will produce the
> exact same outcome

Or the really short way to say it: Which values are cheap and which are
expensive to encode differ between CABAC and CAVLC, thus a good encoder
will take that into account and sometimes choose a different encoding
depending on whether CABAC or CAVLC follows.
Which also means that e.g. while transcoding a file from CAVLC to CABAC
would save some bits it generally won't be as good quality as if it
had been encoded with CABAC in the first place.