[Ffmpeg-devel] Reading bit-reversed VLC codes

[Michael Niedermayer]

Hi

On Sun, Mar 18, 2007 at 07:27:43PM +0000, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > Hi
> >
> > On Sun, Mar 18, 2007 at 07:13:30PM +0000, M?ns Rullg?rd wrote:
> >> I need to read VLC codes that are stored in reverse order in the
> >> bitstream, i.e. a 4-bit code value of 5 is returned by get_bits as
> >> 0xa.  Is there some way to do this using the lavc VLC reader?
> >
> > for get bits the code in asv1.c
> > static inline int asv2_get_bits(GetBitContext *gb, int n){
> >     return ff_reverse[ get_bits(gb, n) << (8-n) ];
> > }
> 
> Non-VLC elements are stored the right way around, so I don't need to
> do anything special with those.
> 
> > might do what you want, for get_vlc() just change the table
> > before init_vlc()
> 
> I tried reversing the bits of each code before init_vlc(), but that
> only gives me an "incorrect codes" message and an error return.  I
> guess I did something wrong, but I'm at a loss figuring out what.

well, you cannot just reverse a vlc like that, maybe you could elaborate
on what exactly you are trying to do (link to codec spec?)
i thought that you wanted to reverse the output of get_vlc() not the
bits of the vlc codes as stored in the bitstream later wont work in general
example:

0   0
10  1
11  2

if you now reverse it

0   0
01  1
11  2

what is

0111111111111111111111111111...

is it

0,2,2,2,...
 or
1,2,2,2,...

it cannot be decided before the end of the stream ...

init_vlc/get_vlc() doesnt support such non prefix codes (for obvious reasons)

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have often repented speaking, but never of holding my tongue.
-- Xenocrates
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