[Ffmpeg-devel] Reading bit-reversed VLC codes

[Michael Niedermayer]

Hi

On Sun, Mar 18, 2007 at 08:10:59PM +0000, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > 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'm trying to write a zlib decoder.  The spec is RFC1951.
> 
> > 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:
> 
> [...]
> 
> > init_vlc/get_vlc() doesnt support such non prefix codes (for obvious
> > reasons)
> 
> No, of course it doesn't.  Do you have any suggestion how to solve
> this?

now without reading the RFC (will do so in a moment) are you sure the
vlc is reversed instead of each byte being reversed?

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you really think that XML is the answer, then you definitly missunderstood
the question -- Attila Kinali
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