[Ffmpeg-devel] Reading bit-reversed VLC codes

[Michael Niedermayer]

Hi

On Sun, Mar 18, 2007 at 08:54:13PM +0000, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > Hi
> >
> > On Sun, Mar 18, 2007 at 09:38:10PM +0100, Michael Niedermayer wrote:
> >> Hi
> >> 
> >> On Sun, Mar 18, 2007 at 08:33:31PM +0000, M?ns Rullg?rd wrote:
> >> > Michael Niedermayer <michaelni at gmx.at> writes:
> >> > 
> >> > > 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?
> >> > 
> >> > It's stored in such a way that if I get_bits() the right number of
> >> > bits for a code, I get the bits reversed compared to the code value
> >> > I'm expecting.
> >> > 
> >> > Here's what the spec has to say:
> >> > 
> >> >              * Data elements are packed into bytes in order of
> >> >                increasing bit number within the byte, i.e., starting
> >> >                with the least-significant bit of the byte.
> >> 
> >> try a
> >> #define ALT_BITSTREAM_READER_LE
> >> before the #incldues
> >
> > and pass INIT_VLC_LE to init_vlc()
> 
> Segmentation fault.

gdb?

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

When the tyrant has disposed of foreign enemies by conquest or treaty, and
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