[Ffmpeg-devel] Reading bit-reversed VLC codes

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> 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?

There's a small bug in init_vlc().  With the attached patch it works
if I reverse the codes and use INIT_VLC_LE.

-- 
M?ns Rullg?rd
mans at mansr.com
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