[Ffmpeg-devel] Reading bit-reversed VLC codes

[Måns Rullgård]

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?

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