[FFmpeg-devel] [PATCH] Split huffman_tables into codes and bits in mlpdec

[Ramiro Polla]

Hello,

On Tue, Aug 12, 2008 at 3:17 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Tue, Aug 12, 2008 at 02:49:46PM -0300, Ramiro Polla wrote:
>> Hello,
>>
>> Attached patch splits huffman_tables into codes and bits tables in
>> mlpdec.c. This will be shared with the encoder. This makes it easier
>> for the encoder to reuse two out of three huffman_bits tables and a
>> third custom table.
>
> why does it make it easier?
> iam not against this patch but i like to understand it first

Hmmm, let me see if I can manage to explain it.

MLP encodes residuals using a codebook for the higher bits, and then
encodes the rest from that as verbatim. It has 3 codebooks. The vlc
code 0 equals -7 for all of them. But MLP also does a sign shift,
which is different depending on the codebook. For codebook 1 it's -2
(so vlc code 0 is effectively -9), for codebook 2 it's -1 (so vlc code
0 is effectively -8), and for codebook 3 it's not on the vlc code, but
on the last bit of the residual from the vlc. For example:

codebook 1:
000 111 + sign_shift = 010 111
vlc code 7 + 2 (010) = 9
residual = 111

codebook 3:
000 111 + sign_shift = 001 011
vlc code 7 + 1 (001) = 8
residual = 011

To simplify the bitcount code, codebooks 1 and 2 only check the higher
bits against the huffman_bits array, since their value doesn't depend
on the rest that will be encoded verbatim. But for codebook 3, it
depends on one more bit. So I made one more array for codebook 3 that
takes that into account, and so has double the size (for the extra bit
being 0 and 1).

For the same explanation in C, look at huffman_bitcount2,
huffman_bitcount, codebook_extremes, and codebook_bits_offset() from
mlpenc.c in the SoC repo.

Ramiro Polla