On 6/15/23 8:46 AM, Paul B Mahol wrote:
> get_vlc2 can be made for get_bits_var(), first table bits (that are still
> int and not uint8_t), the code that picks table index from which to take
> bits.
>
> It is also possible to make it take both first index and rest of it and
> build bigger tables but that is very very advanced step for new
> contributors.
>
> Use INIT_VLC_SPARSE_STATIC, there are myriad examples in libavcodec, one of
> them being imm4 decoder.
>
I will change that table bits array to uint8_t.
I encountered an issue when trying to set up the VLC table with
INIT_VLC_SPARSE_STATIC.
The current get_bits_var:
static int get_bits_var(GetBitContext *gb, const VarBits *var_bits)
{
static const uint8_t bits_used[8] = { 1, 1, 1, 1, 2, 2, 3, 3 };
int code = show_bits(gb, 3); /* value range is [0, 7] */
skip_bits(gb, bits_used[code]);
if (var_bits->bits[code] == 0)
return 0;
return get_bits_long(gb, var_bits->bits[code]) + var_bits->add[code];
}
Changed to use get_vlc2 would be:
static int get_bits_var(GetBitContext *gb, const VarBits *var_bits)
{
int code = get_vlc2(&gb, vlc.table, 3, 1);
if (var_bits->bits[code] == 0)
return 0;
return get_bits_long(gb, var_bits->bits[code]) + var_bits->add[code];
}
The "vlc.table" that INIT_VLC_SPARSE_STATIC needs to output is:
len: 1, 1, 1, 1, 2, 2, 3, 3
sym: 4, 4, 4, 4, 8, 8, 16, 32
INIT_VLC_SPARSE_STATIC would get the same len and sym and this code table as
input:
code: 0, 1, 2, 3, 4, 5, 6, 7
INIT_VLC_SPARSE_STATIC rejects the "len" and "code" because the code
of "2" will not fit into one bit. Regardless of the fact that the desired
output table is not a valid table according to VLC functions, that invalid
table is what is needed.
This goes back to how get_bits_var decodes the bit context.
get_bits_var looks at three bits to get an index in the range of 0-7.
But it may not skip all of those bits. One or two of those bits may also be
read by the final get_bits_long.
code = show_bits(gb, 3);
skip_bits(gb, bits_used[code]);
get_bits_long(gb,...);
The VLC functions to construct tables have validation in place that prevents
the construction of the table needed for get_vlc2() to work with
get_bits_var.
So the only way I can find to use get_vlc2() is to not use any of the vlc.h
initialization functions, and instead hard-code the tables to be used by
get_vlc2() like this:
VLCElem table[8] = { {4,1}, {4,1}, {4,1}, {4,1}, {8,2}, {8,2}, {16,3}, {32,3} };
Before I do that, I wanted to verify this would be acceptable. Would
this be a use of get_vlc2() that could lead to issues in the future
if the GET_VLC macro changed so that it would not skip fewer bits than the
code length?
What is your opinion on this?
Thanks,
-Roy
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