[FFmpeg-devel] [PATCH] Arithmetic decoding in ALS

[Thilo Borgmann]

Am 06.02.10 10:03, schrieb Reimar D?ffinger:
> On Tue, Jan 26, 2010 at 01:44:46AM +0100, Thilo Borgmann wrote:
>> +//#define DEBUG
> 
> That's not used anywhere.
> 
>> +static const unsigned int cf_tables_1[3][129] = {
> 
> All tables seem to fit in 16 bit?
> 
>> +static const unsigned int *cf_table[16] = {
> 
> one const too few.
> 
>> +int bgmc_init(AVCodecContext *avctx, uint8_t **cf_lut, unsigned int **cf_lut_status);
> 
> global symbols all need either a ff_ or av_ prefix, ff_ is appropriate I guess.
> 
>> +static const unsigned int tail_code[16][6] = {
> 
> Fits in 8 bits?

All above done.


> 
>> +        unsigned int B = av_clip((av_ceil_log2(bd->block_length) - 3) >> 1, 0, 5);
> 
> B is not a good name, not only looks it a lot like 8 with some fonts, uppercase is also
> for macros.

Renamed to "b". Yes, bad name but used just once and it is some
fictional entity I don't know a suitable name or abbreviation for. Also
"b" is close to the specs and therefore easy to relate to the algorithm.

> 
>> +            k    [sb] = s[sb] > B ? s[sb] - B : 0;
> 
> FFMAX(s[sb] - B, 0)
> might be faster for some architectures at least.

s[] and b are unsigned, so this might reveal unsigned-issues again on
some compilers. Also, this is done once per "large" sub-block so a
minimal speedup should converge to zero.


> 
>> +                    if (res & 1)
>> +                        res = -res - 1;
>> +
>> +                    res >>= 1;
> 
> Isn't that the usually "level + sign" coding? In that case there should be an
> optimized function for MPEG coding, one of these days I have to search for it
> and write it down...
> Either way, if res has the lowest bit set, so does -res, so your -1 is just
> unsetting the bit you shift away in the next step anyway.

Indeed, Done!

Also, ff_bgmc_end() actually used in alsdec.c.

Thanks! Revision 6 attached.

-Thilo
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