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

Thilo Borgmann thilo.borgmann
Wed Feb 10 16:09:26 CET 2010


Am 07.02.10 01:16, schrieb 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.

If no one objects anymore I will apply this in a few days.

-Thilo



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