[x264-devel] Re: CABAC performance

[Son Minh Tran]

Loren Merritt a écrit :
> On Tue, 29 May 2007, Son Minh Tran wrote:
>
>> Have you ever tested the performance of CABAC in x264. It is strange 
>> that with  toggling between using / not using CABAC (without / with 
>> --no-cabac option) I didn't see the improvement about 16% of bitrate 
>> at the cost of encoder speed fps (complexity) about 25-30%. In fact I 
>> had about 4-11% improvement in bitrate for only max 6% increase in 
>> the fps speed of encoder. You have a optimized CABAC encoder or I 
>> miss some things while turning off the CABAC
>
> Encoding CABAC takes the same amount of time as decoding CABAC. But 
> the encoder does lots of other computations, most of which are 
> analysis only and not duplicated in the decoder. So CABAC is a large 
> fraction of the decode time, and a small fraction of the encode time.
>
> Where did you hear "16% bitrate and 25-30% speed"?
>
> --Loren Merritt
>
Dear Lorent Merritt,
I take these data from the article "Video coding with h.264/AVC: tools, 
performance and complexity" written by Jorn Ostermann, Jan Bormans, 
Peter List, Detlev MArpe, Matthias Narroschke, Fernando  Pereira, 
Thomas  Stockhammer and Thomas Wedi  
(http://iphome.hhi.de/marpe/download/h264_casm04.pdf) See on page 23.
Anyway on any tutorial of CABAC you can see the average bit-rate saving 
is of 9-14%. Cabac is an arithmetic coder, which is time-consuming. 
Maybe the replacement of multiplication with pre-calculated table  does 
reduce this load?  The implementation of  CABAC in x264 seems to me 
quite straight forward, except the fact that you have to allocate a lot 
of tables in the memory.
I have a cross question right now. You said that the max value of option 
-r for x264 is 16. But actually in the standard , it can go up to 31 
(the max value of *num_ref_idx_l0_active_minus1 and 
**num_ref_idx_l1_active_minus1*)?

Thank you
Son


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