[FFmpeg-devel] [PATCH 3/7] avcodec/golomb, h264*: Fix get_ue_golomb_31()

[Michael Niedermayer]

On Tue, Jul 14, 2020 at 10:43:51PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > On Tue, Jul 14, 2020 at 05:34:50PM +0200, Andreas Rheinhardt wrote:
> >> get_ue_golomb_31() uses a LUT of 512 entries; therefore it can be used
> >> to parse exp-golomb codes of length <= 9, i.e. those codes with at most
> >> four leading bits that have five effective bits; this implies a range of
> >> 0..30 and not 31. In particular, this function must not be used to parse
> >> e.g. the H.264 SPS id.
> > 
> > hmm, are you sure ?
> > 
> 
> Yes.
> 
> > 1           0
> > 01X         1-2
> > 001XX       3-6
> > 0001XXX     7-14
> > 00001XXXX   15-30
> > 000001..... 31
> > 
> > we need to read 9 bits for this, we do not need to read the bits marked with
> > a "." because the code is already determined at this point.
> > 
> The code is only determined at the point if one already presumes that it
> can't be anything >31. 

yes, that is the idea of get_ue_golomb_31() its only used when the biggest
valid code is 31.


> Without this assumption, one needs to look at all
> the five "." in order to distinguish it from 32..62. By looking at only
> 9 digits one can not rule out 32-34.
> And if you take a look at ff_ue_golomb_vlc_code, you will see that it
> does not even contain 31 at all. It returns the values 0-30 for the
> codes with length <= 9 and returns 32 for the rest.

const uint8_t ff_ue_golomb_vlc_code[512]={
32,32,32,32,32,32,32,32,31,32,32,32,32,32,32,32,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,
                        ^^
[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

During times of universal deceit, telling the truth becomes a
revolutionary act. -- George Orwell
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