[FFmpeg-devel] [PATCH] mxf umid generation

[Baptiste Coudurier]

On 3/7/2009 5:23 PM, Michael Niedermayer wrote:
> On Sat, Mar 07, 2009 at 04:14:19PM -0800, Baptiste Coudurier wrote:
>> On 3/7/2009 3:36 PM, Michael Niedermayer wrote:
>>> On Sat, Mar 07, 2009 at 02:48:49PM -0800, Baptiste Coudurier wrote:
>>>> On 3/6/2009 7:44 PM, Michael Niedermayer wrote:
>>>>> On Fri, Mar 06, 2009 at 07:28:55PM -0800, Baptiste Coudurier wrote:
> [...]
>>>> Property changes on: libavutil\random_seed.c
>>>> ___________________________________________________________________
>>>> Added: svn:eol-style
>>>>    + LF
>>> intended?
>>>
>>> except these ok
>>>
>> I'm not sure, I'm on windows because products I test with only works on
>> windows, I set ending lines style to unix, but it keeps adding this...
>>
>> It should be ok I think.
>>
> 
>> Btw, is lfg ok for this purpose or should I use something else ?
> 
> to generate these id numbers out of the seed?
> id rather use
> seed += 1LL<<32 :)
> 
> lfg seens pointless complexity for this ...
> 
> and patch ok
> 

Well I'd like to use the defined methods for umid generation, see below:

"A.3.2 Alternative masking methods
The masked material number is an unpredictable number uniformly
distributed over the range 0 thru 2^128-1. Its
effectiveness as a unique identifier relies on this uniform random
distribution, and the exact method of its generation is not
important. Therefore, the use of the reference masking method is not
normative, and any method providing an equivalent
level of unpredictability and uniformity of distribution may be used
with the ?masked method? value in the ?number
generation method? field of the UMID universal label (reference table 1
in 5.1.1)."

And instance generation:

"B.2 24-bit PRS generator (?2h?)
Any suitable psuedo-random sequence (PRS) generator polynomial may be
used provided it has a maximal length of
16,777,215 clock cycles. At the point of creating a new instance of the
material, the 24-bits from the PRS generator are
sampled to gain a new instance value.
PRS generators shall not allow a zero value.
NOTES
1 Any suitable seed may be used to start the pseudo-random sequence
(PRS) 24-bit generator.
2 The PRS generator should use a free-running clock having no time
relationship with the clock used to generate the sampling strobe.
3 The PRS generator clock frequency should be greater than 10 kHz.
4 The number of feedback taps resulting from the PRS generator
polynomial should be between 8 and 16 to ensure the random nature
of the sequence."

What do you think ?

-- 
Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
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FFmpeg maintainer                                  http://www.ffmpeg.org
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