[FFmpeg-devel] [PATCH] FLAC parser

Justin Ruggles justin.ruggles
Sun Mar 29 05:35:52 CEST 2009

Michael Niedermayer wrote:
> On Sat, Mar 28, 2009 at 10:24:04PM -0400, Justin Ruggles wrote:
>> Michael Niedermayer wrote:
>>> On Sat, Mar 28, 2009 at 06:41:02PM -0400, Justin Ruggles wrote:
>>>> Michael Niedermayer wrote:
>>>>> On Fri, Mar 27, 2009 at 01:05:47AM -0400, Justin Ruggles wrote:
>>>>>> Hi,
>>>>>> I finally got a working FLAC parser without resorting to buffering
>>>>>> max_frame_size bytes like the FLAC decoder does.  It requires a slight
>>>>>> change to ff_combine_frame() since the header can be up to 16 bytes long
>>>>>> and ff_combine_frame() currently only supports up to 8 bytes of overread
>>>>>> This works with all samples I've tested, but it would be great to have
>>>>>> more tested as well.  There are quite a few corner cases, and while I've
>>>>>> tried to think of everything I can, I might have missed something.
>>>>> If i understand this correctly,
>>>>> this is a probabilistic parser, that is it will fail once in 4tb of
>>>>> random data at least, but due to the max crc stuff sooner.
>>>>> and as data is not random it could fail more frequently
>>>> Yes. The only alternatives I can see would be decoding twice (although
>>>> inverse prediction, and interleaving could be skipped in the parser) or
>>>> not having a parser, which prevents stream copy to most other containers.
>>> You can do better without decoding twice
>>> first find all valid headers (sync + header crc + a few bits in the header)
>> When you say "all valid headers" do you mean to buffer the whole file?
> no
>> Would it be ok to check sequences in a fixed number of headers or a
>> fixed number of bytes (this would have to be like pretty large to be useful)
> it would be possible for example to add to the buffer until enough headers
> are detected. or until a long enough connected chain is found ...
> (probably the one fitting better in the code ....)
>>> that would give you something like:
>>> H          H      H                      H       H                    H
>>> then calculate the frame crc between each 2 headers
>>> H          H      H                      H       H                    H
>>> --CRC!=0-->
>>> ---------CRC==0-->
>>>           -CRC!=0->
>>>           ----------------------CRC==0-->
>>>                   --------------CRC!=0-->
>>>                   ----------------------CRC!=0-->
>>>                                          -CRC==0->
>>>                                          ------------CRC==0---------->
>>>                                                   ------CRC==0------->
>>> Then find the longest connected chain of valid CRCs, if there is another
>>> non overlapping chain of valid crcs before it pick it instead.
>>> Return the first frame from the chain.
>> I just want to make sure I understand you correctly.
>> H0-->H2 = 1
>> H1-->H3-->H4-->H5 = 3
>> H2 = 0
>> H3-->H4-->H5 = 2
>> H4-->H5 = 1
>> This would select the chain starting with H1, and the data between the
>> start of the buffer and H1 should be returned in this case.
> yes
>> Then the CRCs starting at H0 can be tossed out, and after the next
>> header is found, calculate 2 new CRCs, H4-->H6 and H5-->H6 and
>> re-evaluate the sequences.
> yes, also i just used n->n+1 / n->n+2 for sake of readability, one
> should prossibly consider more

Ah, I see. 2 or more false positives in a row could be pretty likely.

>>> also you should favor chains which dont change samplerate or other such
>>> things.
>> Do you mean to pick a sequence that doesn't change values over an
>> earlier sequence of the same size that does change values?  Or something
>> with more weight than that?
> I think this is not that important ATM, but i would tend to give
> things like samplerate changes more weight.
> i mean, for example:
> h0----->h1------->h2
>   --------------->
> and assuming all crcs match
> but h1 has a different sample rate than h0 and h2 
> here the shorter chain seems more likely correct, but this depends on
> my assumtation that a single lonely packet with different samplerate is
> very unlikely.

The same holds true for bps and channels.  The FLAC format description
does not explicitly disallow changing these values mid-stream, but the
libFLAC reference decoder considers changing some of them from one frame
to the next to be an error.


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