Re: [FFmpeg-devel] [PATCH v3] avformat/framecrcenc: compute the checksum for side data

[James Almer <jamrial@gmail.com>]

On 5/4/2024 5:58 PM, Michael Niedermayer wrote:
> On Sat, May 04, 2024 at 12:16:00PM -0300, James Almer wrote:
>> On 5/4/2024 5:34 AM, Marton Balint wrote:
>>>
>>>
>>> On Thu, 2 May 2024, James Almer wrote:
>>>
>>>> On 5/2/2024 6:23 PM, Marton Balint wrote:
>>>>>
>>>>>
>>>>>   On Wed, 1 May 2024, Michael Niedermayer wrote:
>>>>>
>>>>>>   This allows detecting issues in side data related code, same as what
>>>>>>   framecrc does for before already for packet data itself.
>>>>>>
>>>>>>   This basically reverts c6ae560a18d67b9ddaa25a0338b7fb55e3312e57.
>>>>>
>>>>>   Can you at least add an option which allows disabling dumping the side
>>>>>   data? Changing the format of framecrc output again and again is
>>>>> not very
>>>>
>>>> The framehash/framemd5 muxer is versioned, which is what you should
>>>> use if you want parseable output.
>>>
>>> Okay, but then the question is that why framecrc is using different code
>>> and options?
>>
>> Originally it was framecrc (using AVAdler) and framemd5 (using AVMD5). The
>> latter was renamed/aliased to framehash and made to use the AVHash API,
>> which supports all lavu hashing algorithms, and is versioned.
>> If anyone cared, framecrc could be also made into an alias of framehash that
>> defaults to adler32 output, but it would result in a massive change to
>> reference files, if anything because AVHash initializes adler32 with a 1
>> whereas framecrc does it with a 0.
> 
> normally starting adler32 at 0 is bad as one could prefix by a 0 byte with
> no checksum change, but given we also show the size that spcific case isnt
> an issue
> 
> can we make the initial value for adler32 configureable so as to improve long
> term stability of checksums ? (or add a adler32_1 to AVHash)

av_hash_init() already initializes adler32 with a 1. No need to do 
anything there. It's the framecrc muxer that uses adler32 with 0 as init 
value.

> 
> thx
> 
> [...]
> 
> 
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