Re: [FFmpeg-devel] [PATCH] lavc: deprecate AV_CODEC_FLAG_DROPCHANGED

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2023-07-12 07:14 pm, Anton Khirnov wrote:
> Quoting Gyan Doshi (2023-07-12 15:31:57)
>>
>> On 2023-07-12 06:51 pm, James Almer wrote:
>>> On 7/12/2023 10:14 AM, Gyan Doshi wrote:
>>>>
>>>> On 2023-07-12 06:12 pm, James Almer wrote:
>>>>> On 7/9/2023 9:57 AM, Anton Khirnov wrote:
>>>>>> This decoding flag makes decoders drop all frames after a parameter
>>>>>> change, but what exactly constitutes a parameter change is not well
>>>>>> defined and will typically depend on the exact use case.
>>>>>> This functionality then does not belong in libavcodec, but rather in
>>>>>> user code
>>>> NAK.
>>>>
>>>> The applicable parameters are well defined as set by the implementation.
>>> The implementation defined its own interpretation of what constitutes
>>> a parameter change, yes, but callers may have their own
>>> interpretations too. The result is they either don't set this flag, or
>>> set it but still manually check every frame for other parameters.
>>>
>>>> They are whatever leads to a change in the size of the decoded frame
>>>> payload or the layout/semantics of the elemental unit in a decoded
>>>> frame, so width, height, pixel format, sample format/size,
>>>> interleaving..etc
>>> You give pixel format as an example, which would include a change from
>>> yuv to a yuvj pseudo format. What about decoders that export yuv +
>>> color_range mpeg and then yuv + color_range jpeg? This implementation
>>> will not consider that a parameter change, despite being practically
>>> the same scenario.
>>> Once the yuvj formats are removed, this will be true for all decoders.
>>> And suddenly starting to check for color_range would be a change in
>>> implementation here.
>>>
>>> This is definitely something that should be left to the caller. They
>>> get a frame out of the decoder, they decide if they want to drop it.
>> They can still do that. Don't set the flag (default) and check
>> manually.  Keeping the flag in lavc doesn't prevent callers from
>> inserting their own gate.
> The point is that what this code considers a parameter change is
> entirely arbitrary.
>
> Why does it not consider any of the colorspace parameters? Or
> interlacing, cropping, display transform matrix, or any of the other
> AVFrame fields or side data we might add in the future that someone
> migth consider a parameter change in their use case.

This flag considers the changes to AVframe raster data that can lead to 
out of bounds read/writes or mangling of that data.
All the side data add-ons are irrelevant.

When is major bump to 61 expected?

Regards,
Gyan

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