[FFmpeg-devel] [RFC] libavfilter-soc and variable-frame-size video streams

Jason Garrett-Glaser darkshikari
Sun Dec 14 12:24:15 CET 2008

On Sun, Dec 14, 2008 at 2:51 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> Aurelien Jacobs schreef:
>> On Sat, 13 Dec 2008 12:45:21 +0100
>> Stefano Sabatini <stefano.sabatini-lala at poste.it> wrote:
>>> On date Friday 2008-12-12 16:54:26 -0800, Jason Garrett-Glaser encoded:
>>>>> But maybe I'm just confused and there is a far simpler solution, or
>>>>> maybe we could just decide that supporting variable-frame-size video
>>>>> streams is not worth that hassle.
>>>> This sounds reasonable to me: thinking through what every filter would
>>>> have to do to support such a thing, it sounds completely
>>>> unmaintainable, especially for filters that act temporally: how will
>>>> they deal with the fact that cached temporal data from previous frames
>>>> (such as for temporal denoisers) is no longer the correct size
>>>> matching the current frame?!
>>>> Avisynth gets away just fine without variable frame size support: I
>>>> have actually never seen such a thing as a feature request either.  I
>>>> don't think libavfilter needs it either.
>>> My only problem with this is that this way we cannot support video
>>> stream such as VP6, where the size is detected just when we parse the
>>> first decoded frame:
>>> http://thread.gmane.org/20080830132832.GA6663 at geppetto
>> If this really helps avfilter, I can modify the VP6 decoder so that
>> it don't zero out the container width/height before decoding first
>> frame. But that won't change the fact that width/height can change
>> later.
>>> I'm thinking about some mechanism for which an input filter may detect
>>> that the input size changed, and notify the output filter of that.
>> Without knowing libavfilter very well, I can imagine a simple
>> solution to resolution change.
>> Every filter should have a flag to declare wether it supports resolution
>> change or not. Then when a resolution change happens, the whole filter
>> chain is inspected. As soon as a filter which don't support resolution
>> change is detected, a scale filter is automatically inserted before
>> it. Some people may not like automagic stuff, so this feature could be
>> optionnal, and when not enabled, automatic insertion of the scale
>> filter would be replaced by a hard failure.
>> Aurel
> What is conceptually the difference between the very first valid frame received at a temporal filter and the first frame received at a temporal filter after a resolution change?

By that logic, libavfilter doesn't actually have to support variable
frame size--it just means the filter chain has to be re-initted at a
framesize change.

Dark Shikari

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