[FFmpeg-soc] libavfilter review

Vitor vitor1001 at gmail.com
Sun Nov 4 22:12:00 CET 2007


Bobby Bingham wrote:
> On Sun, 04 Nov 2007 21:33:10 +0100
> Vitor <vitor1001 at gmail.com> wrote:
>> Hi, and sorry to resurrect an old thread.
>> Michael Niedermayer wrote:
>>> Hi
>>> On Sun, Aug 19, 2007 at 12:36:15PM -0400, Bobby Bingham wrote:
>>>> On Sat, 18 Aug 2007 19:22:38 +0200 Michael Niedermayer wrote:
>>>>> On Sat, Aug 18, 2007 at 12:12:08PM -0400, Bobby Bingham wrote:
>>>>>> On Fri, 17 Aug 2007 21:10:15 +0200 Michael Niedermayer wrote:
>>>>>>> the query_formats system is very flexible, it takes a
>>>>>>> AVFilterLink so a filter could have a fixed list of supported
>>>>>>> input pix_fmts and make the output query_formats depend on the
>>>>>>> input pix_fmt or the other was around well i dont think the
>>>>>>> other way around would work but how should the user know that?
>>>>>>> and whats the sense of this overly flexible system if it doesnt
>>>>>>> work with anything but the obvious simple supported output
>>>>>>> pix_fmt depends upon input formats, it would be alot clearer if
>>>>>>> query_formats would take a list/array of input pix_fmts as
>>>>>>> argument (or a array or pix_fmt, flags pairs) where the flags
>>>>>>> could indicate if the pix_fmt can be provided without colorspace
>>>>>>> conversation, but maybe thats not really usefull and a simpler
>>>>>>> prefer first possible format in the list system would work
>>>>>>> equally well
>>>>>>> also what happens in the following case:
>>>>>>> src -> filter -> destination
>>>>>>> src supports formats A and C
>>>>>>> destination supports formats B anc C
>>>>>>> and the filter supports A,B,C inputs and output=input
>>>>>>> if i understood the code correctly this would fail
>>>>>> I think I've got an idea which will be less absurdly flexible,
>>>>>> and will support graphs like your example without requiring
>>>>>> conversion.  The only thing is that I might need to set the
>>>>>> restriction that all the inputs of a filter must always be
>>>>>> operating on the same colorspace, and similar for outputs.
>>>>>> This of course doesn't affect all those filters with only simple
>>>>>> inputs and outputs.  And I expect most filter authors would only
>>>>>> support this case anyway.  But before I start coding it, I want
>>>>>> to check if such a restriction would be acceptable.
>>>>> its probably ok (i cant think of a case where a filter would want
>>>>> 2 inputs with differing colorspace)
>>>> Unfortunately, I thought of a case.  The filtergraph filter, being
>>>> simply a composite of various subfilters, may legitimately have
>>>> inputs with multiple colorspaces.  And doing so may optimize the
>>>> number of conversions.
>>>> I'm afraid the more I look at the problem, the more I realize it's
>>>> not something I can come up with a good solution to by Monday.  I
>>>> think I'll add support for autoconversion to the current system so
>>>> that it at least works in all cases, and I'll have to work on a
>>>> better system after the end of summer of code, because there are
>>>> enough other areas I can actually make improvements on before the
>>>> deadline if I don't get hung up on this one thing.
>>>> Of course, I'm open to any ideas anybody has regarding a solution
>>>> to colorspace negotiation...
>>> ive thought about cs negotiation a little and ive come up with the
>>> following possible solution
>>> each filter link has 2 IDs (integers or  pointers or something)
>>> one corresponding to the input into the link and one corresponding
>>> to the output (alternatively we could have always 2 links with a
>>> dummy filter between)
>>> now if 2 IDs anywhere within the filter graph are equal that means
>>> that the colorspace must be equal and each ID has a list of valid
>>> colorspaces
>>> now to solve it you just merge the 2 IDs on each link
>>> if both the corresponding lists have a common colorspace then merge
>>> is possible if not conversation is needed
>>> example: 
>>> the case from above:
>>>>>>> src -> filter -> destination
>>>>>>> src supports formats A and C
>>>>>>> destination supports formats B anc C
>>>>>>> and the filter supports A,B,C inputs and output=input
>>> src b-->c filter c-->d dst
>>> b={A,C}
>>> c={A,B,C}
>>> d={B,C}
>>> first we merger b and c leading to
>>> src b-->b filter b-->d dst
>>> b={A,C}
>>> d={B,C}
>>> and next: b and d
>>> src b-->b filter b-->b dst
>>> b={C}
>> That will work if the filters have either input_fmt=output_fmt or
>> fixed output format. But for the case of, for example, a gamma filter
>> which would turn RGB24 into RGB32 and BGR24 into BGR32 it won't work.
>> In schema:
>> src --> filter --> dst
>> src supports RGB24 and BGR24
>> dest supports BGR32
>> filter outputs BGR32 if input is BGR24 and RGB32 if input is RGB24
>> The solution without conversion is
>> src BGR24 -> BGR24 filter BGR32 -> BGR32 dest
>> -Vitor
> As far as I'm aware, RGB24 and RGB32 only differ in that RGB32 has an
> unused byte for each pixel, where an alpha value would go for RGBA.  So
> unless I miss something, I don't see why a filter would do that exactly.

You're right... Actually, when I thought about my example I was thinking 
  more of RGBA and YUVA, but I didn't find any PIX_FMT_YUVA...

> But it still raises a good point.  When I had been brainstorming ideas
> for colorspace negotiation, I tried taking such cases into account, and
> it just made the whole thing so much more complicated.  After seeing
> Michael's suggestion, I figured that in almost all cases, a filter will
> either have output format = input format, or will be some sort of
> colorspace conversion filter where the output and input formats don't
> need to be related.
> How common is this case where the possible output formats depend on the
> input format, but do not equal it, really?  If it's rather uncommon
> like I suspect, I'd rather have those filters deal with it themselves
> than introduce all the extra complexity into libavfilter.  For example,
> in your example above, the filter could without too much trouble also
> have a routine to perform the operation on RGB24 -> BGR32.  It's just
> two bytes swapped from what it would normally do.

I agree. For such rare cases, I don't think it is really worth the extra 
complexity. I took some time to find the example of the gamma filter and 
I can't think of any other examples...

> --
> Bobby, who finally found time to implement Michael's suggestion
> today ...

Cool! Looks like we'll start the long "[PATCH]" threads in -devel soon :-)


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