[FFmpeg-devel] [RFC] AVFilter Parser

[vmrsss]

Sorry guys, for some reason my email is badly asynchronous: I receive  
your emails with large delays, which makes the discussion a bit  
clumsy. In particular,  when I wrote my previous proposal I had not  
seen several emails that apparently have been sent before it. This is  
somehow embarrassing.

Below, I'll comment on the new (for me) messages

>> How do
>> you express that out_k is to be fed back to in_h? Or can you only do
>> feedback on intermediate labels?
>
> (out_k) is one of the outputs of the whole filter graph. It can't be
> feed to anything. Maybe I don't understand your question.

The idea to keep feedback labels entirely separated from input and  
output labels is entirely right. But should I not bee able to take a  
filter and perform some feedback after composing as part of a larger  
graph? I'll say more below.

>> Which one? the first? or the last? the one missing from the list?
>> Perhaps I should know the answer, but I am not clear about your
>> intended use of labels. Let me take an example you use below:
>>
>> 	 '(in1)(in2) picInPic, (in3) picInPic(out1)'
>>
>> Is "(in1)(in2)" the same as "(in2)(in1)" ? I expect the answer is no,
>> the second form would swap the two input stream. And (in3) refers  
>> to a
>> third input from the entire chain. Is that fed to the second input of
>> picInPic?
>
> You got all that right.
>
> In order to feed it to the first input, would I have to
>> write the filter this way:
>>
>> 	 '(in1)(in2) picInPic(tmp), (in3)(tmp) picInPic(out1)'  ?
>
> Unless this is a typo, you got this one wrong. The comma would link  
> the
> second (inexistent!) output of picInPic to the first input of the  
> other
> picInPic. It would cause a syntax error. Maybe you meant
>
> '(in1)(in2) picInPic(tmp); (in3)(tmp) picInPic(out1)'  ?

Well, I simply meant to ask: the (1st) output from the 1st picInPic is  
fed to the 1st input of the 2nd picInPic. How do I specify that I want  
that output to go to the 1st input, and (in3) to the 2nd?


> mygraph := (in1) vflip (out1)
>
> vitor at vitor$ ffmpeg -i file1.avi -vfilters '(in1) mygraph (out1)'  
> file2.avi
>
> And yes, textual substitution will give
>
> '(in1) (in1) vflip (out1) (out1)' which is nonsense.

Ah! This I don't understand:

First of all, in -vfilters '.....' (in1) and (out1) carry absolutely  
no information, why would to use them? Only one stream, no confusion  
is possible: just write -vfilters 'mygraph'.

Then, do you or do you not allow (x) mygraph (y) ? I have the feeling  
you are arguing against the substitution, not again that form. (But  
then, if (in1) mygraph (out1) is non-sense after the substitution,  
isn't it non-sense also before?)

Of course there is an important reason to allow (x) mygraph (y): it is  
that you want to capture (that is name) its input or output to do some  
non-trivial re-routing of the streams: eg, I might want to use  
mygraph, feed it with the second of my input streams, and then send  
its output as 2nd input of another filter. How do you propose I do  
that? I think it would be natural to write something of the kind:

	(in2) mygraph (a) ; (in1)(a) picInPic (out1)

Why not? Do you agree? Also, getting back the point above about  
feedback, what would be wrong with something like:

	(a)(in1)picInPic , mygraph (a)

Why not? Is that  allowed?


>> with respect to the form above (A)(B)(C) my_filter1 (out1), if I make
>> a textual substitution I find myself with
>
> As I've already said, textual substitution is not needed (and not
> wanted, at least by me). It's not easier to parse and it's not  
> easier to
> understand. So, unless you give a good argument for textual
> substitution, please stop using it in examples or arguments for how  
> the
> syntax should be.

I don't think I am making an argument for textual substitution, I am  
trying to discuss mechanisms for input/output binding. I am not  
thinking about implementation, I am thinking about combinators, and I  
would like to be able to understand them also at the level of  
syntactic manipulation.

>> Because of course, you'll have situations like this:

>>
>> 	my_f1 = '(in1)something(out1)'
>> 	my_f2 = '(in2)otherthing(out1)'
>>
>> 	movie=f1.avi(A);
>> 	movie=f2.avi(B);
>> 	(A)(B) my_f1 ; my_f2 (out1)(out2)
>>
>
> no. Syntax error. You defined
>
>         my_f1 = '(in1)something(out1)'
>
> That means you defined a filter that gets one (and only one) input,
> filter it with the filter "something" and gives one (and only one)
> output. In
>
>        (A)(B) my_f1 ; my_f2 (out1)(out2)
>
> you are passing two outputs to my_f1. Maybe you meant
>
>  	my_f1 = '(in1)something(out1)'
>  	my_f2 = '(in2)otherthing(out1)'
>
>  	movie=f1.avi(A);
>  	movie=f2.avi(B);
>  	(A) my_f1 (out1); (B) my_f2 (out2)

I'd expect "(A)(B) (my_f1 ; my_f2) (out1)(out2)" and "(A) my_f1  
(out1); (B) my_f2 (out2)" to be exactly the same (perhaps I should use  
* here instead of ; ?) And here is why: "my_f1;my_f2" has got two  
input and two streams A (out1) is the first input (output), B (out2)  
the second input (output). The same happens to (A) my_f1 (out1); (B)  
my_f2 (out2)

(There is however something else I don't understand with (B) my_f2  
(out2): can you use out2? Or should it be out1? There is only one  
output there.)

> Also, looking at
>
>        (A)(B) my_f1 ; my_f2 (out1)(out2)
>
> It gives me the impression you misunderstood the meaning in my  
> syntax of
> the semi-colon.

Yes, I am clearly confused about your syntax. Sorry. I am trying to  
understand.

> For such a line be valid, my_f1 can't have any output.
> I'll try to explain by a drawing:
>
> (etc) filter (etc2) ; (tmp1) filter2 (tmp2)
>
> means
>
> (etc) ------> filter ---> (etc2)       (tmp1) --->  filter2 ---->  
> (tmp2)
>
> another example
>
> (etc) dead_end_filter ; vsrc_movie=a.avi (tmp2)
>
> (etc) ----> dead_end_filter     vsrc_movie ---> (tmp2)
>
> (mind the difference with the coma)
>
> The semi-colon describes two different parts of the filter chain  
> without
> doing any linking. As I said,
>
>        vflip ; hflip
>
> is a syntax error no matter where it appears. It expects vflip to have
> no output, but it has one. (Yes, I know it is related with your '*',  
> but
> it has the lowest precedence)

This I don't understand: everybody knows what these filters are like,  
that is a perfectly fine filter with two inputs (first to vflip,  
second to hflip) and two outputs (first from vflip, second from  
hflip). Why do you want me to over-specify (in1)vflip(out1); 
(in2)hflip(out2) ? As I said above, the only reason I might want to do  
that is if I need to name the output for some non-trivial threading.

>> I think it is important that whatever the syntax is, users are  
>> allowed
>> to omit elements so as to write simple filters as simply as always,  
>> eg:
>>
>> 	... remove_logo,deinterlace,crop,scale,pad...
>>
>> In this sense, it's important to be able to leave streams implicit if
>> possible.
>
> Yes, I'm favorable to that. But how is this related to this  
> suggestion?

Only, is this allowed by your syntax?

> Which convention for implicit linking?
>
> The comma links the last stream of the previous filter to the first of
> the next. This is explicit and well defined.

Well, implicit means that it is not explicitly noted. I don't  
particularly like it, but after all I don't mind. What I still don't  
understand is what do I need to do in order to send the last stream of  
the previous filter somewhere else. Is the answer: use ";" ? That'd be  
quite an overhead, like below.

> If you don't want "implicit" linking, you can do all your links
> explicitly (and named, btw), like
>
>
>                   L1           L3
> (in1) ---> split ---> rotate ------+
>                |                   picinPic ----> out
>                +-----> nop   ------+
>                   L2           L4
>
> (in1) split (L1) (L2);
> (L1) rotate (L3);
> (L2) nop (L4);
> (L3) (L4) picinPic (out);
>
> No comma, no "implicit" linking.