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From: Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>
Newsgroups: comp.os.vms
Subject: Re: Local Versus Global Command Options
Date: Fri, 14 Feb 2025 13:38:59 -0000 (UTC)
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On 2025-02-13, Lawrence D'Oliveiro <ldo@nz.invalid> wrote:
>
> What?s the most complex *nix command you?ve come across? I think it
> has to be ffmpeg. This looks broadly like
>
>     ffmpeg «local-input-options-1» -i «infile-1» \
>         «local-input-options-2» -i «infile-2» ... \
>         «local-output-options-1» «outfile-1» \
>         «local-output-options-2» «outfile-2» ...
>
> The convention is that options apply to the immediately following file
> specification: this is prefixed with ?-i? for an input file, and is a
> plain argument for an output file. Note the lack of ?global? options:
> all settings apply to a particular file.
>
> But that?s not where it ends. Certain of the options can specify
> ?filtergraphs?, which are entire chains of effects operations to be
> applied to a particular video or audio stream. The man page talks
> about ?simple? versus ?complex? filtergraphs, but even the ?simple?
> ones can be pretty complex.
>
> Filtergraphs can also be used during real-time playback, with the
> ?ffplay? command. For example:
>
>     ffplay -autoexit -vf scale=1152:864,setsar=0.9 \
>         'Sun Is Shining (Official Video).mp4'
>
> That ?-vf? option specifies a sequence of video filters, first to
> scale up the video to make more use of my screen, and ?setsar? (?Set
> Source Aspect Ratio?) to fix distortion in the shape of the image
> (everybody looking squashed) from the original digitization of the
> video.
>
> What would DCL-style syntax for ffmpeg look like? I suppose one
> obvious equivalence would be
>
>     ffmpeg -
>         «infile-1»/«local-input-options-1»,«infile-2»/«local-input-options-2»,... -
>         «outfile-1»/«local-output-options-1»,«outfile-2»/«local-output-options-2»,... -
>
> (being very careful about where the commas go), but what about the
> syntax for filtergraphs? Would it be something like
>
>     /vf=(«filter-name-1»=«filter-params-1»,«filter-name-2»=«filter-params-2»...)
>
> Does DCL have provision for this sort of complexity?

Not in any meaningful way. There's no way to validate the syntax or
parameters of a filter or other ffmpeg syntax with DCL. For a simple
example, when merging two files, one with an audio stream and one with
a video stream, into a MP4 output container then specifying the input and
output video stream numbers would be a parameter along the lines of "0:v:0".

How would you even specify in DCL the first and last fields are numbers
and the middle field is a letter from a list of valid values ?

You can use DCL syntax in the way you specify, but the vast majority
of the parsing would still have to be done in ffmpeg as it is at the
moment. DCL syntax doesn't really give you anything extra.

Similar comments apply to mplayer BTW, and to make something clear to
people reading this (which is only implied above), the filters MUST be
executed in the order given. For example, with mplayer, I might first
use a crop filter to get rid of black bars within a frame and then
apply a scale filter to the resulting frame.

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Walking destinations on a map are further away than they appear.