Re: [FFmpeg-devel] [RFC] FFmpeg Execution Graph Visualization

[Stefano Sabatini <stefasab@gmail.com>]

On date Friday 2025-03-21 20:11:29 +0000, Soft Works wrote:
[...]
> > What it's not clear to me is how this builds up on top of text
> > formatters, since they are meant to render a tree structure in a
> > generic way. From this you can have a description of a graph, but then
> > you need specialized ad-hoc logic to convert it to a graph format.
> > 
> > What am I missing?
> 
> 
> Hehe, that's been a bit tricky indeed, but more in terms of figuring
> out that (and how) it can fit into the writer/textformatter
> patterns, the result is rather simple and it requires only some
> moderate extensions to the writer/formatter APIs:
> 
> 
> It is using a Mermaid Flowchart graph with only 3 kinds of elements:
> 
> - Shapes
>   i.e. "boxes", can't be nested
> - Subgraphs
>   Container for shapes, can be nested
> - Links
>   connection from one shape to another
> 
> 
> A simple graph with 2 shapes in a subgraph looks like this:
> 
> ...
> flowchart TD
>  subgraph s1["Subgraph 1"]
>         A("Shape A") --> B("Shape B")
>   end
> ...
> 
> There's a link from A to B - but: it can also be written in a different way:
> 
> ...
> flowchart TD
>  subgraph s1["Subgraph 1"]
>         A("Shape A")
>         B("Shape B")
>   end
> 
>   A --> B
> ...
> 
> This does the same thing. The link definitions can be at a different
> place in the document - which is one of the key points to make it
> work.
> 
> Then there are three new section flags to denote subgraph, shapes
> and links and two fields in the section struct (source_id_key,
> dest_id_key). For link-producing sections they indicate the names of
> the keys to use for building a link. A section can also produce both
> - shapes and links when the flags are set accordingly.  I also use
> another (existing) mechanism, which is the data object that can be
> provided on section start, only that it's a (publicly) defined
> "SectionContext" struct, which is a somewhat alternative
> direction.

So basically a graph can be represented as a collection of subraphs,
each subgraphs containing shapes, and links.

> I kept it for the moment as suggestion because I didn't
> know which ideas you have for the API to evolve, so that's just
> something for discussion.

One of the possible uses is to expose the data printed by
filters. E.g. detection filters are printing the information either in
the stderr using custom formats, this should really be converted to
something easier to consume (whatever formats for which you don't
need a custom parser).

The metadata muxer also might benefit from using a text writer, to
avoid again the need for a custom parser, and probably there are more
use cases easy to spot.

So at some point we want to make this API accessbile from the
libraries, that is to move them into libavutil. Again, it's fine to
expose this at the tools level first so we can experiment and refine
the interface before moving to a stable one.
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